trend of this sort can be seen in Figure In any case the effect should be rather small, as the epithermal flux in the irradiation positions used is not greater than 1% of the thermal flux. Natural Isotopic Abundances. T h e numerical values reported in Table I are taken from a compilation done by Bainbridge and Nier ( I ) . The natural isotopic abundances are thought to be a rather minor source of error, as this quantity is known with a good precision. Decay Constant. If the decay cons t a n t is not precisely known, a systematic error could be introduced for short-lived radioisotopes, as measurements are always made some time after the end of irradiation. An increase in random error should be observed, if the different measurements are made after different decay times. As random errors observed for shortlived radioisotopes are not especially great, this effect should not play a n important part.
7.
ACKNOWLEDGMENT
The suggestions and criticisms of G. B. Bertolini are gratefully acknowledged. Thanks are also extended to R. Fantechi
for the standardization of the radioactive sources used for calibrating the gamma-ray spectrometer. LITERATURE CITED
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