A fast digital filter algorithm for gamma-ray spectroscopy with double-exponential decaying scintillators

Scintillators like CsI(Na), having double-exponential decay times, typically cannot be used in high-count rate applications due to the complicated pulse shapes created by the convolution of their light output decay curves with the decay constant of charge integrating preamplifiers. We present here a novel digital filtering algorithm that is capable of using CsI(Na) at input count rates exceeding 250 kcps, while still achieving good energy resolution. We used a 2.54 cm diameter and 2.54 cm long CsI(Na) crystal, whose scintillation light can be best described by a short component with a 550 ns decay time and a long component with a 4 /spl mu/s decay time. The crystal was coupled to a 2.86 cm diameter photomultiplier tube. The digital filtering algorithm was implemented in XIA's all-digital Polaris spectrometer, in which five running sums were captured from each digitized scintillation pulse and the Polaris's on-board DSP read these sums and used a set of precomputed coefficients to reconstruct the pulse's total light output as a measure of the deposited energy. The algorithm was tested at different input count rates, ranging from 19 kcps to 270 kcps using a 1 mCi /sup 137/Cs source. The energy resolution (full-width at half-maximum) at 662 keV was 10.7% at 19 kcps and 11.7% at 270 kcps with a filter rise time of 1.0 /spl mu/s, and improved to 7.0% and 8.4%, respectively, with a filter rise time of 3.2 /spl mu/s. The energy peak shifted by less than 0.3% for input count rates below the maximum throughput point. Output count rates of 65.3 and 17.8 kcps were obtained with filter rise time of 1.0 and 3.2 /spl mu/s, respectively, at an input count rate of 270 kcps. This algorithm can be easily adapted to other double-exponential decaying scintillators by changing the decay times used in the energy reconstruction formula.