the board by carefully removing the tape. The junctions were collected a t the center of the board by sliding them along the guides, and the thermopile was tied a t several places with fine cotton thread. The guides were then clipped near the thermopile and carefully removed. The therniopile was thus left free and ready for installation. ACKNOWLEDGMENT
The authors thank D. C. Sievers
13) Glover. C. A. Stanlev. R. R..AXAL. CHEW33, 447 (1961). (4) . . Haaer. K. F., Rosenthall, M., Ross, W.j Gas J . 49 ( S o . l o ) , 87 (1950). ‘ ( 5 ) Harris, L., J . O p t . SOC.d m . 36, 597
for his interest and suggestions and J. B. Engleman for assistance in the electrical aspects of the problem.
~I
oil
LITERATURE CITED
(1946). ( 6 ) Mason, L. S., Rec. Sei. Instr. 15, 205
(1) Am. Inst. Physics,
“Temperature. Its Measurement and Control in Science and Industry,” T-01. 1, Reinhold, SeJT Tork, 1941; T’01.(2,1955. ( 2 ) Dykp, P. H., ‘Thermoelectric Thermometry,” Leeds 8: Sorthrup Co., Philadelphia, Pa., 1954.
(1944). ( 7 ) Strong, J., ‘.Procedures in Esperi-
mental Physics)” p. 305, Prentice-Hall, Xew Tork, 1943. (8) Sntter, D. RI., Brock, J. E., Proc. Indiana d e a d . Sci. 6 3 , 266 (1953).
A Method for Determining Barium-140 and lanthanum-140 in a Mixture of the Two Radioisotopes William B. Lane, U. S. Naval Radiological Defense Laboratory, San Francisco 24, Calif.
of barium and lanR thanum iTere used to prepare synthetic fallout for studies of conADIOISOTOPES
taminating events arising from nuclear detonations. Dissolution of the kilocurie supply frequently gave a solution enriched in LaI4o, and it was necessary to know the amounts of the components so that aliquots of the solution could be adjusted to ensure a preselected activity (disintegrations per second) when equilibrium was re-established some six days later. A large number of leaching tests n ere conducted on the synthetic fallout to determine the solubility of the radiotracer and this also involved measuring samples which were enriched in one or the other component. The relative amounts of Ba140 and La140 and the total disintegrations per second \\ere determined with a scintillation counter, using a N a I crystal, and a 4-pi ionization chamber, since these instruments were available at the field test site. Both detecting systems are energy-dependent, as shown in Figure 1 ( I , 2), and nhile scintilla-
desired. When the relative amounts of the two components \\-ere established, total disintegrations per second 17-ere found by referring to the second curve s h o m in Figure 2 . This curve gives 4-pi ionization chamber response as function of composition. Thus if a sample were found to be composed of 20% La140 and 80% Bala the average disintegration per second would produce a n ionization current of 5 x 10-14 ma.
tion crystal response decreases n ith increasing gamma energy, the 4-pi ionization chamber response increaqes. The average energy for the two radionuclides is ( 3 ):
E La-r40 E
Ba-140
=
0.130 m.e.v. per photon
=
0.910 m.e.v. per photon
and this difference is large enough to permit semiquantitative assay of a mixture of the two isotopes. The gamma radiation from a n aliquot of solution was measured in both instruments in a fixed geometry, and a ratio, r , of the two values taken. r =
LITERATURE CITED
(1) LaRiviere, P. D., “Response of a Low-
Geometry Scintillation Counter to Fission and Other Products,” U. S. Saval Radiological Defense Laboratory TR303 (Feb. 4, 1959, unclassified). (2) Miller, C. F., “Proposed Decay Schemes for Some Fission-Product and Other Radionuclides.” E. S. Kava1 Radiological Defense ’ Laboratory TR160 ( M a y 27, 1957, unclassified). (3) Lliller, C. F., “Response Curve8 for USSRDL 4-Pi Ionization Chamber,” U. S. Saval Radiological Defense Laboratory TR-155 (May 17, 1957, unclassified).
ma. (4-pi ionization chamber) counts/second (scintillation counter)
This ratio depends on the composition, and a calibration curve, shorn in Figure 2, was prepared from known mixtures. The slope of the curve shows greater sensitivity for barium-rich mixtures; however, the over-all accuracy of the method was better than lo%, which was adequate for the information
I
I
I 90
01
70
ENERGY IMEVl
Figure 1. Photon response curves for USNRDL 4-pi ionization chamber and scintillation counter ANALYTICAL CHEMISTRY
60
Lo
10
PHOTON
478
80
0
-0
20
30
4o
50
40
50 60 80’~’I P E R CENT1
40
:5
30
20
10
0
70
80
90
100
( P E R CENT1
Figure 2. Ratio curve for determining composition and response curve for determining activity
