Some aspects of gamma-ray scintillation spectrometry using cesium iodide crystals

Abstract

Photopeaks in four crystals, a 3" x 3" NaI(T1), a 3" x 3" CsI(Na), a 2" X 2" CsI(T1) and an unsealed 2" x 2" CsI(T1) were experimentally investigated for ten gamma-ray energies in the energy range of 0.279 Mev to 3.25 Mev (using radioactive sources) and for a number of crystal-to-source distances from 1 cm to 10 cm. These measurements gave directly the relative photopeak efficiencies thus yielding new information about these CsI crystals. The photopeaks in the 3" x 3" CsI(Na) crystal were found to be unexpectedly asymmetric, and we analyzed our spectra from two points of view: (i) referring to the high energy half of the peaks and (ii) referring to the full peaks. The extent of asymmetry was investigated in detail. An important feature in our experimental set-up was the source-holder designed to facilitate accurate adjustments of the crystal-to-source distances (inside the lead chamber) with external manipulations only. We also determined the photofractions of the CsI crystals at 3 and 10 cm (and at 0 and 15 cm by extrapolation) assuming the experimental values of Heath (1964) for the photofraction of a 3" x 3" NaI(T1). In this connection we used the "scaling relations" to compute the absolute detection efficiencies of the CsI crystals from the available information on the NaI crystals. The photofractions were found to be not too sensitively dependent on the crystal-to-source distance. We have compared our photofraction values with the theoretical values of Mller and Snow (1961) for the photofractions of the CsI crystals. Theoretical values were seen to be generally too large. Disagreement with the theoretical values was also seen in the comparison of the ratios of the photofractions of the 3" x 3" and the 2" x 2" CsI crystals.