Standardization of cadmium-109 sources for γ-ray emission rate

Abstract

The standardization of 109Cd sources for 88-keV γ-ray emission rate has been performed using a NaI(Tl) well crystal which gives a nearly 4π geometry. Since the low-energy portion of the 88-keV γ-ray spectrum is obscured by 22-keV X-ray pulses, the total counting rate cannot be found by merely summing the counting rates for all the channels in the distribution. A method, in which the spectrum is broken down into eight components, is used to determine the number of degraded 88-keV γ-ray pulses. The calibration is performed by summing the counting rates of the various components below an arbitrarily chosen energy of 39 keV, and adding them to the total counting rate above 39 keV. The process in which photoelectrons escape into the aluminum liner of the well is found to be responsible for most of the degraded 88-keV γ-ray pulses which fall below 39 keV. The estimated systematic error in the measurement is ±0·9 per cent. Using 57Co sources which have been accurately calibrated by coincidence counting, the 2-in. × 2-in. NaI(Tl) well crystal is shown to have a γ-ray detection efficiency of 100 per cent with an uncertainty of ±0·2 per cent (after corrections have been made for well-wall γ-ray absorption and excape out of the well). The total abundance for the 122- and 136-keV 57Fe photons emitted per decay from 57Co has been measured to be 0·963 ± 0·003.