ACKNOWLEDGMENT
The authors express appreciation to
J. H. Patterson for very helpful discussions, to Barbara Swartz for the mass spectrometer analyses, and to Leon Moore for technical assistance. LITERATURE CITED
(1) Albouy, G.. Ann. phys. (13) 1, 99 (1956). (2) Albouy, G., Teillac, J., Conzpt. rend. 232, 326 (1951).
(9) Inghram, hl. G., Chupka, K. A . . Rev. Sei. Instr. 24, 518 (1953). (10) Jarvis, C., Ross, M., Proc. Phys. SOC. (London) MA, 541 (1951). (11) Kondrat'ev, L. N., Novikov, G. I., Sobolev, Iu. P., Gol'din, L. L., S o w ( Phys. J E T P . 4, 645 (1957). (12) Rotblat,, J., Nature 165, 387 (1950). (13) Shliagin, K. N., Soviet Phys. J E T P . 3, 663 (1956). (14) Worthing, A. G., Geffner, J., "Treatment of Experimental Data," p. 20R, Wiley, Nev Y-ork, 1943.
(3) Asaro, F., Perlman, I., Phys. Rev.88, 828 (1952). (4)Dunlavey, D. C., U. S.$tomic Energy Commission. UCRL-I911 (August . - 1952) unclassified.' (5) Dunlavey, D. C., Seaborg, G. T., Phys. Rev. 87, 165 (1952). (6) Freedman, 11. S., Wagner, F., Engelkemeir, D. W., Ibid., 88, 1155 (1952). ( 7 ) Hollander, J. M., Perlman, I., Seaborg, G. T., Revs. Modern Phys. 25, 609 (1953). (8) Hyde, E. K., "The Actinide Elements," Xatl. Nuclear Energy Series, Div. IV, Vol. 14A,p. 573, LIcGravHill, Sew York, 1954.
RECEIVEDfor review April 28, 1958. Accepted July 28, 1958
Low-Energy Standards for Gamma-Ray Spectrometry GEORGE M. MATLACK, JESSE W. T. MEADOWS, and GILBERT B. NELSON University of California, 10s Alamos Scientific laboratory, 10s Alamos, N. M.
F A simple type of low-energ); Calibration standard for y-ray spectrometry has been developed for the range 5 to 75 k.e.v. Standards are made by placing a target of a single element over a pure beta emitter, such as pro.methiurn-14 7 or strontium-90. Characteristic x-rays of the target element are generated, usually the Ka! radiation, although I x-rays are predominant for very thin targets. For energy calibrations greater than 17 to 20 k.e.v. i t is best to use thin metallic foil targets or pressed oxide disks. Standards for energies less than 17 k.e.v. are best prepared b y mixing a solution of the beta emitter with a few milligrams of a salt of the target element, and evaporating to dryness on a micro cover glass. By this technique, the lowest energy Ka! x-ray clearly observed was that of vanadium at 4.96 k.e.v., using a 1 '/z X '/z inch thalliumactivated sodium iodide crystal equipped with a 0.005-inch beryllium window. The use of these standards affords a simple and convenient technique for checking nonlinear response in the low-energy region for scintillation detector systems, measuring the noise level of multiplier phototubes, and identifying radioactive elements which emit converted y-rays.
100 k.e.v., making it necessary to calibrate a multichannel analyzer in this energy region with only a few of these radioisotopes. Below 100 k.e.v., however, various workers have reported nonlinear response (2,8). Further, this is a n energy region in which it is difficult to use radioisotopes as calibration sources because of the relative scarcity of lom-energy y-rays suitable for calibration purposes. It is possible, of course, to use K a x-ray& resulting from converted gamma transitions, but hereagain from the viewpoint of ready availability and reasonable half life, the variety of
85.9 KEV
. 1
Pm -147 BEFORE PURIFICATION
***
Gd Kat X-RAY .. ..
: '..,**. 105 KEV
* .
.
........................................................................... ,.?h'mbw3
T
use of multichannel pulse height analyzers for y-ray analysis has increased greatly in the past several years. Energy calibration of these instruments is usually accomplished with easily available radioisotopes, such as cesium137, cerium-144, or cobalt-60, which possess y-rays whose energies are accurately known, and fairly long half lives. Most well-designed scintillatormultiplier phototube detection systems show a linear response n-ith energy above
such sources is Ijmited, whereas it is necessary to have a large number of calibration sources, spaced a t small energy intcrvals, if nonlinearity is present in a particular scintillatormultiplier phototube system. Until recently, the use of extcmml radiation-excited K a x-rays for calibration purposes has received limited attcation, although the fact that charactuistic x-rays may be excited by brernsstrahlung, &particles, or other x-rays has been known for a long time. For example, in 1954 Robinson and Jentschke (9) reported the use of the
HE
-
Pm 147 AFTER PURIFICATIQN " ".L-**.
-". I20 KEV D ".,."-
..............................................................................
CHANNEL NO.Figure 1. low-energy y-ray spectrum of promethium-147, contaminated with europium-155, before and after purification VOL 30, NO. 11, NOVEMBER 1958
1753
KCYradiation from tin-113, which decays by K-capture, for exciting KCYx-rays from 0.0025-inch-thick foils of zirconium, rhodium, and silver. Not long afterwards, Fowler and Roos (4) used sulfur35 /%particles to excite KCYx-rays in targets of salts of molybdenum and tin. I n both instances, these techniques were used in the investigation of response of organic scintillators to soft x-rays. The first systeniatic investigation of pparticle-excited x-rays for calibration purposes mas described recently by Kereiakes and coworkers (61, iqho used strontium-90 and thallium-204 to excite x-rays from foils and salts of a number of elements from uranium to bromine, giving a calibration range of 96.6 to 12.0 k.e.v. The present work extends the range down to 5 k.e.v. and illustrates some additional, simple techniques for generating the characteristic x-rays.
I'
.ASS,
t-
2+1#
Figure 2.
Arrangement of @-particlesource and target
-q 75.0 KEV
.;"*
. . *
EXPERIMENTAL
Beta Particle Sources. Both strontium-90 and promethium-147 were used for beta sources. Strontium-90. in equilibrium with its daughter, yttrium-90, was obtained from the Oak Ridge Kational Laboratory and used without further purification. Promethium-147, also received from t h e Oak Ridge Xational Laboratory. was found to be contaminated with about 2% europium-155 activity, plus traces of other rare earth fission products which rendered it unfit for use as a pure beta source. It was purified by adsorption on 200- to 400-mesh Domex-50 cation resin, followed by elution with 1.0Jmf lactic acid buffered a t pH 3.15 with ammonia. (Additional promethium-147 recently received from Oak Ridge was free of other activities.) The low-energy gamma spectrum of the promethium-14i before and after purification is shown in Figure 1. These sources were prepared by evaporating small aliquots on 1-inch square micro corer glasses, mounting on 21/2 x 21/4inch Lucite plates, '/Isinch thick, by means of double-back Scotch tape, and covering with 0.9 mg. per sq. cm. Mylar film. Source strengths were approximately 5 X lo7 beta disintegrations per minute. An extra strong source of i x lo* disintegrations per minute of promethium-147 was prepared for use with heavy element targets, or thick targets. Target Materials. Thin metal foils, from 0.001 t o 0.020 inch thick, lvere used as targets wherever practical These included niobium, molybdenum, rhodium, palladium, cadmium, indium, tin, tantalum, platinum, gold, and lead. Rare earth element targets were made by mixing the powdered oxide with Carbowax 4000-poly(vinyl alcohol) binder, pressing a t 10,000 pounds per square inch, then firin@a t 1000' to 1500' C. These ceramic disks v ere generally about 0.020 inch thick and 1.5 inches in diameter. Ceramic disk targets included cerium, praseodymium, neodymium, samarium,
1754
ANALYTICAL CHEMISTRY
S 0 U A RE
5
*
..'
Pb Ka X - R A Y 0.020", Pm 147
-
*
.
10.5 KEV
Pb LO X - R A Y 0.00I5",Pm I47
-
' . ...... .**
*%..
CHANNEL NO.Figure 3. Lead K a and I x-rays produced from different thicknesses of transmission targets
CRYSTAL-I$
7 4
6o
t
DIA.
x
I:
Figure 4. Response of Harshaw Type 6D6 crystal, with Dumont 6292 multiplier phototube, to radiation between 75 and 15 k.e.v.
STD. AI CAN PHOTOTUBE-DUMONT 6292 STD. DEV.*I.S% (Mo TO Pb)
/ /Od
0
>
1
4 30
z a
/d
0
IO
Mo
30
20
%a
40
50
ENERGY IN KEV
60
70
ea
gadolinium, strontium, yttrium, and zirconium. For target elements below strontium, both foils and pressed oxides or salts were generally too thick for good x-ray intensity. Accordingly, sources were prepared by mixing a few milligrams of the chloride or nitrate of the element with 0.1 to 0.2 ml. of promethium-147 solution containing about l o 7 beta disintegrations per minute; the dissolved mixture was evaporated to dryness on a 1-inch-square micro cover glass, mounted on a 1/16-inch-thick Lucite plate, and covered with 0.9 mg. per sq. em. Mylar film. These are referred to as "mixed" targets. An unusual type of target was made from polyethylene and arsenic powder. The powdered arsenic, 50% by volume, was mixed with -50 mesh Alathon-G (Du Pont) and molded into a slug. This n-as compressed between heated plates to a thickness of 0.008 inch, using shims to control the thickness. Disks 1.5 inches in diameter were cut from the sheet thus produced. This target proved satisfactory.
disk directly on top of the mounted beta source, as illustrated in Figure 2. When this assembly was placed in such a position t h a t the target was between the beta source and scintillator, i t was called a "transmissionJJ target. When i t was placed so t h a t the beta source R-as between the target and scintillator, it was called a "reflection,' target. For study of bremsstrahlung excitation of the heavier element x-rays, the strontium-90 beta source was covered with sufficient cadmium t o absorb the pparticles, which also absorbed most of the bremsstrahlung below 50 k.e.v. Counting Equipment. Two different thallium-activated sodium iodide scintillation crystals were used. The first was a standard Harshaw Chemical Co. crystal, 11/2 inches in diameter X 11/2 inches thick, Type 6D6. This is referred to as the "thick-window" crystal, because the thickness of the aluminum can cause appreciable absorption below 25 k.e.v. The second crystal, made by Harshaw on special order, Type 6HB-2. was 11/2 inches in inch thick, and was diameter X
Beta Source and Target Assembly. The simplest type of assembly used was to lay the metal foil or ceramic
75.0 K E V
.- .. 5 5
8.
-.,..,,,,,.
.,..5..."
Pb K a X-RAY 0.025 'I, Sf -90
..*e"
L
U
...,.~~**n**'
....................................................... ..,.............................,.........
.-*. . *. . 9
.
'
*
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To K a X-RAY
.
0.020'; S r - 9 0
.........
*~+..*----
*P-*.*...5 -.".I)
..........................................................................l,..,,.l...,.*,.,.l....
.. -
'
33.4 K E V
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..
--.h-..r*
.....:fwl-::: ......................................... .*4.*""*
a'.
..-e
-
0.020", Pm 147
. . '
L o Ka X-RAY
ILclr,c, ......................................
*
17.5 K E V
.. . ........ ;.'
Mo Ka X-RAY 0.002", Pm- 147
CHANNEL NO.Figure 5. Typical x-ray peaks obtained from transmission targets, using thick window crystal X-ray p e a k height is 10,000 counts in each case.
Channel width 1 volt
fitted n ith a 0.005-inch thick beryllium n-indow. This is referred to as the "thin-window" crystal. Both crystals nere mounted directly on the faces of Dumont 6292 multiplier phototubes, using mineral oil coupling. The tube used with the thin-window crystal was selected from a group of 12 tubes for low noise level, which n a s negligible aboi-e 2.7 k.e.v., when the tube was operated a t 1050 volts. After suitable amplification through a preamplifier and a Los Alamos Type 250h linear, nonoverloading amplifier, pulses from the multiplier phototube were fed into a 100-channel pulse analyzer, which could be operated a t 0.5- or 1-volt channel widths, equipped with magnetic core data storage, and auxiliary oscilloscope presentation of data. (All figures illustrating shapes of peaks were made directly from Polaroid transparency photographs of the oscilloscope.) The center of an xray peak was located by visual ob,qervation of the peak on the oscilloscope. This was reproducible to within one-half a channel width. The light-tight aluminum assembly containing the scintillator and multiplier phototube was fitted with a series of shelf slots, so that a choice of distance between source and scintillator ITas available. X-Ray Energies. Values of the KOI u-rays were taken from the energies tabulated by Fine and Hendee (3). RESULTS AND DISCUSSION
Target Thickness. As would be expected, target thickness has a pronounced effect on the intensity and shape of the x-ray peak obserrrd. For targets of the correct thickness, the peak is symmetrical and has a maximum intensity. This is true of both transmission and reflection type targets. Muller ( 7 ) has shown that for a transmission target, the optimum thickness will correspond approximately to the reciprocal of the linear absorption coefficient, p , for the KCYx-ray of the target element. A target with a less than optimum thickness gives an x-ray with decreased intensity and distorted shape on the lorn energy side. This distortion has no obvious effect on the location of the peak on the energy axis, although it renders visual estimation of the center of the peak more difficult \Then the thickness is very much greatrr than optimum, the most noticeable effect is again a decrease in intensity, but nithout distortion of the peak. Typical thicknesses for metal foil transmission targets are shown in Table I. Experimentally, it was found that if a target thickness corresponding to l / p produced an x-ray pcak lvith a distorted low-energy side, the distortion disappeared on increasing the thickness to 2/pJ without an appreciable decrease in intensity. I n cases n-here a metal foil was not convenient to use, such as for the rare earths, the ceramic targets were satisVOL. 30, NO. 11, NOVEMBER 1958
1755
I
40
CRYSTAL-I
STD.DEV.*I%(Y TO
1
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-
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-
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0: 40
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60
70
80
N
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Ka ENERGY IN KEV Figure 6. Response of special Harshaw thin-window crystal, with Dumont 6292 multiplier phototube, to radiation between 75 and 5 k.e.v.
1
~
Table 1.
Transmission Target Thicknesses
Target
Thickness, Inch
Pb Au Ta
0.020 0.012
Cd Rh Mo Nb
0.004 .....
Sn
Figure 7. Response of special Harshaw thin-window crystal, with Dumont 6292 multiplier phototube, to radiation between 17.5 and 5 k.e.v.
0,010 0.008
0.004 0.003
0.003
factory, even a t thicknesses of 0.020 to 0.040 inch. This illustrates the fact that to produce a n undistorted peak, target thickness is not critical, provided it is greater than the optinium iralue. -4n interesting result of decreasing the target thickness is the appearance of the L x-ray. Calculation predicts that for a transmission lead target, a thickness of 0.01 t o 0.02 inch nil1 almost completely absorb the LU x-ray a t 10.5 k.e.v. However, as the target thickness is decreased, the K a x-ray diminishes in intensity, while that of the L x-ray increases. I:i Fig ire 3, the top part of the picture shows the K a
..' 5.0 KEV
V Ka X-RAY MIXED, Pm-141
.. ......................................................................
..'*'**. ' 5.9 KEV
Mn Ka X-RAY MIXED, Pm-147
" ...............................,..................
...........................................
b
KU X-RAY MIXED, Pm- 147
CO
.......................................................................(, . . .
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Figure 8. Typical x-ray peaks obtained from mixed targets, using thinwindow crystal X-ray peak height is 10,000 counts for manganese, cobalt, and copper targets. Bremsstrahlung peak height in the vanadium example i s 10,000 counts. Channel width 0.5 volt
,*
'a,
8.0 KEV c
................................................................................................
CHANNEL N O . w 1756
ANALYTICAL CHEMISTRY
Cu Ka X - R A Y MIXED, Pm-I47
.
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r.
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.
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33.4 KEV
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......................................................................
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.
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..
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MO Ka X-RAY
c
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.................................................................................
CHANNEL NO.Figure 9. Effect of promethium-147 vs. strontium-90 excitation on relative peak to background intensity Note virtual absence of molybdenum x-ray when using strontium-90 source
t;
. .
1.0
W
z
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e
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-
...
-
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10
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-
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WINDOW
40
50
CRYSTAL
60
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70
,
80
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I
CRYSTAL
0.7L 1
I
I
I
0
2
4
6
Ka
8
PHOTON
IO
ENERGY,
I
12
14
16
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K.E.V.
Figure 10. Relation between Ka: photon energy and pulse height per unit photon energy, for thick- and thin-window crystals
peak a t 74.9 k.e.v. produced in a 0.020inch lead target by promethium-147. The bottom part shows the L x-ray produced in 0.0015 inch of lead. Thc relatively high abundance of bremsstrahlung to the right of the L x-ray indicates that even 0.0015 inch is too thick to observe the peak a t maximum intensit'y. Using a value of 1500 em.-' for t'he linear absorption coefficient of lead a t 10.5 k.e.v. (5):the calculated optimum thickness turns out to be 0.00067 em. or 0.00026 inch. The virtual absence of the K a x-ray is caused by the fact that the probability of excitation of L x-rays is much greater than for K x-rays. L x-rays were also observed for a few othcr heavy elements. I n all cases the peak position corresponded to an energy between Lal and Lpl. As an example, the peak position of the Pb L x-ray in Figure 3 corresponds t o 11.6 k.e.v., instead of the theoretical 10.5 for La. C'sing an intensity ratio of 10:7 for La:@, the calculated average is 1I .8 k.e.v. Thick-Window Crystal Calibration. A typical calibration for this crystal (Harshaw Type 6D6) is s h o w in Figure 4. I n this case the strontium-90 (5 x 107 d/m) source was used to excite the lead, platinum, and tantalum targets; the promethium-147 (7 X lo8 d/m) source was used for all the others. Metal foils were used for lead, plat'inum, tantalum, indium, palladium, and molybdenum, while ceramic oxide targets were used for the rare earths and yttrium; the iodine x-ray n-as produced from a thin flat crystal of iodine. The transmission type of target was used in all instances. This particular calibration is of interest because of the high count rate entering the cryst'al, resulting in an analyzer dead time of 20 t o 3070, much of which was caused by a high intensity of bremsstrahlung of greater energy than the x-ray peaks; the count rate under the lead peak was 105,000 c.p,m; that under the molybdenum peak was 140,000 c.p.m. In spite of this high count rate, the calibration line is essent,ially straight, n-ith a standard deviation of l.5Y0 from a least squares straight line over the region from molybdenum t)o lead. The yttrium calibration point was not included in the standard deviation calculation because of extreme absorption caused by thc aluminum can of the crystal. Figure 5 shows four of the x-rays 01,served in this calibration. The relatively high background under the lead and tant'alum peaks can be reduced considerably if a &particle absorber of 500 to 1000 nig. per sq. em. is placwl hctwcn the target and crystal. This i y not necessary if the promethium-1 41 soiurce is used instead of the strontiuin-90 source, because of the weak cnwgy (0.22 1n.e.v.) of the promethium-14i betas. The intensity of the iiiolybdenum x-ray is lower relativc t o the VOL. 30, NO. 11, NOVEMBER 1958
1757
higher energy bremsstrahlung than in the case of the other three x-rays because of absorption by the aluminuni can. -4n interesting effect is the slight bulge on the high-energy side of the tantalum and lead peaks, more clearly discernable in the lead peak. This is caused by the K p x-ray, which has a n intensity of about 1/5 of the K a (1). A comparison of half widths showed that the pure americium-241 y-ray at 59.6 k.e.v. had a half width of IS%, while the tantalum KCUx-ray at 57.5 k.e.v. had a half nidth of 26%, indicating the degree of widening by the KP line of tantalum a t 67.2 k.e.v. Thin-Window Crystal Calibration. This crystal was supplied with a 0.005inch thick beryllium window, thus enabling observation of x-rays, without attenuation, below 25 k.e.v., which was the point where absorption by the aluminum can enclosing the standard crystal becomes appreciable. The calibration curve of this crystal from the lead K a x-ray a t 74.9 k.e.v. to the vanadium x-ray at 4.96 k.e.v. is shown in Figure 6. The gain of the amplifier was set to give approximatel?. the same analyzer voltage scale as in Figure 5. Again the calibration is essentially linear down to the yttrium x-ray at 15.0 k.e.v Below this point a marked deviation from linearity is observed, pointing toward a positive intercept of the K a energy scale. Figure 7 shows a second calibration at increased amplifier gain for the K a x-rays of all the elements except krypton, from molybdenum (17.5 k.e.v.) t o vanadium (5.0 k.e.v.). Extrapolation of the straight line intercepts the K a energy scale a t about 1 k.e.v. A possible explanation of this bias may be incomplete reflection, with consequent intensity loss in weak light flashes within the crystal. The targets from vanadium to rubidium were of the “mixed” type. Above rubidium it was found that mixed targets were inferior to thin foil transmission targets because of excessive bremsstrahlung background. Four of the lower energy x-rays observed with the thin window crystal, using 0.5-volt channel widths, are shown in Figure 8, which also illustrates the rapidly increasing attenuation between manganese (3.9 k.e.v.) and vanadium (5.0 k.e.v.). A search for the titanium K a x-ray a t 4.5 k.e.v. revealed a barely perceptible hump, which was unreliable for calibration purposes. The half width of the cobalt x-ray at 6.9 k.e.v. was SO%, while that of the molybdenum x-ray at 17.5 k.e.v. was 35y0,. Choice of Beta Source. For equal target thicknesses and source strengths, the strontium-90 source is
1758
ANALYTICAL CHEMISTRY
more efficient in the production of xrays from the elements tantalum t o lead, because of the energetic 2.28m.e.v. p r a y from the daughter, It has the disadvantage yttrium-90. t h a t a higher bremsstrahlung background is produced, in addition t o the fact that energetic betas transmitted through thin targets must be stopped by an absorber before reaching the crystal. With the promethium-147 source, which has a 0.223 m.e.v. beta, no beta shield was required for any of the targets, and bremsstrahlung under the peaks was greatly reduced, especially for the lighter element targets. Figure 9 illustrates x-rays produced in transmission targets of lanthanum metal and molybdenum by both strontium-90 and promethium147, using the thick-window crystal and no beta shield. No x-ray is observed from molybdenum when the strontium90 source is used. K i t h a thicker target, the molybdenum x-ray becomes visible, but of low intensity. By using a massive molybdenum target in the reflection position, a fairly good x-ray is obtained. Strontium-90 can also be used as a bremsstrahlung source by placing a thick enough absorber of cadmium to stop all the betas. The bremsstrahlung from the cadmium layer will excite K a x-rays in transmission targets, but with greatly reduced intensity when compared t o direct beta excitation. It is felt that promethium-147 is a generally superior excitation source to strontium-90, even though a source strength of -5 X lo* d/m is required for good intensity of the heavy metal x-rays. For the elements between zirconium and the lighter rare earths. a source strength of 5 X lo7 d/m is sufficient. Carbon-14, with its 0.156 m.e.v. beta, would probably be even better, with respect to bremsstrahlung background, for light element mixed targets. Crystal and Multiplier Phototube Nonlinearity. A more sensitive way of detecting nonlinear response is to plot the photon energy, E, against the pulse height per unit photon energy, U ( 2 ) . A detector which gives linear response should give a plot parallel t o the photon energy scale. Figure 10 shows the data from Figures 4, 6, and 7 plotted in this manner, where the pulse height per unit energy has been normalized to 1 for the lead x-ray (74.9 k.e.v.) in curves A and B, and to 1 for the molybdenum x-ray (17.5 k.e.v.) in curve C. I n the thick-window crystal the response is essentially constant in the range 17 to 75 k.e.v. This is also true n-ith the thin-window crystal. But below 15 k.e.17. there is a sudden de-
crease in pulse height per unit photon energy, which becomes even more marked as progressively lower photon energies are detected. GENERAL REMARKS
I n using the low-energy sources described here, one must be careful to have approximately equal counting rates when comparing one source with another. Some multiplier phototubes show a marked shift in pulse height nith change in count rate. Conditions should also be arranged so that the bremsstrahlung in a set of energy calibration sources is a minimum, or at least roughly constant within the set. Although the x-ray peak which is actually observed is mainly the unresolved Kal and Ka2, there is little error involved in assigning it the value of K a l . Because K a z is one half as intense as K O , ( I ) , a weighted average of K a l plus K a z is easily computed. The difference between this average and Kal is 1% for the lead x-ray, and 0.27, for the molybdenum x-ray. The difference becomes even less for lighter element E. ACKNOWLEDGMENT
It is a pleasure to acknowledge the help of Steven D. Stoddard, who prepared the ceramic oxide targets, and James Church, who made the arsenicfilled polyethylene target. The thinwindow crystal was kindly loaned by R. H. Riuller, who also gave much time to helpful discussions. The continuing interest and support of C. F. Metz are gratefully acknowledged. LITERATURE CITED
(I) Compton, A. H., Allison, S. K., “XRays in Theory and Experiment,” 2nd ed., p. 640, Van Nostrand, New York, 192.5. -l__.
(2) Engelkemeir, D., Rev. Sci. Instr. 27, 589 (1956). (3) Fine, S., Hendee, C. F., Nucleonics 13, No. 3, 36 (1955). 14) Fowler. J. M.. Roos. C. E.. Phus. Rev. 98, 996 (1955). ’ (5) Hodgman, C. D., ed., “Handbook of Chemistry and Physics,” 37th ed., p. 2407, Chemical Rubber Publishing Co., Cleveland, Ohio, 1955. (6) Kereiakes, J. G., Kraft, G., R., mreir, 0. E., Krebs, A. T., Nucleonws 16, KO. 1, 80 (1958). (7) Muller, R. H Symposium on Radiochemical AnalyAs, 133rd Meeting, ACS, San Francisco, Calif., April 1958. (8) A-ucleonics 14, No. 4, 46 (1956). (9) Robinson, \Ir. H., Jentschke, R’., Phys. Rev. 95, 1412 (1954). \
I
I
”
RECEIVED for review April 28, 1958. Accepted July 18, 1958. Work erformed Atomic under the auspices of the U. Energy Commission.
E.
