ing efficiencies for more complicated decay schemes. Microscopic observation indicates that much of the loss is still caused by absorption from infinitesimal amounts of inert solids which aggregate into particles of 0.01 to 0.1 micron on evaporation. Similar conclusions have been obtained in other laboratories (1). ADVANTAGES OF 4 ~ S - yCOINCIDENCE STANDARDIZATION METHOD
Only approximate branching ratios need be known in the more complicated decay schemes. No corrections are required for “chance” coincidences or angular correlation, because practically
all of the betas are being counted. Almost any specific activity sample can be assayed. The 4n counting technique can be “standardized” for betaemitters having no coincidences. Theoretically, statistical variations in the true N , and NB-? counting rates should cancel each other almost exactly in the 4ap-y method but not in the ordinary 0-y technique. Different back scattering and counting efficiencies for beta activity in decays involving more than one beta group have no effect on the method. LITERATURE CITED
(1) Campion, P. J., Atomic Energy of Canada, Ltd., Rept. CRP-746 (1958).
(2) Pate, B. D., Yaffe, L., Can. J . Chem.
33, 15 (1955). (3) Ibid., p. 610. (4) Ibid., p. 929. (5) Ibid., p. 1656. ( 6 ) Seliger, H. H., Cavallo, L., J.Research N a t l . Bur. Standards 47,41 (1951). (7) Seliger, H. H., hlann, W. B., Ibid., 50, 197 (1953). (8) Seliger, H. H., Schwebel, A., Nucleonzcs 12, 54 (1954).
RECEIYED for review September 26, 1958. Accepted December 29, 1958. Division of Analytical Chemistry, Symposium on Radiochemical Analysis, Counting Methods and Techniques, 133rd Meeting, ACS, San Francisco, Calif., April 1958. Research supported by the Atomic Energy Commission, under contract AT(11-1)-347.
Precision Calibration of Large 4 Pi Scintillation Crystals for Routine Gamma Counting L. J. COLBY, Jr., and J. W. COBBLE Department o f Chemistry, Purdue University, lafayetie, Ind.
b By use of new techniques in isotope calibration, and scintillation spectrometry, a large well-type scintillation crystal counter has been calibrated very accurately. This now readily available counting system can be used routinely for mixed isotope analysis, for absolute counting of isotopes, and for decay scheme analysis. Accurate absolute counting of electroncapture isotopes is a further possible application.
T
recent use of large crystal scintillation counters (1, 6) has markedly improved both the sensitivity and accuracy of y-ray counting. The efficiencies of these instruments now approach that of high pressure ionization chambers (2, 4) and they are much more sensitive and readily obtainable. Scintillation techniques are highly developed (7) and the instrumentation appears to be reasonably stable with time. I n this laboratory a large crystal, 4?r, scintillation counter is being used for routine, absolute analysis of tracer level fission product activities and for nuclear decay scheme studies. It can also be used with isotopic mixtures. Several on the curves have been published (8,Q) photo efficiencies of crystals based largely on calibration with isotopes standardized by 4np proportional counting and the method of intrinsic
photo efficiencies of P. R. Bell using mass absorption coefficients. Because of the possibility of errors in standardization techniques (6),the calibration curves so obtained do not demonstrate the inherent accuracy obtainable with the scintillation counting method. I n the present research, calibration was achieved by use of the 4np-y coincidence method (3, 6), which is superior to normal 4ap standardization. The calibration curve of y-ray photopeak efficiencies so obtained is believed to be accurate to =t3%.
-
ELL L E A D SHIELD .S”CRYSTAL ‘PHOTO TUBE
:fix41
I FOLLOWER
I
I
PULSE
NALYZER
AMPLIFIER
1
1
-1 supPLy1
SCALER
Figure 1. Experimental arrangement of instruments
HE
798
*
ANALYTICAL CHEMISTRY
*
EXPERIMENTAL
The crystal used in this research was
a cylinder 5 inches in diameter and 5 inches high of sodium iodide (Tl) with a 6/8-inch diameter and 2l/2 inch deep well. This crystal was obtained commercially from Harshaw Chemical and was optically coupled, by using silicone oil, to a Dumont 6364 5-inch multiplier phototube. The instrumentation varied with the need, but one experimental setup is indicated in Figure 1, which facilitates rapid multicomponent analysis and determination of photopeak efficiencies. The system has a resolution of 11.4% for the cesium-137 gamma activity with the source in the well-i.e., a t the center of the crystal. For high sensitivity the background can be reduced by proper shielding; in this laboratory 4 inches of lead is commonly used, resulting in a background of 600 counts per minute under the cesium-137 photopeak. For “low-
level” counting an anticoincidence arrangement would be required. Efficiencies were obtained by first standardizing the isotopes by the 470-7 coincidence method (6) or in some cases in a previously calibrated 4aP counter (6, 11, 12). Appropriate aliquots (10 or 25 pl.) of the standardized solution were then transferred into a 3-inch borosilicate glass test tube which fitted into the well. The photopeak count rate for monoenergetic -prays was obtained by summing the appropriate channels in a 100-channel energy analysis of the sample. I n the case of sodium-22, scandium46, and cobalt-60 the sum peak for the coincident gamma activities was counted, resulting in the product of photopeak efficiencies for the two yrays involved ( I S ) . Such a procedure contributes appreciably to the accuracy of the determination]because this photo-
peak (sum peak) does not require the someli hat arbitrary subtraction of a Compton distribution due to a gamma of higher energy. Further, the sum peak is usually well separated from the rest of the spectra, shows almost perfect Gaussian shape, and has little Compton distribution. The data obtained are summarized in Table I. Through judicious choice of isotopes having overlapping y-ray energies, it is possible by successive approximations on the data in Table I to construct a curve of single photopeali efficiencies a s given in Figure 2. Although a 100channel analyzer facilitated collecting the data, a single-channel analyzer will xork almost as well, the largest disadvantage being the electronic drift over the time required to collect all the necessary information.
Table 1.
Photopeak Calibration and Efficiencies (Primary Standards)“
Isotope
E, 5l.E.V.
Hg203b
0.279 0.412 0.511 0,6616 0.885 1.119 1.17 1.27 1.33
Alu 198 Na22 Cs’37
SC‘6 Sc46 cow Sa22
N
* 10%.
(1) Bell, P. R., “Beta- and Gamma-Ray
7%
€2
...
+
4rpc 4 ~ p coined y 4TP 4 ~ p coinc. y 4 ~ p coinc. y 4TPY
89.6 74.9 69.0 61.4 50.6 41.0 38.6 36.0 35.0
0: i i i o 0; 2075 0.2075
*
0
4rpy
80 -
z70.
?
I L
*
W
8 60:
e
a 500
E
-
40 30 7
’ ah ’ 04
,
06
08
’
a
IO
‘ I2
*
I4
~
I6
Ed
Figure 2. crystal
Photopeak
Spectroscopv,” Chap. V, Interscience, &-en-York, 1955. (2) Borkowski, C. J., ANAL. CHEM.21, 348 (1949).
(3) Campion, P. J., Atomic Energy of Canada, Ltd., Research Rept. CRP746 (1958). (4) Cobble, J. W., Oak Ridge Natl. Lab., ORKL Memo C.F. 50-6-114 (1950). (5) Davis, R. C., Bell, P. R., Kelly, G. G., Lazar, N. H., IRE Trans. on ivuclear Sci. NS-3, No. 4,82 (1956). (6) Gunnink, Ray, Colby, L. J., Jr., Cobble, J . LT., ANAL. CHEW 31, 796 (1959).
( 7 ) IRE Trans. on i2‘uclear Sci. NS-3,
KO.4 (19563.
LITERATURE CITED
Efficiency, el
coinc. 0,1353 4 r p y coinc.d 0.1716 COS0 4 ~ p coinc. y 0.1353 a Two other secondary standards, Ce“3 (0.294 m.e.17.) and 1 1 3 1 (0.364 m.e.v.) gave efficiencies of 87.5 and 78.370, rePpectively, which agrees ne11 nith other data. b Using recent data of Nordling (10). e Using receht data of Wapstra ( 1 4 ) . d Data from coincidence of 0.51 m.e.v. gamma.
DISCUSSION
The very high efficiencies, especially at the lower energies, are apparent from Figure 2. However, for photons below 0.2-m.e.v. absorption in the glass sample holder and aluminum in the walls of the crystal well (-300 of mg. per sq. em. absorber) becomes appreciable (solid line). (By use of mass absorption coefficient data the solid line below 0.279 m.e.v. was calculated.) The dotted line would be the theoretically expected behavior, which in this particular experimental apparatus approached 99.5% of 47. In the resolution of one isotope from another, or in decay scheme work, it is necessary to subtract or “peel off” the Compton distribution of the higher energy y r a y from the photopeak of the less energetic y-ray. The errors as introduced with large crystals are appreciably less, because the Compton contribution is very much reduced. This has been particularly useful in this laboratory for mixed fission product analyses. By using this calibrated crystal as a secondary standard, electron capture isotopes (which are used as standards in calibrating crystals) can be calibrated to a much higher accuracy (=t3%) than previously possible. If only x-rays are emitted, the accuracy ivould be
Calibration Method 4 ~ p coinc. y
(8) Kalkstein,’ M. I., Hollander, J. M., Univ. Calif. UCRL-2764 (1954). (9) Lazar, N. H., Davis, R. C., Bell, P. R., Nucleonics 14,52 (1956).
efficiencies
in 5-inch
well
(10) Sordling, C., Siegbahn, K., Soliolomki, E., Nuclear Phys. 1, 326 (1956). (11) Pate, B. D., Yaffe, L., Can. J . C‘hem. 33,15,610,929,1656 (1955). (12) Seliger, H. H., Cavallo, L., .I. Research A a t l . Bur. Standards 47, 41 (1951). (13) Shapiro, P., Higgs, R. R., RCL.Sci. Zmtr. 28, 939 (1957). (14) Wapstra, A,, Nijgh, G. J., Salomons-
Grobben, N., Orstein, L. Th. &I., Univ. Calif. UCRG1928 (1958). RECEIVEDfor review September 26, 1958. Accepted December 29, 1958. Division of Analytical Chemistry, Symposium on Radiochemical Analysis, Counting Methods and Techniques, 133rd Meeting, ACS, San Francisco, Calif., April 1958. Work supported by the U. S. Atomic Energy Commission under contract AT (11-1)-347.
VOL. 31, NO. 5, MAY 1959
799
