(SANITIZED)TWO SOVIET ARTICLES ON ULTRASOFT RADIATION BY A P LIKIRSKII, M A RUMSH, AND L A SMIRNOV/A POSSIBLE NEW TECHNIQUE FOR INVESTIGATION IN THE SOFT X-RAY REGION(SANITIZED)

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• Collection: General CIA Records
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• Original Classification: C
• Document Page Count: 7
• Document Creation Date: December 22, 2016
• Document Release Date: January 11, 2012
• Sequence Number: 1
• Publication Date: February 23, 1962
• Content Type: REPORT

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262 ADRIANOVA, POPOV, and LAPINA
one another only by the amplitude of vibration.
It can cc concluded from the above investigation that
the phase of the modulation of light from an interfero-
metric modulator retains a practically constant value
along the cross section of a beam of light obtained by re-
flection from one vibration zone of the piezo-electric
mirror. This property of the interferometric modulator
must be considered as an important advantage over the
Kerr cell and diffraction modulators.
1. A. A. Lebedev, V. V. Balakov and V. G. Vafiadl,
Doklady Akad. Nauk SSSR 108, 458 (1956).
2. Yu. V. Popov, Kand. diss. Leningrad (Candidate's
Dissertation, Leningrad State Univ.), 1955.
3. Yu. V. Popov, Izvest. Vysshikh Ucheb. Zavedenii,
Geodez. i Aerofotos"emka (Reports of Higher Edu-
cational Institutions, Geodesy and Aerial Photography)
No. 2, 57 (1957).
4. Yu. V. Popov and I. I. Adrianova, Izvest. Vysshikh
Ucheb. Zavedenii, Geodez. i Aerofotos"emka No. 1,
49 (1959).
X-RAY MONOCHROMATOR FOR ULTRASOFT RADIATION WITH THE RECORDING OF
THE ABSOLUTE NUMBER OF QUANTA
A. P. Lukirskii, M. A. Rumsh, and L. A. Smirnov
Received 26 January 1960
The construction of a diffraction-grating, vacuum, x-ray monochromator for the spectral
region front 15 t_~ o__120 A is described. Methods of adjusting the monochromator and methods of
obtaining electronic regulation of the monochromatic radiation are presented. A method for the
absolute counting of the number of quanta with the aid of a Geiger counter is developed.
The grating, made in GOI State Optical Institute) and
ruled on glass, has 600 lines/mm and a radius of curva-
ture of 1 meter. The grazing angle of incidence of rays
on the grating is equal to 2.5?.
circle between the x-ray tube and the diffraction grating,
and the dianhragm D is set rear the
ti
Th
gra
ng.
e mono-
The monochromator to be described is designed for chromatic radiation is selected by the detector slit Sd,
the study of the efficiency of various radiation detectors, behind which the radiation detector is located. The Gei-
A focusing diffraction grating* is used as the dispersive ger counter is placed between the detector slit and the
element in the apparatus. The source of rZdiatiQ; is a
demountable x-rav tub which makes it possible to ob-
tain a series of characteristic lines without breaking the
vacuum. A drawing of the monochromator is shown in
Fig. l,a. The entrance slit S. is placed on the Rowland
detector to be studied and with the aid of special acces-
sories can be removed from the beam without breaking
the vacuum, thus allowing the radiation to fall, on the
other detector. The detector slit and the detectors are
mounted on the platform P, which can be translated,
without breaking the vacuum, in two mutually perpendicu-
lar directions, thus making it possible to place the de-
tector slit at different points of the Rowland circle. The
platform with the radiation detectors can be rotated
about an axis coincident with the detector slit, also with-
Fig. 1. Construction of the x-ray monochro-
mator. Se-entrance slit; D-diaphragm;
G-grating; Sd-detector slit; C-Geiger.
counter; P-platform; S,-separator slit;
T-x-ray tube; A-anode; K-cathode.
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-7
Fig. 2. Block diagram of the x-ray tube
supply and Geiger counter recording cir-
cuit. HVR-high-voltage regulator; A-
autotransformer; HT-high-voltage trans-
former; V28-800-kenotron; V-electro-
static kiiovoltmeter; mA-milliammeter;
T-x-ray tube; ACR-anode current regu-
lator; C-Geiger counter; HSC-high-voltage
supply for counter; RM-counting-rate
meter; B-2-scaling circuit of a B-2 radiom-
eter. The section within the dotted outline
is located in the vacuum chamber of the
monochromator.
through which radiation from the tube passes into the
monochromator. The volumes of the x-ray tube and the
monochromator are pumped out by separate diffusion
pumps, which are equipped with stainless steel oil baffles
and liquid-oxygen traps. This pumping system permits
vacuums of 5 x 10-7 and 2 x -W-' mm Hg in the x-ray tube
out breaking the vacuum. Such motion is necessary to
point the detectors toward the diffraction grating. The
monochromator is enclosed in a vacuum chamber. Be-
tween the x-ray tube and the vacuum chamber of the mono-
chromator is placed the separator slit S, of area 5 mm 2
stable in time_ The vacuum chamber of the monochroma-
tor is connected with apparatus for filling the Geiger
counter. Construction details of the monochromator are
shown in Fig. 1,b.
X-RAY TUBE SUPPLY CIRCUIT AND RECORDING
CIRCUIT
Circuits for electronic stabilization of the anode
current of the x-ray tube and of the high voltage were
used in order to stabilize the radiation intensity with
time. A block diagram of the x-ray tube supply is pre-
sented in Fig. 2. Regulation of the anode current was
carried out by means of a regulator analogous to the one
in the URS-501 instrument' except that in our regulator
the range of emission currents was considerably wider
Lroui 20 ;z to 1 0 ma). Regulation of the high voltage
was accomplished using a magnetic-amplifier stabilizer
which was controlled by the rectified voltage from a sep-
arate rectifier. This rectifier was fed by the same
alternating potential as was the transformer of the high-
voltage apparatus. The high-voltage regulator had a
stabilization coefficient of about 30. The remaining parts
of the supply circuit are seen in the block diagram.
The recording and supply circuit for the-Geiger
r counter is also presented in Fig. 2. In order to obtain
a good plateau" a forced-quenching circuit of the Neher-
Harper type2 was used. Impulses from the quenching cir-
cuit proceeded to the counting-rate meter, in the first
stage of which they changed sign, and then to the scaling
circuit of a B-2 radiometer.
In adjusting the apparatus with the curved grating
operating at grazing incidence it is necessary to estab-
lish strict parallelism between the entrance and detector
slits and the rulings of the grating. Deviation of the slits
from the required position in planes perpendicular to the
IISC =-229
T i - - - - i,61h3 3sa. ,f I673P1
I
direction of the beam is particularly inadmissable. The
entrance slit, diffraction grating, and drrrrtor clst user
mounted on cones, which were inserted in conical sockets,
The conditions of manufacture were such that the axes of
the conical sockets were strictly parallel to one another.
All three cones were made to be interchangeable, and, in
addition, a fourth socket was placed on the table of an
autocollimator. The slits and the grating were mounted
in turn in this socket. By clamping a small plane-parallel
glass plate to the jaws of a slit (or by holding it between
the jaws), parallelism of the slit and its axis of rotation
was established. Precise setting of the distances between
slits and grating (the distances between axes of the cones
were precisely measured) was insured by coincidence of
the slits with the axes of the conical bearings; this coinci-
dence was set with the aid of a microscope. Each slit
was placed in such a way that its position did not change
as a result of rotation in its conical socket.
In the adjustment of the slits the autocollimator was
set up such that its optic axis was perpendicular to the
axis of rotation of the conical bearing. After this the ad-
justable stage with the grating was placed is the conical
socket on the autocollimator. The diffraction grating was
made so that its rulings were parallel to the boundary
surfaces of the glass block. By fastening plane-parallel
plates to these end surfaces and to the rule surface of
the grating, strict parallelism was established between
the rulings and the axis of rotation of the cone by means
of the autocollimator. The remaining steps of the adjust-
ment (coincidence of the ruled surface and the axis of
rotation of the cone, setting of the necessary angle of
incidence of rays on the grating, adjustment of the separa-
tor slit, and adjustment of the x-ray tube) were carried
out in the monochromator with the use of a light beam.
In order to illustrate the operation of the mono-
chromator, Fig. 3 presents the characteristic K-series
lines of fluorine, off, carbon, boron, and beryllium,
which have wavelengths of 18.' , 23.6, 44, 67, and 1-A,
respectively. These lines were obtained using LiF, Mgo,
aquadag, boron, and beryllium applied to the surface of
the anode in the form of fine powder suspended in alco-
hol (with the exception of MgO, which was obtained by the
combustion of Mg). The use of powders insures stability
of the radiation with time because of the poor heat con-
tact between the powder and the no and the low heat
conductivity, which leads to heating of the powder by the
electron beam. Hot surfaces are coated to a considerably
smaller degree with carbon which results from the de-
composition of residual organic vapors (diffusion pump
oil. etc.) in the electron beam.
The characteristic lines are obtained by measuring
the intensity during translation of the counter along the.
y-coordinate for fixed values of the x-coordinate corre-
sponding to calculated values for the wavelength at the
"center of gravity" of the lines (see Fig. l,a). As can
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II
ORa CR a,(. Brea
f
LUKIR
SKII, RUMSH,
be seen from Fig. 3, the characteristic lines have back-
ground. The considerable excess in intensity of the lines
over the magnitude of the background attests to the favor-
able operation of the grating and correct a justmen .
DETERMINATION OF THE ABSOLUTE NUMBER OF
QUANTA IN THE MONOCHROMATIC BEAM
The use of the Geiger counter for recording ultra-
soft x-radiation is known in the literature 30 With spe-
cial recording circuits it is possible to exclude the main
deficiencies in the operation of counters which there are
in the cited works and to use counters for absolute meas-
urements of intensity. Two drawbacks which have been
met in previously used counters are poor counting char-
acteristics (" llateau-"), which, moreover, shift with
time along the vTe scale ("plateau drift"), and ab-
sorption in the "dead region" of the counter which can-
not be taken into account.
In order to determine the absolute number of quanta
with a counter it is necessary to satisfy the following
conditions: (1) the counter must have a good "plateau"
(i.e., a relatively small number of spurious counts);
(2) there must be no "dead region;" (3) it is necessary
to know the amount of absorption in the counter window,
the absorption coefficient of the counter gas, and also
the "dead time" of the counter is needed in order to in-
troduce corrections for "missed counts."
The construction of the counter used by us is shown
in Fig. 4. As can be seen from the diagram, the window
9 was glued over the slit in the frame 7. This frame was
connected to the casing of the counter 1 through the
rubber gasket 8. The possibility of removing the frame
with the window made it easy to glue on the window and
to determine its transmissivity. In order to do this the
frame and window were mounted in the vertical slider of
the monochromator in place of the Geiger counter and
could then be moved into and out of the beam. Another
radiation detector (e.g., photomultiplier of the Allen
type?) was attached tothe platform, and with it measure-
ments were made of the transmissivit for the charac-
teristic lines Ok, Ck, Bk, an ek. Results of the meas-
urements are given in the Table.
line (Aftf=
Pik
about 0,14 was used as a window. This thin sheet is very
weak, and even w Hen glued on a slit of width 0.3 mm it
can withstand a nr essure difference of only about 150 mm
H,. Therefore we use a special gas filling system allow-
I , the counter to be pumped out together with the mono-
chromator chamber; also the air could be let into both
simultaneously. The gas mixture was prepared in a sep
crate tank, from which it was fed into the counter each
J/ A double sheet of celluloid ith a total thickness of
Fig. 3. Characteristic K-series lines of
fluorine, oxygen, carbon, boron, and beryl-
lium. N-intensity in relative units; y-
translation of detector in y-direction (1
division equals 0.1 mm).
time after pumpin7 out the apparatus. In the operation
of the counter its "plateau" shifted as a result of chane
the composition of the r_as. Forced circulation of the
mixture was used to speed up the establishment of a
"plateau." Experiment showed that forced circulation of
the gas stabilizes the operation of the counter, practically
eliminating "plateau drift" with time. The copper casing
of the counter 1 has two hose connections 6 for filling and
forced circulation. The diameter of the cylindrical bore
of the casing equals 18 mm.
Fig. 4. Construction of Geiger counter.
1-casing (copper); 2-glass insulator;
3-spring; 4-picein; 5-tungsten wire (di-
ameter 0.05 mm); 6-hose connections for
filling; 7-frame; 8-rubber gasket; 9-
celluloid window.
As the counter gas we used either a mixture of argon
and methane (807) A and 20 CH4) or a mixture of arson
and abso u e Iv alcohol (90% A and 10% C2HSOH). The
use olrforced quenching provides n eex ensTon tithe
"plateau" to 250 volts with a slope of about 2(70 per 100
volts. Consequently, "spurious impulses" could be dis-
regarded.
A study of counter efficiency was carried out by
measuring the-intensity for different gas pressures.
Figure 5 gives curves of counting rate as a function of
gas pressure (8011o A and 20% CH4) for K-radiation of
Be, B, C, and O. From the graph it is evident that for
pressures greater an 80 mm Hz, all curves apprqach
"saturation." This attests to the fact that under these
conditions total absorption of radiation occurs within the
gas of the counter. The absence of a falling off for high
pressures indicates that there is no "dead region" in the
counter. Extrapolation of the curves into the region of
small pressures (anticipating a dependence according to
the late 1 - e-kV, where p is the pressure and k the ab-
sorption coefficient) shows that the curves pass through
the or' in. This indicates that photoelectrons from the
window have only a second-order role, whit.: is in agree-
ment w, ith a small absorption and, appar cn:1?y , a. low value
of the ;,..otoeiectric yield in the window. Ther ufor e, the
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X-RAY MONOCHROMATOR FOR ULTRASOFT RADIATION 265
photoeffect in the window was not taken into cons iCarat ion.
Thus in our case many phenomena (the effect of "spuri-
ous impulses," the effect of ,a "dead region") have been
reduced to neoiigibly small ciuantities. The pressure in
the counter is chosen such that practically total absorp-
tion of the radiation occurs within the gas. If under these
conditions account is taken of absorption in the window
and of "missed counts," then the reading of the counter
will lead to the absolute number of quanta incident on the
window.
0 20 e0 60 60 100 120
p(mm Hg)
Fig. 5.. Dependence of relative counting rate
M of Geiger counter on gas pressure p for
constant intensity of radiation.
counting a larger number of quanta an attenuator con-
s.sting of a series of raster screens with about 30-?
mesh laid one on top of another was placed before the
counter. Such an attenuator produces the same attenua-
tion for all wavelengths. A special investigation of the
dependence of the counting rate on the point of incidence
of the beam in different parts of the attenuator showed
that the attenuation was the same in different spots. Cali-
bration of the attenuator was accomplished in the same
way that the transmissivity of the counter window was
determined. A selection of raster attenuators made it
possible to increase the counting rate to 3 x 105 quanta
of monochromatic radiation per second. The possible
error in determining the absolute number of quanta is
estimated to be no more than 10%.
In conclusion we wish to express our thanks to A. A.
Lebedev for discussions and his interest in this work.
1. I. B. Borovskii, "Fizicheskie Osnovy Rentgeno-
spektral'nykh Issledovanii" (Physical Principles of
X-ray Spectral Analysis). Izd. Moskovsk. Univ.,
Moscow, 1956.
2. S. Korff, "Schetchiki Elektronov i Yadernykh Chastits"
(Electron and Nuclear Counters). Izd. Inostrannoi Lit.,
mentally.
Correction for
using the equation'
"missed counts" was introduced
N;=1NIV'
where N, is the actual counting rate, N the measured
counting rate, and r the "dead time." The `dead time,'
was determined experimentally by the method of two
sources6 (as sources we used two radioactive prepara-
tions of Co") and for our counter amounted to about
2.5 x 10-4 sec. It is evident that the maximum counting
rate for such a counter is about 500 impulses/sec. For
J,
A method for measuring the photoelectric yield for ultrasoft x-radiation is described. The
method is based on a system devised for absolute counting of very small numbers of photo-
electrons using an electron multiplier and for absolute counting of quanta using a Geiger
counter. Results of measurement of the photoelectric yield for a series of metals and non-
metals are presented.
Moscow, 1947.
3. I. L Ro-ers and F. C. Chalklin Proc._Phys. SOC.
London) B57 412 19 4).
4. A. ukirskif Radiotekh. i Elektron. 2, 328 (1957).
5. E. R. Piore, G. G. Harvey, E. M. Gyorgy, and R. H.
Kingston, Rev. Sci. Instr. 23, 8 (1952).
6. M. A. Blokhin, "Metody Rentgenospektral'nykh
Issledovanii'' (Methods of X-ray Spectral Analysis).
Moscow, 1959.
MEASUREMENT OF THE PHOTOELECTRIC YIELD FOR ULTRASOFT X-RADIATION
short wavelen tT_uYt_r7_v_iolqLli ht and ultrasoft x-radia-
tion is nterest from the point o iew of both eor~
and application. won cI's "fiSP n teate a possi-
bi ity of using photomultipliers of the Allen type for the
absolute measurement of intensity in these spectral re-,
pions.
The external photoeffect for short wavelength ultra-
violet light has been investigated many times.3-6 In the
contiguous, ultrasoft x-ray region this problem has until
now been discussed in only a few works.9-1 In these
works only the relative photoelectric efficiency of a
series of metals for polychromatic radiation was deter-
mined. The lack of detailed quantitative studies of the
photoelectric yield is explained by the experimental dif-
ficulties connected with (1) the necessity of either obtain
ing powerful monochromatic beams or else measuring
very weak photocurrents, and (2) the determination of
the absolute intensity of beams causing the photoemission.
These difficulties are relatively easy to overcome when
working in the short wavelength ultraviolet part of the
spectrum since discharge in a capillary (Lyman source')
is sufficiently stable and provides the necessary intensity
of the monochromatized beam to allow measurement of
the photocurrent by the usual electrometric methods. In
this case measurement of the absolute intensity is made
tive to an intense source of thermal radiation.
In this work a photomultiplier of the Allen type- op-
erating in a system for counting separate photoelectrons
was used for the measurement of the photocurrents. The
possibility of counting separate electrons enabled us to
use a special x-ray tube of relatively low power as a
radiation source. We used the characteristic K-series
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266 LUKIRSKII, RUMSH, and SMIRNOV
radiations of the elements oxygen, carbon, boron, and
beryllium, which have wavelengths o 23.6,T4,_67, and
113 A, respectively.
A monochromator with a curved diffraction grating
was used for mono chromatization of the radiation.
,Measurements of the intensity of the monochromatic
radiation were made with a specially constructed Geiger
counter, for which the operating conditions that make it
possible to determine the absolute number of quanta
passing through the detector slit were found experimen-
tally. The construction of the monochromator* made the
*The construction and operation of the monochromator
and also the technique of measuring the absolute number
of quanta are described in the preceding article.
following operations possible without breaking the vacuum:
isolation of different monochromatic lines by the detector
slit; placing of either the Geiger counter or the photo-
multiplier behind the detector slit; and small 'shifts of
the counter or photomultiplier relative to the selected
monochromatic beam.
The x-ray tube supplies (i.e., high-voltage and fila-
ment supplies) were regulated by special electronic
stabilizers, which in combination with the high vacuum
in the x-ray tube (5 x 10-4 mm Hg) kept the monochro-
matic radiation sufficiently constant during the measure-
ment time. The width of the monochromator slit was
chosen such that the isolated spectral interval amounted
to 1 A.
An Allen-type multiplier was used to record the
photoelectrons. These multipliers possess a compara-
. ..1, stable .;Lpi:ficat.on factor, very low background,
and also permit air to be let into the apparatus. Electron
multipliers with dynodes .of a Cu-Be alloy 14 were used
in this work.
Thtocathode was mounted in the first dynode in
such a way that it practically duplicated the form of the
plane middle part of the dynode. The angle of incidence
of the beam on the photocathode was equal to about 60?.
The power supply and recording cir nits are shown
in Fig.
T7V
o age was supplied to the dynodes through a
divider which was connected to a source of regulated
voltage, the magnitude of which could be varied between
4100 and 5400 volts
he first resistor of the voltage divider was variable
and was placed outside the monochromator so that the
voltage between the photocathode and the second dynode
could be varied. From the collector of the photomulti-
plier an impulse passed to a preamplifier, which was
placed in the vacuum chamber of the monochromator in
the immediate vicinity of the multiplies. The preampli-
fier consisted of a single stage amplifier and cathode
follower. The gain of the preamplifier was equal to 6 for
impulses no shorter than 5 usec. a ition, the pre-
amplifier produced an approximately 4-5 ?sec lengthen-
ing of impulses arriving from the multiplier. This length-
ening was necessary for triggering of the scaling circuit.
From the preamplifier an impulse was fed to a wide-
band, two-stage amplifier with variable gain (from 0.5
to 50), after which it proceeded to a B-2-radiometer
scaling circuit. The "threshold of operation" of the re-
cording apparatus for 5-?sec rectangular impulses fed
into the preamplifier and for maximum gain of the wide-
band amplifier was equal to 2 mv.
A multiplier with the above described recording cir-
cuit registers all photoelectrons under the conditions
that (1) all impulses of the multiplier are greater than
the "threshold of operation" of the circuit, and (2) all
High-voltage
supply for
I u~ +300 volts.
-105 vote
-.1f I Regulated
7 / Dower
J' a
]~
- - - - - J
Fig. 1. Block diagram of recording circuit
using electron multiplier (part in dotted out-
line is located within the vacuum chamber
of the monochromator).
photoelectrons fall on the second dynode of the multiplier.
In order to find the conditions under which these require-
ments are fulfilled the following dependences were in-
vestigated:
1. The dependence of the number N of registered
impulses on the gain G of the wide-band amplifier was
studied for a sufficiently large voltage between the photo-
cathode and the second dynode for x-rays incid rat on the
middle of the photocathode. This dependence is presented
in Fig. 2. From the graph it is seen that beginning with a
certain value Go of the gain, the number of registered im-
pulses ceases to increase (saturation), which verifies that
practically all impulses are being counted. Thus, for
G > Go practically all impulses are recorded by the de-
scribed apparatus.
o e a Ya G
Fig. 2. Dependence of the number of im-
pulses registered by the multiplier an the
gain of the wide-band amplifier.
2. The dependence of the number N of registered
impulses on the voltage U applied between the photo-
cathode and the second dynode was studied for values of
G > Go and for rays incident on the middle of the photo-
cathode. This dependence was investigated with regard
to different wavelengths and for all photocathodes. Figure
3 presents curves corresponding to a Ni photocathode for
the extreme wavelengths Ka of 0 (A = 23 A) and K,, of
Be (a = 113 A).* From the graphs it can be seen that be-
r ccording to the data of Rud erg, " $ ;o of the photo- -
electrons arising under the action of carbon K-radiation
have energies of 20-30 ev. Such an energy distribution
of photoelectrons exxp ains the shape of the curves of Fig.
3, which exhibit a sharp rise as the magnitude. of.,.U.ap-
proaches a value that insures the collection of photoelec-
trons at the second dynode.
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I
~..wJ
0 es 202 300 400 .00 0 100 200 lOJ 400 500
a U (volts) b U (volts)
Fig. 3. The dependence of the number of
electrons registered by the multiplier on
the magnitude of the voltage between the
photocathode and the second dynode. a-for
Be K-radiation; b=for 0 K-radiation.
conditions under which all photoelectrons are collected
at the second dynode, the dependence of the number of
impulses N on the point of incidence of rays on the photo-
cathode was studied. This dependence is presented in
Fig. 4. From* the graph it can be seen that there are re-
gions on the photocathode, considerably larger than the
width of the x-ray beam, from which photoelectrons are
completely collected by the second dynode. Thus, if the
gain of the wide-band amplifier G > Go, the voltage be-
tween cathode and second dynode U > U0, and the beam
falls at a spot on the photocathode corresponding to the
middle of the horizontal part of the curve in Fig. 4, then
all electrons will be registered. Possible sources of er-
ror may be "miscounts," which depend on the finite
length of the impulses, and the natural background of the
multiplier. If, however, counting rates of less than 1000
impulses/sec are used, then for impulses 10-5 sec long
missed counts amount to a fraction of a percent. The na-
tural background of the multiplier. although negligibly
small, can easily be measured and taken into account in
the final results.
790 790 Z00 210
D (rel. units)
Fig. 4. Dependence of the number of regis-
tered electrons N on the point D of inci-
dence of the monochromatic beam on the
photocathode. One division corresponds to
a translation along the cathode of 0.18 mm;
the width of the beam is no more than 1 mm.
Thus, in general, the number of registered impulses
equals the number of photoelectrons if we neglect the
probability of the appearance of two electrons as a re-
sult of the action of a single quantum.
MEASUREMENT OF THE PHOTOELECTRIC YIELD
In the light of the above, measurement of the photo-
electric yield in our case amounted to counting with the
multiplier the number of photoelectrons arising each
second and determining the intensity of the x-ray beam
in number of quanta per second using the Geiger counter.
Since measurements of photoemission and x-ray intensity
were conducted successively in time, they were repeated
many times, and average values were used in the calcula-
tion of the photoelectric yield. The ratio of the number of
ginning with U = Ua = 240 volts, all photoelectrons fall
on the second dynode and lead to the appearance of photo-
current impulses. In order to make more precise the
impulses recorded by the multiplier in some interval of
time to the number of quanta incident on the photocathode
for the same interval was taken as the magnitude of the
photoelectric yield. The value thus obtained represents
the fraction of quanta leading to photoemission from the
cathode. This quantity is equal to the photoelectric yield
with a precision set by the probability of the appearance
of several electrons under the action of a single quantum.
For large photoelectric yields this probability is, appar-
ently, considerably different from zero, and-the actual
value of the quantum yield may prove to be higher than
the values determined by us.
Results of measurements for various photocathodes
are presented in the Table. It should be noted that'the
metallic photocathodes which we used were not subjected
to outgassing (their surfaces were cleaned with a fine
abrasive). The nonmetallic photocathodes were made by
vacuum evaporation on a nickel substrate.
Values of Photoelectric Yields of Various Cathodes (%)
avelength of c aracteristic line (A)
1 2:3 3.7 49
lV 3.8 4.31 2.2
0.94
t.il . I 6.0 17.0 C. 1.0
N..F . . . . . . . . . . . . 3.2 12.5 _ito 95.0
Ca F. . . . . . . . . . . I 15.9 7.1 14.2 2:5.0
SrF2 2^_.0 31.0 27.11 24.0
NaCl . . . . . . . . . . . . . .. 13.5 1:3.5 I 13.5 27.0
We must also note that the results obtained should
be considered as preliminary since, according to the data
of ref. 15 for ultrasoft x-radiation and ref. 9 for short
wavelength ultraviolet radiation, a strong dependence of
the photoelectric yield on the degree of outgassing of the
photocathode surface should be expected. It is very prob-
able that for sputtered layers a dependence of the photo-
electric yield on the thickness of the layer should also
be observed.
In conclusion, we wish to express our gratitude to
A. A. Lebedev for discussions and for his constant in-
terest in this work.
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