THE SOVIET JOURNAL OF ATOMIC ENERGY NO. 4

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP10-02196R000100090006-9
• Release Decision: RIFPUB
• Original Classification: K
• Document Page Count: 211
• Document Creation Date: December 27, 2016
• Document Release Date: April 3, 2013
• Sequence Number: 6
• Publication Date: May 29, 1956
• Content Type: REPORT

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AT0NHa5I
p.rHM.
Number 4, 1956
The Soviet Journal of
ATOMIC
ENERGY
IN ENGLISH TRANSLATION
CONSULTANTS BUREAU, INC.
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ATOMNAYA ENERGIYA
Academy of Sciences of the USSR
Number 4, 1956
A. 1. Alikhanov, A. A. Bochvar, V. S. Fursov, V. F. Kalinin,
G. V..Kurdyumov, A. V. Lebedinsky, 1. 1. Novikov (Editor in Chief),
V. V. Semenov (Executive Secretary), V. 1. Veksler, A. P. Vinogradov,
N . A . V l a s o v (Acting Editor in Chief)
The Soviet Journal
of
ATOMIC ENERGY
IN ENGLISH TRANSLATION
CONSULTANTS BUREAU, INC.
227 West 17th Street
New York 11, N. Y.
Printed in the United States Annual Subscription $ 75.00
Single Issue 20.00
Note: The sale of photostatic copies of any portion of this copyright translation is expressly prohibited by the
copyright owners. A complete copy of any article in the issue may be purchased from the publisher for $12.50.
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THE SIX-METER SYNCHROCYCLOTRON OF THE INSTITUTE OF NUCLEAR
PROBLEMS, ACADEMY OF SCIENCES USSR*
D. V. Efremov, M. G. Meshcheryakov, A. L. Mints, V. P. Dzhelepov,
P. P. Ivanov, V. S. Katyshev, * * E. G. Komar, I. F. Malyshev, N. A. Monoszon,
1. Kh. Nevyazhsky, B. I. Polyakov, A. V. Chestnoi
The chief characteristics of the six-meter synchrocyclotron of the Institute of Nuclear
Problems of'the Academy of Sciences, USSR, which provides proton acceleration to an
energy of 680 Mev, are described.
INTRODUCTION.
The construction of powerful high-energy accelerators for heavy.particles became feasible with the
announcement by V. 1. Veksler (1944) and. McMillan (1945) of the phase stability principle in connection with
particle motion in cyclic resonance accelerators.
In order to further research in the physics of high-energy particles and gain experience in synchrocyclotron
acceleration techniques a large five-meter synchrocyclotron was built at the Institute for Nuclear' Problems of the
Academy of Sciences, USSR using this machine it was possible to accelerate deuterons to an energy of 280 Mev
and cc-particles to an energy of 560 Mev. In 1950, protons with an energy of 500 Mev were obtained with. this
machine [1-3]. The construction of the accelerator was preceded by studies on a working; model which clarified
a number of questions connected with the start-up and operation [4].
Studies of the operation of this machine and experience acquired in carrying out research with it made
it possible in 1953 to modify the accelerator so that it became feasible to accelerate protons to an energy of
680 Mev. The average current in the outermost orbit in this operation was 0.3 pamp. A general view of the
six-meter synchrocyclotron is shown in Fig. 1.
In rebuilding the machine a new vacuum system was installed, the diameter of the pole-pieces of the
electromagnet was increased to six meters and a new radio-frequency resonance system was'developed. The
six-meter synchrocyclotron provides intense beams of positive and negative it -mesons with energies up to
400 Mev and neutrons with energies up to 600 Mev. By making certain minor changes in some of the compo:=
nents of the radio-frequency system in this machine it is also possible to obtain deuterons with energies of
420 Mev and a-particles with energies up to 840 Mev [5], [6].
The Electromagnet
It is well known that the stable motion of ions in the gap of an electromagnet is limited to the region
.in which the index denoting the decay of the magnetic field intensity in the radial direction .
dlnH
n _ ding 0. 2,
*Reported at the All-Union Conference.on the Physics of Hi gh-Energy Particles, May 14, 1956.
*Deceased.
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so that for ii = 0.2 there Is observcd a parametric resonance between the free vertical and radial' oscillations of
the ions being accelerated.
General view of the accelerator.
In actual accelerators, because of the lirnitatioh of the vertical dimension of the aperture of the .dee,
the region in which parametric.resonance niay.arise determines the. maximum radius of the region of magnetic
field in which the acceleration of jOns is feasible. This was kept in mind in designing so as to obtain the largest
possible stable orbits for the accelerated ions.
In the five - meter magnet the pole tips were ' fabricated from. soft iron. in the form of solid discs. The
pole tips now in use, which are six meters in diameter, are welded together from several pieces.
The pole tips simultaneously serve as the upper and lower walls of the vacuum chamber located in the
gap between the poles of the iiiagnet. The gap. space between the pole tips at the center of the chamber is
600 mm. The magnetic field intensity at the center of the' vacuum chamber is 16,600 ,Nauss. The length of
the electromagnet is 18 m, height about 10 m ,and weight'7.,000 tons. The yoke was made from ordinary carbon
steel. The exciting winding of the electrorria;~net consisted of .air cooled copper straps. The, direct-current
generator required 'to obtain the nominal field intensity had a power rating of 1000 kw. The current in the ex-
citing winding was stabilized to within. ? 0.1 ?fo.
A great deal of laborious computational and experimental work was devoted to the magnetic field correc-
tions in the acceleration region. In addition to the use of shims 'at the periphery of the pole pieces a great deal
of effort was made to insure the coincidence of. the surface at which the radial component of the -magnetic field
was zero with the median plane of the acceleration chamber.
In carrying out the work on the correction of the magnetic field configuration it was found necessary to
develop apparatus suitable for accurate magnetic measurements; an instrument for measuring the decay of the
magnetic field in the radial direction [1]; an instrument for determining the azimuthal asymmetry of the-mag-
netic field [7], and 'apparatus for determinf; the location of the surface at which the radial component of the
magnetic field vanishes.
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As a result of these studies and the corrections of the magnetic field of the six-meter synchrocyclotron
the radius of the region in which stable motion, of the accelerated ions could] be guaranteed reached a value of
279 cm.
The machine also had a number of auxiliary non-symmetrical exciting windings fed fron a separate gen-
erator thus making it possible, while the machine was in operation, to vary the location of the plane containing
the trajectory of the accelerated particles; this feature was used in determining the optimum operating conditions
The decay of the magnetic field intensity in the radial direction from the center out to the limiting radius
needed for focusing the particles in the vertical plane was 4.9%. The displacement of the center of the outer-
most orbit did not exceed 2 cm. Careful adjustment of the magnetic field of the accelerator made it possible
to accelerate protons to an energy of 680 Mev [8]. To enhance ion focusing in the. initial acceleration stage
-steel. cones were placed at the center of the pole tips.
Means were also provided for relatively fast changes of the polarity of the electromagnet (15 min) thus
making it easy to get an output of tr+ or n mesons of various energies through the collimators in the apertures
in magnet supports.
Resonance: system :and Radio-Frequency Generator [9)
The design of the rf system for this machine was dictated by the necessity of obtaining an accelerating .
voltage of some 15 kv and a frequency change from 26.5 to 13.6 me for proton acceleration. In this connection.
it is necessary that the metal rotor of the variable condenser, 'which determines the frequency, be located in a
region of weak magnetic field in order to avoid high eddy currents. On the other hand the removal of the rotor
to a location at which the leakage field of the magnet is weak is undesirable because it is impossible to keep
the system compact. Therefore extensive precautions were taken to see that the variable condenser, which was
located at a point at which the field intensity-is 600-800 gauss, is well shielded magnetically. Inside the shield
the field intensity is no greater than 30 gauss. It turned out, however, that under these conditions the distance
from the center of the pole pieces to the variable condenser is approximately equal to a half-wavelength at
the high-frequency end of the operating region. Thus to tune up the system in this region it was necessary to
raise the upper resonance frequency both by changing the wave impedance and by constructional changes which
resulted in a slight increase of the current paths in the system. The expansion of the operating range on the low
frequency side was accomplished by increasing the. wave impedance of the system in the immediate neighbor-
hood of the variable condenser. This part of the system can be considered a "lumped" inductance.
The radio-frequency resonance system consists of the dee with. its grounded frame, ,the variable condenser
and the line which connects them. Close to the variable condenser the latter assumes the form of a coaxial line;
it is connected to the dee through a smooth transition section (Fig. 2).
The rotor of the variable condenser, which consists of '6 discs with 10 fins in each is braced against the
inner conductor of the coaxial line by insulators and is electrically connected to it by a semicylindrical (6 pairs
of cylinders) condenser with a capacity of 20,00.0 ?f. The.shaft and rotor are held firmly by metal bearing-
supports which are located radially at the center of gravity of the system. To keep the reactance of the supports
high over a wide range of frequencies they are made in the form of cylindrical spirals, i. C. chokes; these are
fabricated from special hollow steel tubing which is water cooled and copper-plated to reduce the radio-frequency
resistance. The inductance of these bearing supports is in parallel with the line and increases the upper frequency
of the operating range to some extent.
The present design provides a wide, range of operating frequencies with relatively small values of current
and voltage in the variable condenser. It has been feasible to use an accelerating voltage amplitude of more
than 15 kv and an accelerating-cycle repetition rate of about 100 cps.
The semicylindrical condenser effectively shunts the ball bearings of the rotor and its contact brush which
is a slotted bronze collar which rides on the steel shaft of the rotor. Although the peak current through the con-
denser reaches 3,000 amp the current through the bearing and contact brush does not exceed 100 amp. 'The
*The control of the location of the plane also made it possible to reduce smoothly by a factor of several thousand
the intensity of the external proton beam; this was necessary in order to carry out several physics experiments.
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apparatus has been operated without any trouble, for more than 10,000 hours without replacerrient of bearings.
Fig. 2.' The dee removed` froth the .vacuum chamber.
The rotor design permits the capacity of the resonance system of'th.e radio-frequency generator to be
varied and gives optimum values of.the system impedance over a wide range of frequencies.
Parasitic oscillations were virtually suppressed by using a "band-pass" oscillator system in which positive
feedback occurs only in a specified frequency region; this system allows wide; independent control of the ab-
solute magnitude of the positive feedback in the extreme high and low frequency regions. The system makes
use of a series of LC circuits , between the plate and the cathode input and. between the input grids and cathode
of two ultrahigh, frequency oscillator triodes (GU-12A) which are of the grounded type and also employs out-
put inductances, interelectrode condensers, and cathode chokes.
The operation of the resonance system and the radio-frequency'generator have been described in a sepa-
rate report-[61,
The radio; frequency systeili can be cut off during the non-operating dart of the cycle by a thyratron unit
which controls the,grid of the oscillator [9]. The system can be,operated so that. the acceleration cycle is con-
trolled.by a frequency which is several times-smaller than the modulation frequency; it is also possible to obtain
single acceleration pulses, thus allowing the system to trigger'a Wilson cloud chamber or some other detection
device. All modes of operation arc controlled by a special timer unit [10] which maintains strict time sequence
between the pulses which trigger the ion source, the thyratron control system and the.other components. Use
of the timer also makes it possible to synchronize the operation of the accelerator' with that of other instruments
and devices being used in the various nuclear research projects. All the counting.sequences are controlled by
the frequency-change cycle of the radio-frequency oscillator which drives the resonance system. The switching
of the radio-frequency voltage during each cycle is accomplished by means of a photoelectric system in which
a light beam incident on a photoelectron multiplier is interrupted by the fins, on the rotor of the variable con-
denser.
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Vacuum System
The operating stability of the accelerator as well as the intensity of the accelerated ion beam at the out-
put are determined to a large degree by the vacuum conditions in the volume of the chamber and frequency vary-
ing, device. The accelerator vacuum chamber has the form of 'a rectangular parallelepiped 675 x 675 x 100 cm3.
This chamber is built from brass plates 100 mm in thickness. The steel top and bottom of the chamber also
serve as the pole tips. The vacuum seal between the brass plates of the chamber and the pole pieces, as in the
other vacuum apparatus, is achieved by means of rubber gaskets.
Following appropriate conditioning of the chamber the minimum pressure of the residual gas is 2.10-6 mm
Hg. With the introduction into the ion source of the working gas the pressure does not exceed 6-7. 1.0-6 mm Hg.
The volumes of the chamber, the resonance line and '.the variable condenser are contiguous. The total volume in
which a high vacuum is maintained is about 35 m3,. The vacuum chamber is evacuated by two oil-jet pumps
with a total capacity of 80, 000 liters per second at a pressure of 1. 10-5 min Hg. The housing of the variable
condenser is evacuated by an auxilary pump with a capacity of 10,000 liters/sec at a pressure of 1. 10-5 mm Hg.
The baffles of the high vacuum pumps are cooled to a temperature of -20? C to freeze out the oil'vapors.
The quality of the vacuum gaskets is such that under normal conditions the increase in the chamber pressure
which arises due to the leakage of air from the outside and outgassing from the inner surfaces amounts to 0.2-0.3
mk/hour.* . If a preliminary vacuum of the order of 10-3 mm Hg is maintained in. the chamber then the opera-
ting vacuum for the conditioned accelerator can be reached in 25-30 min after opening the vacuum valves on the
oil-jet pumps.
The vacuum chamber of the accelerator is provided with various devices which are used to introduce targets
into the chamber and to locate them at a given radius. This operation is carried out remotely without disturbing
the operating vacuum in the chamber.
Ion Source and Particle Extraction
The ion source in the six-meter synchrocyclotron is the usual arc type with a thermal tungsten cathode.
A cold cathode was also used quite successfully. In this case a considerably greater stability in the magnitude
of the ion current is achieved at normal intensities. In the cold cathode source the discharge is excited by the
radio-frequency field in the dee by secondary electron emission from an aluminum or beryllium cathode.
Extraction of the proton beam from the vacuum chamber into the external region is accomplished by
excitation of radial oscillations of the accelerated particles in, the outermost orbit and the ejections of protons
through a magnetic channel [11j. The ion current in the external beam is 5-7ufo of the current in the circulating
beam. A large of number of beams of neutrons and charged ir-mesons of both sign is also extracted. The extrac-
tion of each beam is accomplished by means of special devices located in the accelerator. A separate report
[12] is devoted to information on the extraction of particle beams from the vacuum chamber.
Arrangement of the Accelerator Facilities.
All the accelerator facilities are located in two buildings. The first building contains the equipment which
cannot be_ separated from the accelerator by any appreciable distance if efficient operation is to be achieved..
In this building there is a laboratory with experimental and monitoring apparatus which is used in research on the
external particle beams. '
In order to provide the most favorable conditions for carrying out research with the accelerator the control
room of the accelerator is isolated from the main laboratory by two concrete shielding walls with thicknesses of
4 and 2 to and a ceiling 1.5 m thick. The passage of the particle beams through the 4-meter wall which separates
the accelerator -room from the measurement enclosure is allowed by a porthole with collimators (Pig. 3).
The electric power facilities and the water cooling installation for the' resonance system and the radio-
frequency generator, and the control console for all circuits and components of the accelerator are located in
the second building.
Probably equals 10-6 mm Hg/hour.-Translators note].
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Synchrocyclotron room
Th rotr,,n Radio-frequency gen-
Local control
panel
Vacuum -1
pump Collimator
N IGS
vacuum
chamber
Electromagnet
Concrete
Fig. 3. Plan of the six -meter- synchrocyclotrori building.
Control of the' Synchrocyclotron
Because of the radiation which accompanies the operation of the machine the presence of personnel in
the main room in the immediate vicinity of.the accelerator is not possible. The monitoring functions involved
in the operation of the machine and the control of the. facilities are carried out remotely by the attending en-
gineer and technician who remain in a room containing the control console in the second building (Fig. 4).
For this purpose there are provided a number of instruments and devices which permit automatic remote control
of the facilites.
The synchrocyclotron and its facilities have operated without trouble for many thousands of hours without
requiring any significant shutdown. for repairs or modification.
Main Trends of Nuclear Research
In the research which is being carried on at the synchrocyclotron of the Institute of Nuclear Problems of
the Academy of Sciences USSR most attention as being devoted to the nucleon energy region 380-660 Mev and
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to the following three types of nuclear processes: the elastic scattering of protons by protons, neutrons by protons,
and neutrons by neutrons; the production of charged'and neutral ir-mesons in nucleon-nucleon collisions, and the
interaction of if-mesons with nucleons. Cxperimental investigations are also being devoted to a study of the inter-
action of nucleons and ir-mesons with nuclei. A.dis.i~ussion of these investigations isbeyond the scope of the pres-
ent report and is to be found ;1.n appropriate- papers.
Fig. 4. Main control console..
At the synchrocyclotron of the Institute of Nuclear Problems, which at the present time is the largest
machine of ifs type in the world, research in the energy region up to 700 Mev is being carried on by many.
physics and chemistry institutes of the Academy of Sciences USSR.
The accelerator is operated regularly from 100-105 hours per week. It is possible to carry out research
on 13 external beams of protons, neutrons, and 7r-mesons of high energy.
The construction of this accelerator is the result, of the combined efforts over a number of years of a large
group of scientists, engineers, and technicians. Many plants participated in- the construction of these facilities;
particularly. manufacturers of electrical equipment.
The design and development of,the various components of the six-meter synchrocyclotron involved 'a good
deal of research in,the realms of physics, radio-engineering, electronics, power engineering and vacuum tech-
niques. As a.result of this research it has been possible to avoid,a great deal of the difficulties involved in the
start-up operation, although certain of these remained and were overcome in the initial operating stages.
It is apparent that the synchrocyclotron acceleration, technique is extremely convenient in this region of
particle energy. The experience acquired in operating actual synchrocyclotrons and calculations which have
been made indicate that the upper limit on the energy for this- method of accelerating protons is approximately
1,000 Mev.
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? [1] M. G. Meshcheryakov, A. V. Chestnoi, V. P. Dzhelepov, V. S. Katyshev, A. A. Kropin, V. P. Dmitrievsky,
et al. (Report Inst. Nuc. Prob., Acad. Sci. USSR) (1951).
(2] L. M. Gurevich, N. K. Kamins]cy, I. G. Klyatskin, I. Kh. Nevyazhsky, B. I. Polyakov, N. K. Titov, and
V. F. Trubetskoi (Rep. RAL . Acad. Sci. USSR) (1947-1950).
[3] D. V. Efremov, E. G. Komar, and N. A. Monoszon (Report NIIEFA) (1950).
[4] M. G. Meshcheryakov, G. I. Budker, V. P. Dmitrievsky, V. S., Katyshev, A. A. Kropin, and A. V.
Chestnoi, et al. (Report Inst. Nuc. Prob.,Acad. Sci. USSR) (1947-1949).
[5] M. G. Meshcheryakov., A. V. Chestnoi, V.. P. Dzhelepov, V. S. Katyshev, M. F. Shulga, and V. T.
Dmitrievsky (Report Inst. Nuc. Prob., Acad. Sci. USSR) (7.954).
[6] A. L. Mints, I. Kh. Nevyazhsky, and B. I. Polyakov, Certain Characteristics and Information Con-
cerning the Radio Frequency System of the Six-Meter Synchrocyclotron (Report to the All-Union Conference on
the Physics of High Energy Particles) (Moscow, 1956).
[7] M.'P. Zeldovich, and S. M.'Rubchinsky (Report RAL Acad. Sci, USSR) (1949).
[8] A. V. Chestnoi, V. S. Katyshev, V: P. Dmitrievsky, A A_.Kropin, B. I Zamolodchikov, T. N. Tomilina,
and V. B. Mukhina (Report Inst. Nuc: Prob., Acad. Sci. USSR) (1953j,;
[9] A. D. Vlasov, G. P. Grudinskaya; G. I. Zhileiko, B. T. Zarubin; B. G. Kulman, V._M Lupulov, I. Kh.
Nevyazhsky, and B. I. Polyakov (Report RALAcad. Sci. USSR) (1948-1955).
[10] M. M. Veisbein, G. I.. Kiryanov, and A. K. Kotlyakov (Report RAL Acad. Sci. USSR) (1954).
[11] V. P. Dmitrievsky; V. I. Danilov,'Yu. N. Denisov, V. S. Katyshev, A:: A. Kropin, and A. V. Chestnoi,
Extraction of the Proton Beam from the Six-Meter Synchrocyclotron (Report-to the All-Union Conference on the
Physics of High-Energy Particles) (Moscow, 1956). .
[12] V. P. Dzhelepov, V. P. Dmitrievsky, V. S. Katyshev, M. S. Kozodaev, M. G. Meshcheryakov, K. I.
Tarakanov, and A. V. Chestnoi, The High-Energy Particle Beam from the Six-Meter Synchrocyclotron and its
Utilization(Report to the All-Union Conference on the Physics of High-Energy Particles) (Moscow, 1956).
Received May 29, 1956.
457
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HIGH ENERGY PARTICLES FROM THE SIX-METER SYNCHROCYCLOTRON
AND THEIR UTILIZATION*
(REVIEW ARTICLE)
V. P. Dzhelepov, V. P. Dmitrievsky, V. S. Kat.yshev, M. S. Kozodaev
M. G., Meshcheryakov, K. I. Tarakanov, and' A. V.'Chestnoi
The problem of increasing the efficiency of utilization of the six-meter synchrocyclotron
at the Institute of Nuclear Problems; Academy of Sciences USSR'.is reviewed, .The method by
which a large number of particle beams is obtained and collimated is described; using these
beams it is possible to carry on several simultaneous experiments. Characteristic beam data
are-'presented.
INTRODUCTION.
Accelerators which produce particles with energies of several hundreds of millions of electron volts offer
wide possibilities for carrying on research: on the most important problems of contemporary nuclear physics, for
instance studies of the structure and properties. of elementary particles, clarification of the nature of their inter-
action, determination of the characteristics of their exchange reactions and so on.
High energy accelerators represent large scale industrial efforts and require the expenditure of large sums
for their construction and operation. Thus the question .of the efficient utilization of these machines assumes
major importance.
The present paper is devoted to a short description of the approach to this problem which has been adopted
at the. synchrocyclotron of the Institute of Nuclear Problems, Academy of Sciences USSR [1) which accelerates
protons to an energy of 680 Mev;
Basic Methods of Increasing the Efficient of Utilization of the Accelerator
The chief objective of the scientific research at the six-meter synchrocyclotron is the study of elastic
and. inelastic nucleon-nucleon interactions in the 300-660 Mev energy region and the scattering of Tr-mesons
by nucleons and deuterons. Since the cross sections for the majority of these processes range. from several
millibarns to some tens of rillibarns the acquisition of accurate quantitative data entails the expenditure of
a considerable amount of accelerator operating time.
The efficiency and scope of the utilization.of the synchrocyclotron in nuclear research with high-energy
particles depends to a large degree on the rational solution of two problems: the extraction from the acceler-.
ator vacuum chamber of intense beams of various types of high energy particles and the reduction of the back-
ground caused by the accompanying. radiation.
*Presented at the CERN symposium on high energy accelerators and meson physics (Geneva, June 1956).
* *Deceased.
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In this connection we may n-iakc Mention of the following features of the facilities at the six-meter
synchrocyclotron:
a) the beasts of high energy protons, neutrons, and ir-rrtesons are extracted from the accelerator chamber
through the shielding wall in thirteen directions;
b) there area rncasurementenclosureand a special laboratory, shielded from the background radiation, for
work with it-meson beams;
it is possible to carry on simultaneous experiments with several beams of the same or different particles;
the experimental equipment. is operated automatically by remote control;
nuclear events are recorded by means of amultichannel electronic system.
The High Energy Particle Beasts.
In principle it,is possible to obtain beams of high-energy particles from any point in the orbit of the
accelerated protons. The realization of this possibility,thowever, depends strongly on the design of the acceler-
ator.
It is important that the accelerator vacuum chamber and its side walls be free from any devices which
might hinder the extraction of the particle beams. The extent to which this condition has been realized may
.be seen from Figs. I and 2.
N&A
Fig. 1. The vacuum chamber of the synchrocyclotron showing the side from which the proton, neutron and
tr-meson beams are-extracted.',
460
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nceeh'r,tjon nice trode
vacuuw chflnda?r
probe
Iupptcrnentnry
,hte)dtng
Fig. 2. The high-energy particle beams from the six-meter synchrocyclotron.
The first of these shows the front wall of the chamber. Almost the entire length of this wall, at the level
corresponding to the proton orbit, is provided with a window for the extraction,. of -the particle beams; these are
duraluminum diaphragms approximately 1 mm thick.
In Fig. 2 is presented a general diagram. of the accelerator chamber which shows.the particle-beam con-
figuration, the shielding layout and the measurement apparatus.
Most of the high energy particle-beams which are ejected into the atmosphere are directed into the measure-
ment enclosure which contains the experimental apparatus. The collimators are located in the porthole through
the four-meter concrete shielding wall. The latter are steel pieces of square cross section, 3.6 m long, which
have circular openings through the center, the diameters of which vary from 10 to 150 mm. The space between
the collimators is filled with cast-iron blocks, providing good shielding.for the porthole as well as the possibility
of making changes in the arrangement of the collimators. Fig. 3 shows a longitudinal section through the build-
ing which houses the synchrocyclotron and the supplementary shield of concrete blocks which is in front of the
four-meter wall of the measurement enclosure (in Fig. 2 this shield is indicated by dotted lines). The figure
also shows a collimator of circular shape located in one of the neutron beams which is designed for work with
circular scatterers.
Shielding of Synchrocyclotron Operating Personnel From Accelerator Radiation
An idea of the amount of shielding for the measurement enclosure and the room containing the recording
and detection apparatus can be obtained from the following data. The general radiation level in the measure-
ment enclosure for proton energies of 680 Mev with currents at the external target of 0.2-0.3 pamp varies from
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0.1-0.2 mC/sec (in the right side of the enclosure) to 1.5-3mC/sec (in the left side of the enclosure in which
there is a dome in the ceiling to allow the operation of a crane); the corresponding fast-neutron intensity (energies
of 0.5 Mev and above) varies from 1-2 neut./cm?sec to 60 neut./cm2see.. The flux of neutrons with energies
higher than 50 Mev is less than 5 neut./cm?sec [2].
Fig. 3. Radiation shielding for the six-meter synchrocyclotron.
In the room which contains the recording instruments the radiation is negligibly small. This is indicated
by the fact that when the accelerator is operated at high intensities the Geiger-counter count in any part of the
room is not greater than double the cosmic-ray background[2].
'Characteristics of. High-Energy Beams Extracted From The Accelerator Chamber
Unpolarized proton beams. For obvious reasons great effort was expended in increasing the intensity of
the external proton beam. An earlier method of proton extraction based on the scattering of protons by a uranium
target was replaced by a new scheme developed experimentally at the Institute of Nuclear Problems, Academy
of: Sciences USSR [4]'and the Nuclear-Physics Laboratory, of the.-. University of Liverpool [5] in which radial
oscillations-of the particles are excited by a local. inhomogeneous magnetic field.
The local magnetic field inhomogeneities are obtained by placing an iron mass in the vicinity of the last
accelerator orbit. An exciter unit is used to start the radial oscillations. In the vicinity. of the exciter the mag-
netic field intensity, falls off radially. The adjustment of the precession of the centers of curvature of the particle
orbits is achieved by means of a second excitation region ("regenerator"). in which the magnetic field intensity
along the radius increases rapidly.
The magnetic field- configuration in the excitation zone and.the radial extent of this zone must be chosen
so as to provide a sufficiently large jump. in the last orbit (about 40 mm [6]) while not causing any perturbation
of the stable particle motion in the vertical, plane.
The extraction of the particles from the chamber was accomplished by means of a magnetic channel con-
sisting of two iron plates of varying cross section.
In order to control the angular spread of the external proton beam, following the magnetic channel there
is a focusing device which reducers the beam spread in the horizontal plane [7]. At a distance of 7 m from the
magnetic channel the beam diameter is approximately 8 cm. The angular spread of the external beam is less
than ? 0.5?.
The total intensity of the external beam and the flux at various distances from the chamber along the
particle trajectory was determined from the induced s -activity in the reaction Cl2(p, pn) Cu. These measure-
ments showed that the total proton current at egress from the magnetic channel is 7.1010 protons/sec; this is
5-6% of the average particle current in the vicinity of the last orbit.
Using a bending magnet the external beam can be directed into one of the three collimators (6, 7, or 8)
located in the concrete shielding wall (Fig. 2). Thus, on the one hand, the space in which experimental instru-
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merits for proton studies can be located is enlarged considerably, while on the other, most favorable conditions
for operation of the instruments are created.
The proton flux in the measuren-ient enclosure, at a distance of 15 m from the output window of the chamber,
is (1-2). 10s protons/cm2 sec. The energy spread in the proton beam is very small. The deviation of particle
energy from the mean value E av = 657 Mev, as determined by range measurements in copper, was no greater
than f 5 Mev.[8].
Because of the high intensity of the external proton beam, it was possible, outside of the accelerator
chamber, to obtain abeam of lr-mesons with energies up to 400 Mev at fl uxes of 60 mesons/cm2sec(at energies
about 240-270 Mev). In these experiments a ,target of liquid hydrogen or polyethelene is placed in the proton
beam, in front of the bending magnet (Fig. 2). The momentum and direction analyses of the 7r-mesons are
carried out in the measurement enclosure'. through collimators 8 and 9. At energies of 300 Mev the energy spread
of the ir-mesons in these beams was f 5 Mev [9]. hi Fig. 4 is shown a typical ,r. - meson energy spectrum obtained
from proton-proton collisions at 657 Mev [9]. (The it-meson emission angle is 24? in the laboratory system).
Neutron Beams, y Rays and Polarized Proton Beams. As in all other particle accelerators, at the six-meter
synchrocyclotron neutrons.' are obtained by proton bombardment of internal targets. (usually beryllium),which are,
.placed on probes (Fig. 2, probe III) in the vicinity of the outermost orbit. Four narrow "rays" are, obtained from
the wide beam of neutrons emitted by the target through the use of collimators 10; 11,12 and.15.. The first three
100 200 300 E, Mev
Fig. 4. Energy spectrum for 7r +- mesons in (p-p) collisions.
neutron beams are unpolarized. At.incident proton energiesof 680 Mev the flux of neutrons with energies greater
than 400 Mev at the location of the experimental apparatus is approximately 2. 104 neut./cmzsec.- The fourth
neutron beam (collimator 15, emission angle 18?) is partially polarized. It has been shown experimentally that
the polarization is approximately 15% [10]. The energy spectrum of neutrons emitted through collimator 11
(emission angle 0?) is presented in Fig. 5 [11]. The spectra of the neutrons emitted through the other collimators
are similar although the maxima are shifted somewhat toward the low energy region.
The beam emitted through collimator 15 exhibits the largest shift in the main maximum and this amounts
to about 80 Mev [11].
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In addition to the four neutron beams there are obtained from the same target four high energy y-ray
beams which arise in the decay of the neutral 7r-mesons which are produced. The y-ray flux for energies
higher than 10 Mev in the measurement enclosure is about 2.103 quanta/'cm2 sec (collimator 11). The energy
distribution is shown in Fig. 6 (12]. The study of these y-rays serves as a means of obtaining information on the
nature of the interaction of,r?-mesons with matter.
Since experiments with polarized nucleon beams are of great interest, two beams of polarized protons are
also extracted from the six-meter synchrocyclotron. One of these, which passes through collimator 4, consists
of protons which experience diffraction scattering on beryllium nuclei within the vacuum chamber. The beam
is polarized to about 60/ [13]; the proton energy is 635 ? 15 Mev. The other beam, which passes through colli-
mator 5, is comprised of protons which are quasi-elastically scattered by nucleons inthe beryllium nuclei. The
polarization of this beam is about 301 [13]. The intensities of both beams are approximately equal and are about
14-10 4 protons/cm2 sec.
d17?
de?
O 100 200 300 400 . 500 . 600. 700 tp, Mev
Fig. 5. The neutron energy distribution.
The 7r-meson Beam. In order to create the most favorable ' conditions for work with negative 7r-mesons,
a "meson laboratory" was built directly behind one of the vertical supports of the yoke of the magnet. The
iron in the yoke was about .3 in thick and provides good shielding against the direct radiation from the acceler-
ator chamber. The concrete walls. and, ceiling ' of the laboratory, which. are 1 m thick, serve as shields against
scattered neutrons and trays.
To provide an exit for mesons into this laboratory there are three collimators (1, 2, 3) in the magnet
yoke; by means of multiple magnetic shields the magnetic field in these collimators is reduced.from 1600 to
1-2 gauss.
7r-mesons which are produced in a beryllium target (10 mm in thickness) located inside the accelerating
electrode (dee) are directed into these collimators. The target is controlled remotely. By changing.its azimuthal
and radial coordinates in conjuction with appropriate changes in the direction of the magnetic field in the magnet
gap it is possible to obtain 7r-mesons of both signs and of various energies. Thus there are available 7r.--mesons
with energies ranging from. 140 to 410 Mev and it?-mesons with energies from 140 to 245 Mev. The .7r - flux
in the "meson laboratory" varies with energy from 200 to-2-3 mesons/cm2 sec [14]. The background radiation
level in this laboratory is quite high, however; the general radiation level is 1-2mC/sec, thermal neutron level
500 neut/cm2 sec, and neutrons with energies En > 50 Mev. 3 newt/cm2 sec [14]. .The thickness of the concrete
shielding should be increased to 1.5-2 m to reduce this background.
In carrying out experiments with Wilson cloud chambers and diffusion chambers the meson flux should
not exceed 20-30 particles/cm2 sec and the background of spurious radiation must be extremely low. In this
work 7r-mesons with energies from 150 to 400 Mev are utilized; these enter the measurement enclosure through
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collimators 13, 14, and 16. The intensities of these ir?-meson beams vary from 40 per cm2 sec (E,?.-= 230 Mev)
to 1-2 per cm2 sec (rlr-= 40.0 Mev). The energy spread is ? 6 Mev for r,-= 230 Mev [15]. The meson source
is a second mobile target the positioning mechanism of which is located on the front wall of the accelerator
vacuum chamber.
dN
dey dcd
/.0
O 100 200 300 400 500 219
Ey, Mev
Fig. 6. The energy distribution for y-rays from the decay of it ?-mesons
produced in the collision of 670 Mev protons with carbon nuclei.
In concluding this description of the particle beams we may note that the difficulty of locating the charged
n-meson beams was substantially reduced through the use of a scheme in which a. current-carrying wire under
tension is employed to determine the particle trajectories [16].
Irradiation of Samples Inside the Synchrocyclotron Chamber
In addition to the mobile targets located in the chamber the accelerator is provided with four probes by
means of which it. is possible to introduce (or extract) into the vacuum chamber samples. of various materials
for irradiation by the proton beam accelerated to the desired energy. A similar method of sample irradiation
is widely used in radiocheinical research.
,Simultaneous Operation With Several Particle Beams
In the experiments with high energy particles several different techniques are employed: electronic par-
ticle detection (scintillation counters and Cerenkov. counters in conjunction with photomultipliers), thick-
layered emulsions, magnetic spectrometers, Wilson cloud chambers, diffusion chambers, bubble chambers and
so on.
At the disposal of the experimenters there are ten electromagnets with pole diameters ranging from 30
to 100 cm, powered by a system which is capable of operating five magnets simultaneously. There are also
available several thousand concrete blocks weighing up to 50 kg for the construction of local shields.
The arrangement of the experimental apparatus in the measurement enclosure is shown in Fig. 7. All
terminal equipment for the detection apparatus (counting circuits, recorders, mechanical registers, etc.) and the
remote control panels for the experimental equipment and the magnet-current regulator, which are located in
places unsafe for personnel (the measurement enclosure and the "meson laboratory") are placed in a special
detection apparatus room which is separated from the measurement enclosure (see Fig. 3) by a concrete wall
2 m in thickness. The equipment located in these two rooms is interconnected by a system of radio-frequency,
high voltage, and control cables. The use of a patching system allows any of the detection instruments to be
used in combination with the experimental equipment in any part of the measurement enclosure.
When the machine is in operation no one is permitted in the measurement enclosure; the doors are self-
latching. Scientific personnel engaged in nuclear research at the accelerator remain in the room containing
the detection apparatus while the machine is in operation. As has been indicated earlier the background
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radiation level In this room Is extremely low thus allowing people to remain for an unlimited time:
Fig. 7. The measurement enclosure with experimental equipment. in place.
The present system of collimators and-shielding is arranged so that most advantage can be taken of parallel
(simultaneous) operation of groups of experimental instruments with several beam's-of the same or of different
particles. For instance over. a 'long period of time there were conducted experiments with 10-12 instruments on
four neutron'bearims and two polarized proton beams. It is also possible to conduct simultaneous experiments
with two cloud chambers (a Wilson chamber and a diffusion chamber) with the negative it-meson beams which
enter the. measure ment enclosure.
In this connection we may note that it has been feasible to pulse the accelerating voltage of the six-
meter synchrocyclotron [1]; this allows an interruption of the radio-frequency oscillations at the appropriate
instant of time and dtusparticlesemerge from the accelerator in bursts at times when the chambers are pre
.pared for detection.
In working with Wilson cloud cha'mbei- and diffusion chambers on experiments with maximum energy
tr -mesons, the :flux of which is quite low, to increase the intensity of the pulsed ir-meson beam, use is~ made of
the so-called "storage technique" [17]. This technique is based on the following: for several acceleration
cycles (three-four) protons are accelerated only out to a radius of 160-180 cm (with a corresponding energy of
240-300 M(,v);.then on the fourth or fifth cycle the accelerating voltage is applied over the entire frequency
range (25-14 me) as a result of which the protons are accelerated out to the limiting radius (278 cm) at which
the target is located: By this means it has been possible to increase three-fold the intensity of the s-meson
beam and hence to enhance the operating efficiency of the detection chambers.
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The extraction from the synchrocyclotron chamber of a large number of high-energy particle beams creates
the possibility of carrying on research on a wide front.
By carrying on several experiments simultaneously, by the use of a multichannel electronic system and
through the use of remote control of apparatus located in spots which are unsafe for personnel it has been pos-
sible to increase substantially the utilization factor of the machine and to keep non-operating time to a mini-
mum. The later is not greater than 7-8% and is determined chiefly by the time required for changes in the
experimental apparatus and modification of the machine.
LITERATURE CITED
[1] D. V. Efremov, M. G. Meshcheryakov, A. L. Mints, V. P. Dzhelepov, P. P. Ivanov,.V. S. Katyshev,
E. G. Komar, I. F. Malyshev, N. A. Monoszon, I. I 0 (the amplitude of deviations from equili- Fig. 2.. Construction of the compensating
brium is exaggerated. magnets.
Total number of magnets
120
Number of compensating magnets
15
Number of radial oscillations per, turn
13.752
Number of vertical oscillations per turn
12.744
Length of the radial focusing magnets
10.99
m
Length of the vertical focusing magnets
10.69
Gap length between magnets
1.518
Radius of curvature of the regular magnets
166.1
Radius of curvature of the compensating magnets
-296.6
Distance from the chamber axis to the asymptote of the hyperbolic pole
the regular magnets
pieces of
404.0
thm
Internal half-height of the chamber
60
Internal half-width of the chamber
100
Utilization factor for the magnetic field
0.805
Logarithmic, derivative,. of orbit length with respect to momentum
- 8.2. 10-4.
Opening angle of the beam
2.10-3
radian
Amplitude of the radial oscillations caused by compensation
40
mm
Allowable momentum deviations A-p/p
0.5%
Field tolerances . H/H
0..25%
Gradient tolerances Agrad/grad
0.31
Tolerance on magnet displacement
1.0 mm
Tolerance on vertical displacement (amplitude of 13th harmonic)
0.5 "
Data on the magnet power supply is given in Table 3.
Peak powers of the order of 100 kva will be obtained from generators with flywheels. The nominal
capacity of each unit is 37 kva. ' There is a 12-phase ignitron inverter system. In order to reduce the ripple.
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in the rectified voltage it is planned to use a filter and a special scheme for reducing ripple by negative feed-
Fig. 3. Construction of the regular accelerator
magnets.
Growth time for the magnetic field
Number of cycles per minute
Maximum exciting current
Maximum voltage
Resistance of exciting winding at 15?C
Maximum inductance of exciting winding
Peak power
back.
The injector will be a 100 Mev linear accelerator.
The magnetic field at injection is 90 gauss. Any devia-
tion of particle momentum from the nominal value leads
to compensating radial oscillations and causes the fre-
quency of the free transverse oscillations to approach
the approximate resonance value. This situation deter-
mines the allowable momentum deviations -5.10-3
The accelerating system is supplied with an rf
voltage the frequency of which is 30 times greater than
the rotational frequency of the particles. The basic data
on the radio-frequency system is given in Table 4.
As can be seen from Table 4 in the terminal accel-
eration cycle the tolerance for frequency deviations is
very small. In this last cycle however, the frequency
change. is small and thus the problem is not too serious.
We are considering a design In which the frequency
is controlled by=the beans. ? This system will be tested in
a .7 Bev accelerator, which Is under construction.
3.8 sec
6
12,000 amp
8,000 V .
0.31 ohm
1.8 henry
96,000. kva
Frequency of the accelerating field at the. beginning of the acceleration cycle 2.624 me
Frequency of the accelerating field at the end of the accelerating cycle 6.068 me
Tolerance on the deviations (slow) of the frequency from the specified frequency pf/f
beginning of cycle 2. 10-3
end of cycle 2.6 ? 1076
Corresponding accuracy of magnetic field pH/H
beginning of cycle 2.5 ? 10`3
end of cycle 10-2
Frequency of small synchrotron oscillations F
beginning of cycle 5150 cps
end of cycle 24 cps
Tolerance on resonant harmonics 6 f/f
beginning of cycle 31.10-T
F 50 cps 4.10-g
end of cycle 12.5-10 -9
Tolerance on noise modulation of the frequency 4.8- 10-3 cps2/cps
?i
481
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In the accelerator under design the protons acquire approximately 1.00 key per turn. The sum of the
accelerating voltages is 200 kv. As accelerating elements we propose to use transformers with ferrite cores.
The power of the radio-frequency source will be about 500 kw.
[11 V. V. Vladimirsky and E. K. Tarasov, "On the possibility of eliminating the critical energy in a
strong-focusing accelerator:`Collection; Certain Questions in the Theory of Cyclic Accelerators (Acad. Sci.
USSR Press, 1955).
Received May 29, 1956
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LUMINESCENT CHAMBER
E. K. Zavoisky, M. M. Butslov, A. G. Plakhov and G. E. Smolkin
This paper gives a more detailed description of the components of the luminescent
chamber and their development since the time at which an earlier report [1] was, published;,
new experimental data are also presented.
One of the basic components of the chamber (Fig. 1) is the electron-optical image -converter (EIC), which
utilizes.,the principle of the cascade electron-optical light amplifier. Although this principle was reported in
the literature long ago [2], the only successful experimental realization of the idea up to the present time has
been that of M. M. Butslov.' A multicascade EIC of this type has an electron multiplication factor ne which
is no smaller than that of multipliers with resolving powers (at the screen) of d p 10-2 cm and makes it possible,
to detect photographically one electron emitted from the input cathode.
The diagram of the EIC is shown in Fig. 1. The converter consists of an input section and several amplifier
cascades which are coupled by optical contact through thin transparent sheets. There is a fluorescent screen 1
on one side of the sheet and a photocathode 2 on the other. The electron image is kept in focus in the multiplier
cascades by the homogeneous magnetic field of the solenoid 3. In the input section electrostatic focusing is
employed. With appropriate voltages on the photocathode 2 and the diaphragm 4, an. electrostatic lens is formed.
Electrode 5 provides fine focusing of the electron image on the screen 1.
PG -3 []
Fig. 1. Diagram of the.luminescent chamber. SC) Scintillation
crystal; 0) objective; EIC: 1) fluorescent screen; 2) photo-
cathode; 3) solenoid; 4) diaphragm; 5) focusing electrode;
6) 't.deflectibn plates; 7) 8) pulsed electron gate; Ph) photograph-
ic apparatus; PG-1) pulse generator for the electron gate; PG-2)
pulse generator for the high voltage; PG-3) pulse generator for
control of the photographic apparatus; SWG) sweep generator.
*The EIC.described in [3] has only one amplifier cascade and an amplification factor of less than 100.
ARR
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In the space between the diaphragm and the screen there are two pair of deflection plates 6 which provide
the high-frequency sweep. Electrodes 7 in conjunction with the diaphragm 8 comprise a pulsed electron gate.
Fig. 1 shows a set-up which incorporates a radio-frequency generator and resonance circuits for the deflection
plates as well as a pulse generator PG-7. for the gating section.
Because there is no time delay this pulse scheme is an improvement over the analogous arrangement using
a magnetic deflection system described in [4]. It provides a sweep "frame" with a resolving time of 10-8 sec and a
continuous radio-frequency sweep with a resolving time 3.10-12 sec [5].
Fig. 1 also shows the arrangement of the high-voltage power supply for the EIC. In dc operation, the
supply for all sections is obtained from a single high-voltage source through voltage dividers. In recording the
tracks of cosmic particles and other random phenomena, a pulsed supply is used for the output section in order
to reduce the background. For this purpose a gating electrode such as that described above is used in the out-
put section of the EIC. More frequently, however, use is made of a high-voltage pulse supply from a special
electrical set-up. ? In the figure this unit is designated by PG-2. The pulse length was varied to fit the emiss-
ion time of the output screen.
In recording tracks of relativistic particles and other weak signals, even with pulsed supplies on the out-
put section it is impossible to eliminate the background due to the inherent noise of the EIC. To reduce this
background a pulsed supply was used on the electron gate 7-8 in the input section; this is PG-1. The length of
this pulse is determined by the electron time-of-flight in the interelectrode gap ( p 5. 10-9 sec) and the dura-
tion of the signal itself. For example in working with CsI(Tl) r a 3. 10-6 sec. In this case the noise of the
photocathode is completely supressed. It is, of course, also possible to pulse all the other sections of the EIC.
Synchronization of the gating pulses was accomplished by.means of a scheme consisting of a photomulti-
plier PM, a discriminator D and a coincidence circuit CC with a. resolving power of 4.10-8 sec. Following the
operation of the pulse generators the film in the photographic apparatus was shifted automatically.
The operation of the luminescent chamber is strongly dependent on certain important properties of the
crystal such as light output, transparency to its own luminescence, and also the relative degree of correspondence
between the crystal, luminescence spectrum and the spectral. sensitivity of the photocathode. Unfortunately it
.was necessary to ,use crystals of NaI(Tl), CsI(Tl), anthracene and other low quality crystals. However, even in
these it was shown experimentally that the amount of light emitted along the track of the charged particle is
adequate for detecting even relativistic particles.
This can be shown from an elementary calculation. A typical relativistic singly-charged particle loses,
in a CsI(Tl) crystal, about 5 Mev per cm of track.
The number of photons. emitted in this segment of track into an angle 41r is
F 1.,4.1.05,
where e is the energy required for the emission of one light quantum in CsI(Tl) which is taken to be - 35ev[6].
The number of quanta incident on the photocathode of the EIC is determined by the solid angle intercepted by
the objective
z
s = 16 B2 (1 +k)2 .
16
Here B and k are the relative aperature and the magnification of the objective, g is the ratio of the correspond-
ing solid angles in the crystal and in air - this may reasonably be taken as 1/n2 where n = 1.79, the index of
refraction of a CsI crystal. Typically b = 1/2 and k = 1 so that we get s a 170 photons/cm.
*A detailed description of suitable circuits, developed by the authors, will be given elsewhere.
*It is assumed that the limitation on the emission time of the screen is the speed of the photographic apparatus
and not the resolving time of the instrument.
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The spectrum of the CsI(Tl) emission is almost entirely contained within the spectral sensitivity curve of
a cesium-antimony photocathode. Taking the quantum efficiency of the photocathode to be b 10-1 [7], we
obtain the number of electrons emitted from 1 cm of length of the image of the track on the photocathode.
m pi :1.7.. Since the resolving power of the EIC is dry 10-2 cm it is easily seen that the image of the track will.
consist of different points with spaces between there. Each point corresponds to one electron at the input of the
E.IC. The space I between the points is
16 (1 + z,
d---- --
M 1+?Ilz?Ic't?S_
The useful thickness of the crystal is determined by the depth of focus of the objective and can be com-
puted from the formula
- d_ ~rtz k213z 14 (1 1 Jc)zl - kz131
k2B
All other conditions being equal the gaps in the track are a.function of the ionizing power of the particle.
It is convenient to use this relation in conjunction with the range -energy relation in the crystal to identify the
to -1
l0- 10',
'
Range mm
Fig. 2. Range'energy curve for a CsI crystal.
a 10-2 J0-~ 1 10 102 J
Range mm
Fig. 3. Range-light output curves for a CsI(Tl) crystal.
particle. In the case of strongly ionizing particles which produce
1 heavy tracks' the spaces. which are used for identification pur-
poses are obtained from the optical density of the negative along
the. track. In Fig. 2 and Fig. 3 are shown range -energy curves
2 and range-light output curves for CsI(Tl) which have been cal-
culated for this purpose. The calculation was based on the well-
known Bethe formula for energy loss by ionization. In the low-
3 energy region, where the particle velocity remains comparable
to that of a k-electron in an atom, data on energy loss in silver
[8] and experimental results on atomic, stopping power of the
t
4 d 9
l
Fig. 4. Photographs of proton tracks in a track. The reason for this seems to be fluctuations in the light
CsI(T1) crystal. flux and the quantum efficiency of the crystal and photocathode.
1CM
I emen
s were use [ J. I tna
ly we present photographs (Pig. 4)
of proton tracks in a CsI(Tl) crystal obtained with the neutron .
beam of the synchrocyclotron of the Institute of Nuclear Problems,
Academy of Sciences USSR. The tracks are arranged in .
*We may note that for 1 = d and even for 1> 2e ac holds, the thermal
motion of the nuclei is accounted for as if the-substance under investigation were in a gaseous state [5] (Oae is
the. characteristic Debaiev temperature, which determines the acoustic oscillations spectrum of the crystal
matrix). ' In cases of crystal structures consisting of several kinds of atoms the distortion in the shape of the
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a total, barn
X>00
?f;0
E.ev
Fig. 2. Complete :neutron cross section of radium
Ra 228
resonance absorption curve could also be caused by optical oscillations of the matrix, with a corresponding
temperature eop, and the condition of applicability of the formulas, correct for gases, will be different. The
theory of this problem has not yet been developed. Frequently the crystal structure of the compound is character-
ized by a single temperature e determined experimentally from the thermal conductivity at low temperatures.
Information on the characteristic temperatures of radium compounds is not available. If we take the value of
e for the compound RaBr2 as not higher than 150? K we can apply the relationship which is correct for the
gaseous compounds. For the detected level the Doppler.width p was 0.015 ev.
TABLE 3
Parameter designation
Experimental parameters,
Resulting values
Computed parameters
of the level
first approx-
second
imation
approximation
E0, ev
0.537 f 0.006
a0. barn
2300
3170
3500
3600 1. 150
T', ev
0.045
0.032.
0.0297
0.029 f 0.001
a o 1'2, barn ? eve
3.0 :h 0.2
rn, ev
(2.1 t. 0.1)?10-'
'For example, for KBr e = 177?K, for AgBre = 140?K [6].
A
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In accounting for the finite resolving power of the equipment, the resolution function was approximated
by a triangle with the additional factor (Ef E)-1, which accounts for the nature of the spectrum E-1 and the
dependence of the sensitivity of the detector on the energy E-1/2
B , barn
d000 r---
0.450 0.490 0.530 0.570 0..5/0
E,ev
.Fig. 3. Resonance region in the radium cross
section. I.) Theoretical Breit-Wigner Curve.
II) Computed from the theoretical curve
corrected for Doppler spread and resolution
.of the equipment.
A. P. Tsitovich, who secured uninterrupted operation
The method of successive approximations was
applied to thirteen experimental points in the resonance
region obtained with the thin sample, No. 3.
Experimental and computed results of the values
of the parameters in the resonance region are show: in
Table 3.
The solid curve in Fig. 2 is computed according
to the Breit-Wigner formula using parameters Eo = 0.537 ev.
oo = 3600 barn, t = 0.029 ev, ascat = 6.5 barn. The
absorption cross section for 0.025 ev computed accord-
ing to the Breit-Wigner formula with above parameters
is 7 (0.025 ev) = (13.5 t 1.5) barn.
The. resonance, region is shown in Fig.13.
In the energy interval of 2 to 50 ev. there is
observed a large scattering of experimental points.
The accuracy of rneasurements.in the energy region
is 15 to 2010.
The obtained experimental data do not enable
us to. present reliable conclusions concerning the average
interval between the levels of radium. Apparently this
value is not smaller than 30 ev.
The magnitude of the radiation width for 0.537 ev
rya P,-- 0.029 ev agrees with the general law of variation
of F with atomic weight [7, 8].
The authors thank the radiotechnical group under
of the electronic equipment during measurements, A. M.
Gonchukova and V. A. Chodakov for. their help in the mathematical processing of the data, and V. I. Mostov
for his participation in the discussions of the results. '
LITERATURE CITED
'[1] D. Hughes, Neutron Cross Sections, AECU-2040 (Russian edition:with supplement: Atlas of Effective
Neutron Cross Sections of the Elements edited by'Yu.V. Adamchuk,(Acad. Sci.USSR Press, 1955). Also:
D. Hughes, J. A. Harvey, Neutron Cross Sections (McGraw Hi11 Co., N. Y. 1955.).
[2] V. I. Mostovoi, M. I. Pevsner, A. P. Tsitovich,"Physical investigations; :Reports of the Soviet Delegation
at the International Conference on the Peaceful Uses of Atomic Energy. ( Acad. Sci. USSR Press, Moscow, 1955 )?
[3] L. J. Rainwater, W. W. Havens, J. R. Dunning, and C. S. Wu, Phys. Rev. 73, 733 (1948).
V. L. Sailor, Phys. Rev. 91, 53 (1953).
W. Lamb, Phys. Rev. 55, 190 (1939).
F. Zeitz, Modern Solid State Theory, (State Technical Press, Moscow-Leningrad, 1949). Chap. III.
D. Hughes, J. A. Harvey, Nature 173, 942 (1954).
H. H. Landon, Phys. Rev. 100, 1414 (1955).
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DIRECT MEASUREMENT OF THE ENERGY VARIATION OF n FOR
U233, U235, AND Pu239
H. Palevsky, D. J. Hughes, R. L. Zimmerman, and R. M. Eisberg' ?
Brookhaven National Laboratory, Upton, N. Y. ? ?'
A technique is described that measures directly the energy variation of 11,
the number of fission neutrons produced per neutron absorbed. When combined
with total cross sections, the method is capable of giving fission cross sections
as well. Results are presented in the energy region near thermal, of importance
to reactor design, for U233, U235, and Pu239. Comparison with ti computed from
total and fission cross sections shows good agreement for U233 and U235 but a
disagreement outside experimental error for Pu239. An auxiliary experiment
demonstrates that v, the number of neutrons per fission; is constant with energy
in the region of interest for Pu239, hence that the discrepancy cannot be ascribed
to a v variation.
1. Introduction,
The variation with energy of n the number of neutrons emitted per neutron absorbed by a fissionable
material, is a quantity that is,important to the understanding of the fission process as.well as reactor dynamics.
By definition,
aF
oa
where v is the number of neutrons per fission,
OF is the fission cross section, and
Oa is the absorption cross section (radiative capture plus fission).
On the basis of the liquid drop model for fission (Bohr and Wheeler, 1939), the incoming neutron shares its
energy with the other nucleons and the fission process takes place before the nucleus loses its excitation energy
by radiation. The "prompt neutrons emitted are "evaporated" from the highly excited fission fragments shortly
after they separate. In such a model v as a function of the incoming neutron energy is expected to be constant
for slow neutrons because the kinetic energy. of the neutron is small compared to the excitation energy of the
compound nucleus. From such considerations it was first expected that n also would be constant with energy
for low energy neutrons, because of the constancy of v and the supposed high probability of fission relative to
capture.
*Work performed under contract with U. S. Atomic Energy Commission.
? *Now at. University of Ninnesota, Department of Physics, Minneapolis, Minn.
? [The following is a reproduction of the original American paper, and not a re-translation from Russian -
Publisher's note].
521
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In 1.944 at Los Alamos (McDaniel et al, 1944) resonances in the fission cross sections were observed whose
parameters indicated that fission widths were comparable to radiation widths.. From these experiments and the
work at Columbia (Rainwater and Havens, 1945) on the total cross section measurements of the fissionable materials
it was clear that the ratio of fission to absorption changed from level to level, hence that n would vary with
energy.
At the time the present experiments were undertaken the variation is il with energy in the thermal region
(needed for reactor calculations) had been obtained only from the ratio of fission to total absorption cross sections,
involving the assumption of a constant v. For U23i the variation of rl deduced from such considerations, however.
was in gross disagreement with results of studies of the temperature coefficient of reactivity made at the start-
up of the Oak Ridge and Hanford reactors. The present direct method for measurement of the rl variation was
developed to resolve this difficulty. The results were in agreement with the observed reactor temperature coef-
ficients and it was later shown that the previous disagreement was a result of error in the early fission cross section
measurements.
2. Principle of the Direct n Method
In this experiment the variation of 71 is measured directly. Under exactly the same experimental conditions
two measurements are made: 1) the emission rate of fission neutrons from a thick foil of fissionable material
as the incident neutron energy is varied, and 2) the change with energy of the flux of neutrons incident upon the
foil. Thus one obtains directly from the ratio of two counting rates the change with energy of n. The fission
neutrons are detected by means of proton recoils detected by a scintillation counter (Hornyak, 1952). A thick
sample of the fissionable material is used, so that nearly all the incident neutrons are absorbed and.a maximum
counting rate is obtained in the scintillation detector. The incident flux is measured by a boron detector in the
form of a BF3 proportional counter that absorbs only a few percent of the incident beam. The boron cross section,
in the energy range of the measurements, varies as 1/v (Carter et al, 1953; Egelstaff, 1954), and since the detector
is "thin" the efficiency of the counter varies very nearly as the cross section.
The relationship between tl and the measured counting rates, assuming,the sample is black, is
CF ?F CB CB
kF v o a . kB n kB
where CF is the counting rate of the scintillation counter measuring the fission neutrons from the uranium, and
CB is the counting rate in the,BF3 counter measuring the incident flux. kF and kB are the efficiencies of. the
scintillation and BF3 detectors, respectively. kF is constant in the.energy region of these measurements. kB is
proportional to 1// El where El is the incident neutron energy; therefore
1<
n=
CF
C.B
where k is a constant independent of the neutron energy. In these experiments no attempt is made to make an
absolute determination of 71, and the results are normalized at E _' 0.0253 ev to the values of n given in the
Brookhaven cross section compilation, BNL-325 (Hughes and Harvey, 1955).
In practice the sample is not quite "black" to the incident neutrons, and the absorption cross section is
needed for computing a small correction for the number of neutrons not absorbed in the sample foil. However,
since nearly all the incident neutrons are absorbed, the experimental results depend only in an insensitive way
on the absorption cross section. In .treating the experimental data small corrections are made for isotope impurities
(1 to 3 percent), departure of the BF3, counter response from the 1/v law 1 percent), and for loss of neutrons
scattered out of the foil ( < 1 percent).
The scattering correction, although very small in the thermal region, is important in the application of
this method to higher energy measurements, where the absorption is smaller. For a black sample only those
neutrons that are scattered back from the front face of the foil escape, all the other scattered neutrons being
absorbed. A calculation shows that for a black sample the effective scattering cross section is only 15% of the
true scattering cross section. This reduction in the scattering effect is of value for measurements at higher energies
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where the scattering cross section becomes an appreciable part of the total cross section. The definition of
the effective scattering cross section and a curve giving its value for various sample thicknesses is given in the
appendix.
The energy of the incident neutrons is determined by a time-of-flight method using the Brookhaven slow
and fast choppers to interrupt a beam of pile neutrons. The direct t7 measurement is not restricted by the method
used for determining the incident neutron energy. Following the time-of-flight experiments at Brookhaven,
similar measurements using a crystal monochromator for energy selection were reported from the Hanford lab-
oratory (Leonard, 1955), and at the Geneva meeting in August 1955, several additional results using the same
method were reported (Harvey and Sanders, 1956).
The direct n measurement offers a simple and accurate method for obtaining knowledge of the variation
of n with energy. It is difficult to obtain from cross section measurements the variation of n in the region of
resonances because good energy resolution is required for the proper interpretation of total and fission cross
section data. The fact that tl varies much more slowly with energy than does either Q& or of is the reason
the direct measurement is the logical way to investigate this variation. Experimentally the direct method is
attractive because the use of a thick sample yields the highest counting rate and at the same time gives a result
that is fairly insensitive to the absorption cross section and sample thickness. These characteristics are in contrast
to the cross section measurements where the samples must be thin in the resonance region to give the correct
n variation. The present method also allows the use of the same apparatus to investigate all the fissionable
materials, one foil being substituted for another when a measurement on a different isotope is required. Further-
more the alpha "pile up", which is a serious problem when materials of high specific activity are measured in
ionization chambers, creates no difficulty in the direct n method.
3. Fission Neutron Detector
The fission neutrons are detected by means of proton recoils produced in a suspension of ZnS in lucite.
This phosphor mixture' is molded in the form of 2 in. diameter "buttons" approximately 1-1/2 in. in length. One
end has a slight spherical depression to give intimate contact with the face of the RCA 5819 photomultiplier.
The buttons are made by heating a mixture of 70 grams of lucite molding powders and 4 grams of 40 u ZnS-Ag
to 120?C in a die under 2500 lb/s q.. in. pressure (Higinbotham and Handloser, 1954).. A mechanically sturdy,
nonhygroscopic cap is formed, which can be. easily handled for mounting on the 5819 tube face.
The dimensions of the collimated neutron beam from the Brookhaven slow chopper .are 1-1/2 in. x 3-1/2 in.
Six 5819 photomultipliers are so located along the periphery of this area to surround the sample foil. The arrange-
ment is shown in Fig. 1. The calculated fractional solid angle of the sample foil subtended by the six scintillation
counters is 1010.
The operating point for the scintillation counter is determined by a compromise to satisfy two criteria.
First, the efficiency of the counter for fission neutrons should be as large as possible. Second, the sensitivity of
the scintillation counter for y-rays from the (n, y) process in the fission foil must be so low that less than one
percent of the counts recorded are due to this process. From the work of Hornyak, 1952, and others at Brookhaven
it was well established that the ZnS-lucite combination had low y-ray efficiency; however, in order to be certain
of the second criterion the following test was run. With the detector set up to perform the experiment, the fission
foil is replaced by a gold sample that is of a thickness calculated to produce approximately as many (n, y)
events as are produced in the foil of fissionable material. Then a combination of counter voltage, amplifier
gain, and discriminator bias is found so that the counting rate is about 0.1% of the counting rate with the fission
foil in place. The extra factor of 1/l0 is used because of the uncertainty about the number and energies of the
y's produced by (n, y)reactions in gold and the particular isotope under investigation. Only a small loss of
neutron efficiency results since the discriminator curve for y-rays falls off much faster than the neutron curve.
Under the above operating conditions the over-all efficiency for counting fission neutrons from the U235
foil was approximately 10-3, determined from a measurement of the incident flux and the known fission cross sec-
tion of U235. Therefore, since the solid angle is 10%, the efficiency of detection of fission neutrons incident
upon the buttons is one percent. The probability for a 1 Mev neutron to make a recoil proton, of any energy,
in the traversal of the button is about 1/4. Consequently only about 1/25 of the recoil protons produce a large
enough light pulse to be counted above the discriminator level.
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VOLTAGE DIVIDER AND
CATHODE FOLLOWER
. PHOTOMULTIPLIER
VOLTAGE DIVIDER AND
CATHODE FOLLOWER
Fig. 1. Schematic drawing of the scintillation detector.
4. Results
By means of this direct method, measurements have been made with the Brookhaven slow chopper (Seidl
et all 1951) in the thermal region for U235, U233, and Pu239: For Pu239 the fast chopper (Seidl et al, 1954) at
Brookhaven was used to extend measurements through the 0.3 level. The results will now be presented and their
connection with the cross section measurements for the same isotopes will be duscussed briefly.
4.1 U 235
The foils used were made of uranium metal enriched in U235 to a concentration higher than 90% and also
contained minor amounts of U238 and U234. Two sample thicknesses were used, 0.030" and 0.045". The data
were taken in a series of repetitive cycles as follows:
C1) The counting rate of a thin-walled BF3 counter which gives the distribution in energy of the incident
flux of slow neutrons.
C2) The background counting rate pertaining to measurement C1, obtained, at the slow chopper, by a
0.020" Cd foil inserted in the beam. This foil is essentially black to the incident neutrons in the energy range
investigated in the experiment.
C3) The counting rate of the fission neutrons detected by the scintillation counter.
C4) The background counting rate pertaining to measure C3, obtained by inserting a 0.020" Cd foil in
the incident beam.
The variation in ty is then obtained from the ratio
C3 - C4 _ CF
C1- C2 CB
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Twelve cycles of data were taken, which permits one to demonstrate the general reproducibility of the
results and also averages.out the short term fluctuations in the sensitivity of the equipment. The statistical
accuracy'of the points varies from about one percent in the thermal region to about three percent at 0.16 ev.
The corrections applied to the data are as follows: departure of BF3'cotinter from 1/v, about 1.21o; U23a and
U238 impurities, about 3.5%; and scattering correction, about 0.7%.
Figure 2 shows. the combined results of the experiments performed with U235 at Brookhaven and Hanford.
The Brookhaven data are normalized to n = 2.08 at thermal (E = 0.0253 ev), the value quoted in the Brookhaven
compilation (Hughes and Harvey, 1.955). The Hanford data are normalized to Brookhaven data in the energy
region of 0.10 ev. The dashed line is computed from the ratio of of/ad , Eq. (1), assuming v constant, and
using the most recent cross section data, given in BNL-325. The agreement between the direct measurement and
the variation of n computed from fission and total cross sections is everywhere within one percent. U235 is the
one fissionable isotope at present where the total and fission cross sections are known with sufficient precision so
that then variation deduced from these measurements has an accuracy comparable to the direct measurement.
The observed n variation in U235 is a good example of the accuracy obtainable by this new method of
measurement. The change in 71 in the thermal region is very small'(. 1% from 0.01 to 0.1 ev). From the total
and fission cross sections it is not possible to, predict with certainty the sign.of the slope of the 17 curve in the
thermal region.
The resonance parameters of the 0.29 ev level give a ratio of radiative capture to fission cross section that
is greater than that measured at thermal by spectrographic methods (Inghram., 1956). This fact had led to the
belief that tl would decrease with increasing energy in the thermal region. The results of the present 7 measure-
ments at Brookhaven, however, showed the opposite to be true. This situation arises because contributions from
at least two negative levels, in addition to those,frorn the positive levels, are required to fit the cross section of
U235 in the thermal region. From the observed variation of n and the total cross section an estimate may be
made of the ratio of capture to fission for the negative levels (Harvey and Sanders, 1956).
4.2 U235
The foil was made of uranium metal with a U233 content greater than 90% and a thickness of 0.025". The
same experimental procedure was used in obtaining the U233 data as was outlined above for U235. The corrections
for isotopic impurities, scattering, and the departure of the BF3 counters from 1/v were each about one percent.
u235
1 BROOISIAVEN DATA
{ HANFORD DATA
Figure 3 gives. the results of the measure-
ments on U233 normalized to 77 = 2.31 at
E = 0.0253. rf is seen to be constant within
statistical error (. 1%) from 0.01 to 0.1 ev.
The dashed line is computed from the curves
drawn through the total and fission cross
aloF ~~ _L '-}; T ~Q1e compilation again with the assumption of a
-"-I . 40 -015 in this case is good within the large statisti-
as a function of for U235. The experimental
Fig. 2. 71 energy P
data are normalized to tl =:2.0$ at, thermal.- The dashed line
gives n as calculated from the absorption and fission cross
sections assuming v is constant.
For the Pu239 measurement a 0.033"
plutonium metal sample was used, which
contained over 90% Pu239 and about one per-
cent aluminum. The corrections for scatter-
ing and departure from 1/v. are small (- 1%). Below 0.35 ev the correction for isotopic impurities is also less
than 1%. However, above 0.4 ev this correction rises rapidly and amounts to 28% at 0.7 ev.
Figure 4 shows the results of the measurements made with plutonium in the energy interval from 0.015 to
0.7 ev. Here, in marked contrast to U233 and U235, ,0 changes rapidly in the thermal region. The data from
1.90 I. I 1
001 0.02 005 W 02 0.5 10 cal errors in the fission cross section data,
ENERGY IN e which would easily allow a f 5% variation
in 77 over this energy interval.
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thermal up to 0.15 ev were obtained with the slow chopper and normalized ton = 2.03 at thermal. The fast
chopper was used to obtain the higher energy points and these were normalized to the slow chopper points in
the neighborhood of 0.15 ev. The dashed line again is computed from BNL7325, assuming v constant. From
the direct n measurement the ratio of n at 0.3 ev ton at thermal is seen to be 0.75 * .02. The cross section
data assuming v is constant gives for the same ratio 0.85 t .05.
--T--T,--r
u233
005
I I I I I I I 1 1
02 .03 .04 .05 .06 .07 .08 .0910
ENERGY IN ev
Q15
0.2
ENERGY IN ev
Fig. 3. n as a function of energy for U233. The Fig. 4. n as a function of energy for Pu239. The experi-
experimental data are normalized ton = 2.31 mental data are normalized ton 2.03 at thermal.
at thermal. The dashed line gives n as calcu- The dashed line gives n as calculated from the absorp-
lated from the absorption and fission cross sec- tion and fission cross sections assuming v is constant.
tion assuming v is constant. .
The 12% disagreement between the direct measurement and the cross section results. is seen to be outside
the experimental error, and for this reason a series of experiments were performed to see. if there was an unassigned
source of systematic error. In order to check whether the experiment was dependent on the energy of the fission
neutrons the bias on the scintillation counter was increased until, the counting rate fell to one half of the value
originally obtained. The slow chopper points were re-run with the higher bias settings and the results were in
agreement with the first set of measurements. Next the shielding in the vicinity of the. counters was re-arranged
to make sure that scattered neutrons.were not giving rise to spurious counts. No effect was found within an upper
limit of 2%/0. As a final check the transmission of a 0.009" Pu foil was measured using the scintillation counter
as a detector. The measured transmission agreed with the known. cross section data again proving that the timed
neutrons were of correct energy.
At first this disagreement was taken to imply that cross section measurements were in error. However
recent theoretical (Bohr, 1956) and experimental (Pitcher, Harvey, and. Seth, 1955) work have indicated that
there is a possibility that a fissile:,nucle.us can have more than one mode of fission.: On the basis of such con-
siderations it would be possible that v might be different for the different levels. Therefore it was decided to
investigate whether the assumption of the constancy of v in the energy interval of the experiment was valid.
The measurement of the energy variation of v was made by comparing the ratio of fission neutron to
fission fragment counting rates in the thermal region and a small energy interval centered at 0.3 ev. The same
counter as used in the tl experiment was used to measure the fission neutrons, but a thin (0.001") foil was sub-
stituted for these measurements. In order to measure the fragment counting a gas scintillation counter containing
10 mg of Pit was. constructed (Palevsky, Larsson, and Zimmerman; 1956). The neutron energies were selected by
means of filters, using the beam of pile neutrons collimated by the fast chopper in a stopped, open position.
The thermal data were obtained by Open-Gd differences using Gd(NO3)3 dissolved in D2O as a filter. The atomic
concentration of Gd in the filter was 5 X. 10-4 atoms/cm2. The energy interval centered on the resonance was
obtained by taking Gd-Pu differences using the same filter and a 0.009" Pu foil. Because the plutonium foils in
both the fission fragment and fission neutron detectors were thin the result is fairly insensitive to the shape of
the neutron spectrum. The result of this experiment is as follows:
W0.3 CV
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where uncertainty in the neutron spectrum constitutes one percent of the error and the residual error is statistical.
It is therefore clear that the discrepancy between the direct n measurement and the cross section derived values
cannot be attributed to a change in v. This same conclusion has been reached by means of similar v ineasure-
ments at Saclay (Auclair, Landon, and Jacob, 1.955), Hanford (Leonard et al, 1956), Argonne (Bollinger et al,
1956), and Moscow (Pevsner et a1, 1.956).
The constancy of v constitutes a definite advantage of the present direct rt method be-
cause the t) measurement gives at the same time the energy variation of a, the ratio of capture to fission
cross sections. It also implies that the relative fission cross section can be obtained from the 17 measurement
together with the absorption cross *section (Eq. 1), thus avoiding the often difficult direct fission measurement.
5. Conclusions
For U2'15, where the total and fission cross sections have been most accurately measured, the agreement
between the direct p measurement and the energy variation of n calculated from the cross sections, assuming
v is constant, is excellent. Such a comparison for U233 does not have much meaning at present because the
cross section data do not have sufficient statistical accuracy. In Pu249 the disagreement between the direct
measurement and the cross section calculation is about twice the standard deviation of the error associated with
the cross section measurements. It should be mentioned that direct n measurements performed by Sanders at
Harwell (Harvey and Sanders, 1956) using a crystal spectrometer are in excellent agreement with the present
measurements. In the Harwell measurements the fission neutrons were moderated in paraffin and detected by
BF3 counters. Because of the widely different nature of neutron sources and detectors in the two experiments,
one would expect that the kinds of systematic errors arising in the Harwell experiments would be entirely differ-
ent from those of the Brookhaven measurements. The excellent agreement of the results, therefore, indicates
that the energy variation of n has been correctly determined. The discrepancy between n and the cross sections
for Pu239 cannot be ascribed to a change of v with energy-because the recent experiments at various laboratories,
performed to check this possibility, have all verified the constancy of v.
Even though both 71 and the cross section for Pu239 have been measured at several laboratories it seems
most likely at present that the discrepancy, which is about nine percent when based on world average values
(Harvey and Sanders, 1956) of all quantities, is a result of combined experimental errors. It is important that
further measurements be made in order to resolve this discrepancy so that the present 17 method can be pushed to
higher energies with confidence, and perhaps be used to obtain fission cross sections as well.
0.4
0,3
0.2
U.S
00 t
Acknowledgements
The authors wish to thank Dr. T. I. Taylor of
Columbia University for his assistance in taking the U235
data. We are also indebted to W. A. Higinbotham and
J. A. Handloser of the Brookhaven Electronics Department
for preparing the ZnS-lucite scintillation caps.
The Effective Scattering Cross Section
The counting rate in the scintillation counter used
0,64
to detect the fission neutrons from a fissionable material
.is proportional to the average neutron flux in the material.
2 N6t 3 4 5 For a foil of thickness t the average flux is given by
-~
Fig. 5. K, as a function of sample thickness Not.
Ktrs is the effective scattering cross section.
t
F(x)dx,
F (t) - ? I =F (0)
c dx
0
-Nl at
1_c
Neat
527
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where F (x) = F (0)e-Noa t if there is no scattering in the foil. N is the number of nuclei per cm3 and as the
absorption cross section. To include the effect of scattering let K(x) be the probability that a neutron scattered
at the distance x will leave the foil without being absorbed. Thus K(x)as, the effective scattering cross section,
represents the fraction of the scattering cross section that is effective in contributing to the attenuation of the
flux. We then have, including scattering,
d r(x) - r(x)Na,1-;-K(x)6Q) . (6)
Figure 5 shows the average value of K for various foil thicknesses in units of Naat, computed from Eq. (6).
The curve represents the numerical solution for K (x) of Eq. (6) and subsequent averaging over x.
LITERATURE CITED
Auclair, J. M., Landon, H. H., and Jacob, M. (1955) Compt. rendu 241,.1935.
Bohr, N., and Wheeler, J. A. (1939) Phys. Rev. 56, 426.
Bohr, A.. (1956) Proceedings of the International Conference on Peaceful Uses of Atomic Energy, Paper P/911.
Bollinger, L. M., Cote, R. E., Hubert, P., Leblanc, J. M., and'Thomas?G. E. (1956) Bull. Am. Phys. Soc.
'1, No. 4.
Carter, R. S., Palevsky, H., Myers, V4 W., and Hughes, D. J. (1953) Phys. Rev.92, 716.
Egelstaff, P. A. (1954) J. Nuclear Energy 1, 57.
Harvey, J. A., and Sanders, J. R. (1956) Progress in Nuclear Energy Series I, Vol. 1, Chapter 1 (Pergamon
Press, London).
Higinbotham, W., and Handloser, J., (1954) Rev. Sci. Instr. 25, 98.
Hornyak, W. F. (1952) Rev. Sci: Instr. 23, 264.
Hughes, D. J. and Harvey, J. A. (1955) Brookhaven National' Laboratory Report 325 Neutron Cross Sections
(Superintendent of Documents, Washington, D. C.).
Inghram, M. (1956) Proceedings of the International Conference on the Peaceful Uses of Atomic Energy,
Paper P/596. .
Leonard, B. R. (1955) reviewed by H. Palevsky, Proceedings of the International Conference on the Peaceful
Uses of Atomic Energy; Paper P/587.
Leonard, B. R., Seppi, E. J., Friesen, W. J. (1956) Bull. Am. Phys. Soc. 1, No. 1.
McDaniel, B. D., Sutton, R. B., Anderson, E. E., and Lavatelli, L. S. (1945) Los Alamos Scientific Laboratory,
University of California (unpublished).
Palevsky, H., Larsson, K. E., and Zimmerman, R. L. (1956) Rev. Sci. Instr. (in press).
Pevzner, M. I., Donelyan, L. S., and Adamchuk, Yu. V. (1956) private communication; Kalashinikov, V. I.,
Tebedev, V. I., Mikallyan, L. A. and Pevzner, M. I. (1956) private communication.
Pilcher, V. E., Harvey, J. A., and Seth, K. K. (1955) Phys. Rev. 100, 1248A.
Seidl, F. G.'P., Palevsky, H., Randall, R. R., and Thorne, W. (1951) Phys. Rev. 82, 345.
Seidl, F. G. P., Hughes, D. J., Palevsky, H., Levin, J. S., Kato, W. Y., and Sjostrand, N. G. (1954) Phys.
Rev. 95, 476.
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FUEL BURN-UP IN NUCLEAR REACTORS
The paper describes the method of calculating fuel burn-up in nuclear
reactors, taking into account the capture and multiplication of neutrons while
slowing down. In the calculations, account is taken of the burn-up of U235 and
the build.-up and burn-up of Np239, Pu239, Pu240, Pu241 and of the fission fragments.
INTRODUCTION
The economic characteristics of nuclear reactors designed for the production of electrical energy depend
to an important degree on the quantity of raw material required to produce a given amount of electrical energy.
or, in other words, on the permissible degree of nuclear fuel burn-up.
If we assume.that the problem of maintaining continuity of operation of reactor fuel elements has been
solved, then the permissible extent of fuel burn-up will be determined by the initial excess reactivity in the
reactor and by the laws of its decrease during the reactor's operation.
In this paper is considered the change in reactor reactivity with time (the kinetics of burn-up) for the
case of extensive fuel burn-up. Numerical results are obtained for natural uranium systems with heavy water
moderation.
Multiplication During Slowing Down
In burn-up kinetics a leading role is played by the accumulation of Pu239, Pu240, and Pu241. These isotopes,
and Pu239 in particular, have large neutron absorption and fission cross sections in the epithermal region. For
this reason it is necessary to take into account in the kinetic equations not only the capture of neutrons in the
epithermal range but also the deviation of the cross section from the 1/v law in the thermal region.
We will consider, to begin with, certain questions involved in calculation of the capture and multiplication
of neutrons during the slowing down process.
Let us assume that the neutron spectrum (on the energy scale) has the form of the Maxwell distribution at
the temperature T:
N (E) dE = NT T7'-3/2e-T l/E dE
(NT is the total number of thermal neutrons), which extends up to the energy Ejo* and of a Fermi. distribution of
slowing down neutrons f(E)dE, extending from the energy Ejoto the energy spectrum of fission neutrons. The
"joining" energy Ejo is determined as the energy of intersection of the two spectra. In such an approximation,
no account is taken of the influence of chemical binding (which becomes important near the joining energy) on
the neutron spectrum. Comparison of the spectrum obtained with experimental measurements [1] shows that the
*The factor y, which differs little from unity, takes account of the circumstance that the Maxwell spectrum
must be normalized to unity in integration to Ejo rather than to infinity.
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error introduced by such an approximation is comparatively small. The spectrum of the slowing down neutrons
is determined by the equation
(E) f" q(T)w(L)9
where Is is the scattering mean free path, g is the mean logarithmic energy decrement, dr = 3lEE , and
g
Aq= aT YI2(C)
where L(r) is the diffusion length of neutrons of age T.
In the Equation (3) it is assumed that the 'moderator does not contain "any hydrogen, and that the total
neutron absorption cross section is small compared to the scattering cross section, thereby assuring the applica-
bility of the diffusion approximation. The existence of strong resonance absorption (for example by the resonance
levels of U238 or Pu240) is taken account of in equation (2) by the factor cp(E), which represents, the probability pf
escaping resonance absorption by the strong resonance levels, and which therefore cannot be' included in L2 in
Equation (3). The initial conditions for the function q(r) can be obtained through calculating the number of fast neutrons
originating in the capture of thermal, as well. as of slowing down neutrons, by the fissioning nuclei, and has the
form
e(E-s)
q (O) ?vT viaiTp5NiT I J Lz (T) q (T) d
p
Here p I is the concentration of the ith isotope, a iT is the 'absorption cross section of thermal neutrons, v'i is
the number of secondary neutrons per absorption, NiT is the density of thermal neutrons at the location of the
ith isotope, is the fast neutron multiplication constant,k(r) is the multiplication constant for neutrons of age r.
The thermal neutron cross sections entering into the first member of Equation (4) are assumed to have been
averaged over the Maxwell spectrum in accordance with the equation
E,jo
S -N (E,) dE
aT?T = at1 = Eio
J N(E)dE
where VT = 2200. m/sec, is the standard speed of thermal neutrons. As is well known, in this averaging (see, for
example, [2]) what is actually averaged are not the cross sections as and the transport lengths It, but the mag-
nitudes o av and the' diffusion coefficients D - ltv.
*The division of resonance capture into that by strong and weak levels is somewhat provisional. Subsequently
'we will include in cp only resonance capture in U238 and Pu240, the remainder being included in the term q(r)/L2(r)
in equation (3). .
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The Equations (3) and (4), together with the equation for thermal neutrons
RANT_NT = -Q (To
(Tc is the thermal neutron lifetime) determine fully the conditions for reactor critical size, and for the neutron
density distribution in the reactor.
If the reactor does not have a reflector, the solution of Equations (3), (4) and (6) can be obtained through
the substitutions
q (r, T) = q (T)el r,
which gives
-0 2 T l%2T
4(T)'=q(0)e ?L (T)
Tip dT 2
~ (x2+__)NT(o)=Jf_q(0)e ? L c2)
T
NT (r) = NT (0) ef-r,
where LT is the diffusion length for thermal neutrons (calculated through averaging over the Maxwell spectrum
by the method described above), while ~p is the total resonance escape probability (by strong levels) in neutron
slowing down to the energy Ejo. The substitution of Equation (7) into (4) leads to the characteristic equation
for determining the Laplacian of the system K 2;
1 + %2Lz, =
- ~? d,c . 2
-% T ?o
kre 0 L2 (;) J
T.o dT . 2
k (T) dT a-~ L2 1-%
L2(T)
kT = RT1?O
is the thermal neutron multiplication constant.
From the equations (1), (2) and (7) we can determine the joining energy Ejo
- 5-o- _
e T?= (1-fx2L`) is(I o) ((
T 2
2 eirr \ o )
X (Essoo 11/2
T )
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where 'CT = mean absorption length at the given temperature, while E2200 is the neutron energy corresponding
to a neutron speed of VT 2200 m/sec. Now it is easy to deduce the burn-up equation for any isotope. Let
pi he the number of nuclei of the ith isotope. The number of neutrons captured by the nuclei of this isotope per
unit time are determined by the expression
Ea
NTOTa TPi+Pi voi(1-')! (Is)dE
Ljc,
(E0 is the fission spectrum energy). Strictly speaking, the neutron density in equation (11) should be considered
as varying for the different isotopes although for convenience we will ignore this fact in our notation, assuming
that the appropriate corrections have been included in the cross sections. Inserting the Equation (7) into the
Expression (11), we obtain the equation for the burn-up of the ith isotope:
dpi_
dt - - NTVTaiTpi
- NTVT -
dT 2
-x
-
Z
0L
CO
i- k(T) dT e
L.2 (t)
0
at (E) L2 L2 CO E ?(E)e
dE+Q-a,ipt
where Q is the number of nuclei of the given isotope formed as a result of neutron capture by the preceding
Isotope;. Xi is the disintegration constant (if the given isotope is radioactive).
It should be noted that for an infinite system an equation of the type (12) can be easily obtained also for
the case of moderation by hydrogen since in this case the kinetic equation for slowing down can be solved
exactly.
In the sequel we will be investigating thermal neutron reactors for which the magnitude
is dE
S Eli (E) L
Ejo
Tjo
(' di
o
can be considered to be fairly small. Then the equation (12) goes over into
Eo
dpi - - NTVTaSTpi NTvT?rP Vh?hTPh Pi ?i(E) } Q-a.ipi?
h EJo
? 4= vh ahTPh
It .
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As is known [3], [4], the probability of a neutron resonance capture in its moving through a distance 1
within a cylindrical fuel element of radius R is determined, for the case of a single resonance, by the expression
? T E smod~ lcr C 2 lcr L. 1? (2lcr
Kr L
jl C I for)) .
where r is the level width, while lcr is the absorption length in the center of the resonance It' is necessary to
average this expression over all directions of neutron motion in the fuel element. Let us make such an average
approximately, replacing 1 by the mean path of the neutron in the fuel element. 1- = 2R. This approximation
has sufficient accuracy for practical cases, since in the limiting case of I/l,cr>> 1 the expression (19), after the
substitution 1 = 2R, differs from the exact expression only by 2%.; in the case of the other limiting situation. of
1/lcr i1:
~R-RZ S N (R) R dR
Changes In Reactivity
The reactivity of the system is characterized by the effective multiplication constant, small changes in
which are proportional to small changes in the Laplacian. The expression for the effective multiplication con-
stant can be easily obtained starting with the equation (8). For this purpose let us employ the circumstance
that absorption during.slowing down takes place primarily in the energy region near to thermal energies. The
factor a-x=- can be taken out from under the integral sign in the denominator at the value T]o = .r (Elo). The
expression (8) assumes the form:
1-{- x2L3 = ke -x2TJo ,
k=kT(1a-1A)+
+ S (1- a-u') x2 LT),
dc
Q
tQJ= La
(T) ,
In case of low absorption during slowing down
dT
J(~ k (t) :.. dT a L2(T)
L (T)
dr dT
dT Q L2(T)
L2 (T) e
Tjo dT
C ` Lz'(T) ? 1
in the expression for S one can drop, in
Te dc
-J La
both numerator and denominator, the factor e 0 To the extent that these factors are dropped in
S.M
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both the numerator and denominator the value of S practically does not change even for not very small
TJ,
TJo
S k (T) L2 (T)
8- o
(' dc
kl SILz(~)
The multiplication constant for thermal neutrons, kT, can be written in the following way:
kT = i''Ps`fo`Pff . X
'4sasTPr, 4eaoTP9-i-'JiaiTPi
TPa+.asTP5 + a9TPo+QOrPo+QiTP1+1arP+(arP)ff
.In the Expression (25) go = e`O 0, EOTP is absorption in the moderator and in the construction materials(aTp.)'ff
is absorption by the fission fragments, off' eOff ff is the probability of resonance absorptioiY`in the fissio.i
fragments. Poisoning due to the isotopes Xe 135X e and $.m149 the concentration of: which rapidly attains
equilibrium levels, we will include in Ea Tp and not in (' 01 P)ff-.. The ratio of the value of kT at the time s,.
to its value at the beginning of the fuel reloading cycle; is equal to.'
kT (s) f (s) 1
kT (p) - ~~ 1-A (s) PPo (S) `P ff (S) X
X
cff...,
1+(1.+.ce)(1+;2c)(1-A) ,
1
A(s)= (1-1- CO) Ec) X
X11 - p6 a9T Po -. a'oT Po - aiTPi},
Ec = EoTP cff = (arP)f
.asTPsO+aeTPs' . a6TP6?
(26)
Here, as previously,' it. is assumed that if the mean neutron densities are different in various materials, then the
cross sections must be multiplied by appropriate factors.
The magnitudes, w and 6 are simply expressed through prior introduction of the probability of neutron
absorption during slowing down:
W -'rb+'r9+ lg ? U
t Smod
X a5Pb2~E[cs(1-{-EC)-1-EC
Jo
r,r s~, +yB09T
V6 Q f--' I6 of j9
(1+cs)(1+Ec)(1-A) X 5T sT
w f(s)
d,c
f2 (,) Then
(29)
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To obtain the value of the effective multiplication constant, it is necessary to calculate the change in the
multiplication constant induced by charge in 0 " E which appears on the left side of the Expression (23.). Since
2 0
1-A(s) '
eff(s) -k(s)
eff. (0) k(o)
x2Lr (0) A (s)
-A (s) + iOLT (0)
where the connection between k and 1 1;
2) weak absorbers with of 100). Calculations carried out on the assumption that the scattering at these
a (90?)
energies is of purely diffraction character lead to a proton radius of ro 0'.7.10-13 cm. A similar analysis
carried out by other authors for smaller energies led to a proton radius .r0 P (0.5-0.7) ? 10 cm.
Results of measurements of the total interaction cross sections of nucleons with nucleons and with deuterons
were reported by V. I. Moskalev. The total cross section of the (p, p) interaction increases by a factor of 1.5 in
the energy range from 380 to 660 Mev. The total cross section of the (n, p) interaction remains constant and
equal to (34-37) ? 10-27 cm2. At energies. below 580 Mev the (p, d) interaction cross section agrees within the
limits of error with the sum of the (n, p) and (p, p) scattering cross sections, and at larger energies is somewhat
smaller. This effect can evidently be explained by the screening of the nucleons in the deuteron.
Analyzing the energy dependence of the total cross sections for scattering of nucleons by nucleons, K.
Brueckner (USA) came to the conclusion that meson production at energies 500. to 1000 Mev occurs only in
collisions of nucleons in states with T 1. At higher energies, when double meson production. begins, mesons
are also produced in collisions of nucleons in states with T = 0, and the total'cross section of the interaction of
nucleons in this state begins to increase. This effect can be explained on the hypothesis that meson production
occurs mainly through intermediate states with T = 3/2 and J = 3/2. V. P. Dzhelepov remarked that,
according to data obtained by his group, at neutron energy about 600 Mev the meson 'production occurs
equally intensely with T = 1 and. with T = 0. The conclusions of Prof. Brueckner are based on the analysis
of. data relating to the difference between "the (p, d) and (p, p) cross sections. The nonadditivity of the
(p, d) interaction cross section, which increases with the energy, could' thus lead. to a lowering of the
cross sections found in this way for meson production in states with isotopic spin T = 0.
The morning and evening sessions on May 18 were devoted to the interaction of 7r- mesons with nucleons
and nuclei. In the reports of N. A. Mitin, A. I. Mukhin, and I. V. Sokolova accounts were given of experiments
on the scattering ofir mesons by nucleons and on the carrying' through of the phase-shift analysis of this process.
The report of Prof. E. Clementel (Italy) was also devoted to this latter question. The experiments were carried
out in the meson energy range from 176 to 310 Mev. The phase-shift analysis was done both with and without
the inclusion of d waves. It was shown. that the contribution of the d waves to the scattering is small in com-
parison with those of the s and P waves. The dependence of % on the meson momentum was found to be
almost linear right up to 307 Mev. From the results obtained it follows that the interaction radius ins states
with T = 3/2 cannot be appreciably greater than the value r = 6.5 -14 cm. At energies larger than 240 Mev
it was not possible to obtain agreement of the energy dependence of the phase resonance with the Chew-Low
theory. The results of the. phase-shift analysis permit one to assert that the conclusions from the dispersion
relation agree with experiment right up to 310 Mev energy. R. Marshak told about the scattering of low energy
7r- mesons by protons. A comparison of the experimental results with the Chew-Low theory led the speaker to the
conclusion that the hypothesis of charge independence might be "unjustified" in this energy range, since two
effects depending on charge come to be of great importance: the Coulomb interaction and the nucleon mass
difference. K. Brueckner stated that the dependence of the total interaction cross sections of 7r-mesons with
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nucleons is not monotonic at high energies. At low energies the meson, as it were, "does not notice" the meson
cloud of the nucleon and interacts only with its central core. With increase of the energy the meson cloud be-
conies transparent and collisions occur at large impact parameters. The resonance nature of the interaction
between mesons leads to the nonmonotonic behavior of the energy dependence of the cross sections for inter-
action of mesons with nucleons.
The interaction of 7r--mesons - with the nuclei of various elements was the subject of reports by R. M.
Sulyaev, N. I.. Petrov and A. E. Ignatenko. In collisions of 330 Mev 7r - mesons with ilea nuclei the mesons
interact mainly with the separate nucleons of the a particle, but multiple processes also turn out to be important
here. The angular dependence of the elastic ( 'R , a) scattering is evidently nonmonotonic in the region of
small angles. This fact may possibly be evidence of a difference of sign of the Coulomb and nuclear scatterings,
unlike the case of energies below 200 Mev, where the signs are the same. The elastic scattering of 7r -mesons
by nuclei of carbon and lead is satisfactorily described by means of the optical model. The inelastic collisions
mainly have the character of the interaction of mesons with the individual nucleons, the collisions being mostly
single for the range of scattering angles from 120? to 180?, while in the scattering at angles from 0 to 60? the
mesons make several collisions in the nucleus. The total interaction cross sections and the`.total inelastic colli-
sion cross sections with mesons in the energy range 140 to 400 Mev were determined for a number of nuclei. The
experimental results,agree well with calculations carried out on the basis of the optical model, with use of the
dispersion relation to find the optical parameters of the nuclei. The radii of the nuclei agree well with the
expression R = 1.43 ? 10 -iq At/S cm.
V. V. Krivitsky told about the creation of 7r +- mesons by collisions of 308 Mev ic -mesons with carbon
nuclei. The total cross section for this process was found to be equal to oc = (2.6 t 1.3) ? 10-27 cn72.. A rough
estimate of the cross section forthe similar process in ( 7r p) collisions leads to vp . 10-27 cm2..
In the discussion of questions connected with the interaction of fast, particles, with nuclei, considerable
attention was given to polarization phenomena. G. D. Stoletov reported on the polarization of proton beams
arising from the scattering of 660 Mev protons from beryllium nuclei. The experiments were conducted with
primary scattering angles of 18? and.9?,in the laboratory reference system. The degrees of polarization found
here were 33 and 66% respectively. It was found that the maximum value of the degree of polarization in
elastic scattering of protons from beryllium nuclei is considerably higher than in the case of quasi-elastic
scattering and inelastic collisions. The polarization in elastic scattering at angle 9? does not change perceptibly
with increase of the atomic weight of the analyzer. The polarization of proton beams emerging from quasi-
elastic. collisions and meson formation decreases markedly with increase of.atomic weight.
A report by R. Marshak: was also devoted to the question of studying the polarization occurring in the
scattering of protons by protons and by complex nuclei.
I. I. Levintov told about a determination of the ratio of the real-parts of the spin-orbit and central inter-
action potentials of nucleons with nuclei. This quantity can be found from data on the polarization. at high
energies, on the basis of information about the levels of certain nuclei and about the scattering at low energies.
The values of this ratio found in different ways agree among themselves.
N. A. Guliev told about a calculation of the polarization appearing in the scattering of nucleons by
nuclei. In this work the distribution of the nucleons in the nucleus was given and the potential corresponding
to this distribution was found by means of scalar meson theory.
Several reports were devoted to-the scattering of high energy particles and to the nuclear reactions pro-
duced by these particles. In his report L. Rosenfeld (England) discussed the possibilities that the study of the
nuclear scattering of fast particles provides for the study of nuclear structure. The data on the scattering of
electrons -provide evidence that the parameter To in the expression for the nuclear radius must be regarded as
dependent on A and equal to ro = 1.4 ? 10-13 cm for heavy nuclei and ro = 1.2.10-13 cm for light nuclei. The
calculations show that with further increase of the accuracy of the measurements it will be possible to observe
effects due to the fact that the charge is not' continuously distributed in the nucleus, but concentrated in the in-
dividual protons. On the basis of the results of experiments on the scattering of ?-mesons the conclusion is
drawn that the anomalous scattering of these particles either does not exist at all, or else is much smaller than
had been expected previously.
628.
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M. Levy (France) spoke about the scattering of 550 Mev electrons by protons and deuterons. At large
scattering angles the measured cross sections are 10 times greater than the Rutherford values. Better agreement
is obtained by taking into account the distribution of charge and magnetic moment in the proton. To explain
the data on (e, d) scattering it is necessary to take the neutron radius much smaller than the proton radius, but
this contradicts the data on the magnetic moments of nucleons. Better results can be obtained by using a nucleon
model with a central core, but here the size of the core has to be taken equal to that of the whole proton. It
is possible that the results of these experiments give evidence that the interaction of point charges at small
distances (r Pj 0.5 ? 10 cm) departs from the Coulomb law.
V. I. Moskalev reported on measurements' of total cross sections and-inelastic interaction cross sections of
neutrons and protons with nuclei. The total cross sections of light nuclei at energies of 660 Mev for protons. and
630 Mev for neutrons are equal'to each other, which is evidence for the charge symmetry of nuclear forces. In
the range of neutron energies from 380 to 630 Mev a small increase of the cross sections of light nuclei is'ob-
served. The total cross sections of the heavy nuclei remain practically constant in this range. The inelastic
interaction cross sections of. protons with nuclei remain almost constant in a wide range of energies from 150 to .
850 Nlev.
El-Nadi (Egypt) considered in the Born approximation the theory of reactions in which the incident nucleon
captures two nucleons from the nucleus. The results of the calculations agree 'qualitatively with the experimental
data (presence of. a maximum in the small angle region). But the further course of the cross section does not
agree with the results of the calculation. The carrying out of further experiments will show whether the cause of
the discrepancy is the use of the Born approximation.
An account of the elastic and inelastic scattering of high energy neutrons and deuterons by extended semi-
transparent nuclei wa's'given by K. A. Ter-Martirosyan. The formulas and curves obtained by his work may be
useful in determining the sizes and shapes of nonspherical nuclei.
N. A. Perfilov reported on the emission of fragments with Z >'4 in the destruction by protons of the nuclei
of an emulsion. In the energy range from 350 to 660 Mev the yield of the fragments increases by a factor of
2.5. The observed effect cannot be explained by the splitting of Ag or Br nuclei, nor by the evaporation of
fragments from strongly excited nuclei.
J. Filbert (France) told about a study of the interaction of:1. Bev protons with the nuclei of a photographic
emulsion. The total interaction cross: sections were studied, and also the yield of a particles from light and
heavy nuclei and the probabilities of formation of stars, with various numbers of charged particles.
The discussion of the problems connected with: photonuclear reactions began with a survey report by A. M.
Baldin, devoted to the photoproduction of 7r-mesons from protons and deuterons. An account of the photopro-
duction of slow ir-mesons from complex nuclei with y-rays of maximum energy 260 Mev was given by N. G.
Semashko. Comparison with the results of the calculations. of Baldin and Lebedev showed the necessity of
taking into account the finite dimensions of the nucleus. The fact that the cross section for photoproduction of
it mesons is proportional to A2/3 is interpreted as evidence that the formation of mesons occurs mainly at the
surfaces of nuclei. The conclusion is' drawn that the decrease of the meson yield for small Z is due to the
nuclear reaction (repulsion) in the. system ir-meson-nucleon.
A. A. Abrikosov directed his talk to a consideration of a number of quantum electrodynamical effects at
high energies. A. B. Migdal explained that, because of the necessity of taking into account large longitudinal
distances in the production of pairs and Bremsstrahlung at high energies the ordinary shower theory turns out to
be altogether inapplicable in dense media at energies greater than 10-13 ev.
In a number of interesting reports W. Panofsky (USA) told about experiments on multiple photoproduction
of sr- mesons from hydrogen, about the photoproduction of p-meson pairs, about the direct production of mesons
by electrons and about Bremsstrahlung at high energies. At electron energies of 575. Mev the production of pairs
of ir-mesons was observed with a cross section of 10-33 cm2. The system (ir+, p) is formed .mainly in the p
state, and the system (ir-', p) in the s state. The distribution of energy between the components of a pair turns
out to be extremely uneven: near the threshold the ir-mesons take up almost all of the energy. The results
of the experiments obviously agree with the hypothesis that the production of mesons goes through the inter-
mediate state J = 3/2, T = 3/2. The results of another series of experiments enable us to conclude that direct
creation of pairs of p-mesons is evidently an?existing process. Here the experimental cross sections agree better
629
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with the calculated values if the finite size of the nucleus is taken into account. In these experiments no effects
were observed that could he ascribed to the action of nonelectromagnetic coupling of p-mesons with nuclei.
B. M. Pontecorvo told about attempts to detect a nuclear interaction connected with the interchange of
p-meson pairs. It was shown that if such an interaction exists at all, its contribution to forces does not exceed
10- 2 % of the contribution due to 7r,= mesons.
Reports by N. B. Delone and V. S. Roganov, and also by R. Wilson (USA) were devoted to various questions
connected with the photodisintegration of the deuteron. Studies have been made of the angular distributions and
the spectra of the neutrons and protons appearing in this process. It is noted that the angular distributions of the
neutrons from the reactions (y, d) and (y, C) agree near 0 = 45?. It is concluded that with light nuclei reab-
sorption of mesons is unimportant. The data on the photodisintegration of .the deuteron, for energies up to 50 Mev
agree in their general features with the theory of Schiff. At larger energies the experimental points lie considerably
higher than is predicted by this theory. A simple model is proposed to explain the behavior of the cross section
for this process at high energies. Rough agreement with the experimental data can be obtained if one supposes
that the production of mesons from the nucleons in the deuteron is the same as,from free nucleons. If the pro-
duction of the mesons occurs at large distances from the nucleon, they escape, and actual meson production is
observed. For meson production at small distances, the mesons are emitted and reabsorbed. This effect leads
to a rise of the cross section for photodisintegration of the deuteron near the threshold for meson production.
Similar conclusions, reached in a study of the photoproduction of Tr- mesons from deuterons, were reported by
M. I. Adamovich.
A. N. Gorbunov reported on experiments on the photodisintegration of helium nuclei at high energies. It
was found that the (y, p) and (y, n) reactions are of resonance character with a maximum near 27 to 30 Mev.
The cross section of the (y, p) reaction is constant at energies 40 to 75 Mev and falls sharply at the latter energy.
In this range the cross section of the (y, n) reaction decreases monotonically. The angular distributions of the
neutrons and protons are identical, and an energy-dependent asymmetry is found around the angle 90?. In the
process of absorption of quanta by nuclei of helium higher multipoles play a part even at low energies.
The last session of the section on elementary particles and their interactions was devoted to questions
about new particles. M. Menon (India) told about a cloud chamber study of s-particles at a height of about
2500 m.
A. I. Alikhanyan reported on experiments with a. mass spectrometer used together with two Wilson cloud
chambers. In the composition of the cosmic radiation, besides. protons, deuterons, 7r-and p-mesons, and positive
and negative K-particles, there was observed a group of particles with mass about 560 times the mass of the
electron. The intensity of these particles is equal to about 1% of the number of p-mesons in the same interval
of ranges.
V. A. Lyubimov reported on measurements of the spectrum of: K-particles at a height of about 3200 m. In
the range of momenta up to 450 Mev/c there are far fewer K-particles than ?r-mesons N7r 6.5 Jo in the
interval of momenta up to 900 Mev/c the ratio of the intensities of K-particles and ir-mesons rises to 20 1o, and
in the interval of momenta up to 1200 Mev/c it reaches 40 to 501o. At momenta from 900 to 1000 Mev/c the
yield of K particles is a maximum, and their number exceeds the number of Tr-mesons. The results of the ex-
periments. make it seem possible that K-particles, like 7r-mesons, are quanta of the nuclear force field. L.
Smith reported on experiments with the cosmotron, in which during bombardment of carbon and lead nuclei
with 1.9 Bev 7r-mesons a search was conducted for joint production of A- and 0-particles. The lifetimes of
0?-and T?-particles were found to be greater than 10-9 sec, and that of K-particles equal to 10-9 sec. It was
shown that the scattering cross sections for the different kinds of K-particles are the same.
Wang.Han-Chang (China) told about some results of work in which heavy mesons and hyperons were
studied with a Wilson chamber at a height of 3185 m. In 30,000 exposures 8200 nuclear reactions were found,
and 200 heavy mesons and hyperons. Several cases of pair production of these particles were observed.
B. S. Neganov put forward a proposal to consider the nucleon as a system consisting of a..hyperon and K -
particles. In this case the K-particles must be regarded as structural units entering into the composition of the
nucleon, and not as quanta of the nuclear field. On this assumption the process of production of hyperons and K-
particles in pairs must be regarded as a process of dissociation of the nucleon.
630
Declassified and Approved For Release 2013/04/03: CIA-RDP10-02196ROO0100090006-9
 Declassified and Approved For Release 2013/04/03: CIA-RDP1O-02196ROO0100090006-9
Professor Wang Han-Chang (China) presents' a report at the A11=Union Conference on iligh Energy
Particle Physics in Moscow, May 22, 1.956.
J. Steinberger (USA) presented a report on the production of "strange" particles froth hydrogen by 1.3 Bev
tr-mesons, Decays of .E--and,K-particies:were observed, which were due, in the opinion of the speaker, to
electromagnetic processes. A? considerable number of cases were observed of "unusual" decay of A and 0 ---
particles, in which visible tracks of decay particles were absent. The lifetime of E--particles was found to
be 1,4'10-10 sec, Analyzing the experime ntal'data, the speaker came to the conclusion that they do not
provide an indication of a higher spin for the hyperon.
.The report of R. Peierls (England) was devoted to hypernuclei, i.e., -nuclear systems in which bound
hyperons occur as constituents. Similar questions were discussed in. the report of N?. N. Kolesnikov at one of the
meetings of the theoretical section. Here also L. B. Okun described the results of calculations of the cross
sections for exchange collisions of K-particles in hydrogen and deuterium, and also for reactions in which K-
particles are captured by deuterium; the purpose is,to obtain evidence about the spin and parity of 1-0.4 ev) is equal..to half the capture. cross, section at thermal energy.
For nuclei of intermediate atomic weight (50