Gamma ray line shapes from cadmium zinc telluride detectors: An interim report

CdZn telluride (CZT) gamma ray detectors, operating at ambient temperatures, are potentially useful for safeguards applications such as assay and isotopic analysis of nuclear materials in the field. To analyze the complex gamma spectra, the gamma line shapes, including the amplitude and shape of the tail, must be understood. We have studied the line shapes in spectra taken with a CZT 5x5x5 mm detector. Standard gamma sources covering the energy range 59 to 661 keV were used. After background subtraction, the peaks were fit to a sum of a Gaussian and an exponential tail confined to the lower energy side of the Gaussian. The energy dependence of the various parameters that describe the gamma lines are graphed. The variance of the Gaussian increases linearly with gamma energy. Slope of the exponential tail varies strongly with energy at low gamma energy and then becomes nearly constant. Tail amplitude is larger for CZT than for a Ge detector. Tail cutoff parameter is not a very sensitive parameter for describing the line shape. Using these line shape parameters, gamma spectra at 90-100 keV from small uranium samples with 2 to 75% 235U were analyzed: the x rays from 235U decay and the gammas from 238U-234Th were used to determine the 235/238 ratio. This ratio could be determined to a few percent accuracy if the 235U enrichment did not exceed about 50%