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

Scintillators, like CsI(Na), with double-exponential decay times typically cannot be used in high-count rate applications due to their complicated pulse shapes created by the convolution of scintillator decay times and decay constant of charge integrating preamplifiers. We present here a novel digital filter algorithm which is capable of using CsI(Na) at input count rates exceeding 250 kcps and achieving good energy resolutions. We used a 1" diameter and 1" 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 μs decay time. The crystal was coupled to a 1 1/8" diameter PMT. The digital filter algorithm was implemented in XIA\´s all-digital Polaris spectrometer in which five running sums of each digitized scintillation pulse were captured, and the Polaris\´s on-board DSP read the energy sums and reconstructed the pulse height using a set of pre-computed coefficients. The algorithm was tested at different input count rates, ranging from 19 kcps to 270 kcps using a 1 mCi ¹³⁷Cs source. The energy resolution (FWHM) at 662 keV was 7.0% at 19 kcps and 8.4% at 270 kcps with a filter rise time of 3.2 μs, and increased to 10.7% and 11.7%, respectively, with a filter rise time of 1 μs. The energy peak shifted by less than 1% over the entire input count rate range, which reflects good system linearity. Output count rates of 65.3 kcps and 17.8 kcps were obtained with filter rise time of 1 μs and 3.2 μs, 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.