Direct Estimation of Gamma-Ray Abundances in Radionuclide Mixtures Complement Subtraction Method WILLIAM LEE
U. S.
Novo1 Radiological Defense Loborotory, Son Francisco 24, Calif.
b A method is described for directly determining the absolute abundances of gommo-emitting radionuclides in complex mixtures without manipulation of dota. A pure standard of known disintegration rate is subtracted directly from pulse height d a t a stored in the Argonne-type 256-chonnel onalyzer. Abundance is determined b y comparing counting times read directly from a timing scaler. The technique can b e applied to any mixture in which the photopeak of interest can b e definitely identified. As the correct end point i s determined b y visual inspection of the analyzer display screen, other constituents of the mixture need not b e known. The method i s adequately demonstrated b y quantitative estimation of each component in four-component mixtures.
F i g u r e 1. 256Channel pulse height analyzer A.
6. C. D.
Precision time bore Timing scaler Gross count scaler Viewing screen and S0"tlOl
panel
A
the y-ray scintillation pulse height analyzer is finding increasing acceptance as a standard piece of equipment in the nuclear research laboratory (2, id), its more general application t o the routine cpmtitative analysis of gamma-emitting radionuclide mixtures has been hindered by the lack of a . convenient method for estimating abundanccs from the pulse height distribution data. Methods which have been used require that operating conditions of the equipment, such as voltage gain, linearity, channel width, sample geometry, and detector characteristics be accurately standardized as a prerequisite for precision (IO). Pulse height distribution data once obtained must be subjected t o some type of graphical or numerical analysis for interpretation (1,3,6,@. The present paper describes a technique which permits rapid, direct estimation of gamma-emitting radionuclides in mixtures relatively independent of the requirement for maintaining standard measuring conditions. A unique "complemenbsubtraction" feature of the Argonne-type 256-channel analyzer combined with a specially designed deadtime correcting device makes possible a direct nournathematical procedure. I n using this procedure, a pure calibrated standard sample of each radionuclide LTHOUGB
800
ANALYTICAL CHEMISTRY
sought most be n i d i h l c in a form similar to the sample being analyzrd.
gcstrd as :I nicans of subtracting back-
ground from an accumulated spectrum (13).
EQUIPMENT AND APPARATUS
The "complement" function of the Argonne-type 256-channel analyzer converts the information which is stored in ,each channel of the memory to the full capacity (65,535 counts) minus the number of counts accumulated in the channel (IS). The result of this mode of operation as viened on the monitoring screen is a complete inversion of thc pulse height distribution curve. If suhsequently more counts from the same source are added to the complemented memory, these additional counts will, in effect, be subtracted from the original data stored in the memory-hence the term "complement subt,raction." When a number of counts exactly equal to that ahich was stored in the memory before eomplemcnting has been added, the result as viewed on the monitoring screen is 8. complete erasure of the distribution curve. This technique has been sug-
It was felt that the same procedure could be used to subtract a single photopeak .from a complex spectrum containing several such component peaks. I n this case the result of subtraction would not be complete erasure of the spectrum but merely the removal of the one peak. The end point for complete removal could be determined by close observation of the expanded photopeak trace on the monitoring screen during the process of subtraction. If the source of the subtracted counts were an accurately calibrated standard of the radionuclide in question, the amount subtracted ~ o u l be d a direct measure of abundance. Before any such subtraction method could be applied quantitatively, however, i t was necessary to provide means for correcting the inevitable discrepancy in instrumental dead time due to differences in counting rate between samples and standards. This was further complicated by the fact that in the A r m n n n -
*
(A)
NORMAL SPECTRUM
.
(c)
CsI3' SUBTRACTED, OUT
. .
..
. ..
.. _ ..
*
.
ing device and the scaler registering total counts. Abundance of the radionuclide present in the sample is readily computed from the observed counting time and the known disintegration rate of the standard. Figure 2 s h o w the actual steps in subtracting cesium-137 from a mixture as viewed on the monitoring screen. EXPERIMENTAL RESULTS AND DISCUSSION
Studies on Single Radionuclides. To obtain a reasonable estimate of (6)
COMPLEMENTED SPECTRUM
.'
,.".: ., - .. .r
i i * i
.. ....
