Utilization of adaptive flattop time with large area silicon drift detectors

Abstract

Energy-dispersive detection of X-rays using silicon drift detectors has a wide range of applications, especially for non-destructive material analysis. As X-ray spectra are acquired by signal processing of detector output signals, a key element is a shaping filter used for determination of X-ray energy values within digital signal processing. A flattop region in this filter prevents degradation of spectra by ballistic deficit effects. Detectors with large active areas demand high flattop times due to slow maximum signal rise times, causing pile-up effects and loss of count rate capability. In this work an approach using an energy filter with variable flattop time is presented, adapting the flattop time on a pulse-to-pulse basis to signal rise time. Implementation in hardware and experimental investigation is done using a 150mm2 active area silicon drift detector. Spectroscopic performance is investigated using a 55Fe source operating at 1 × 106counts/s. Peaking times of 50ns and 100ns are considered. In two series of measurements, influence of ballistic deficit is investigated for static flattop times between 150ns and 800ns, and for adaptive flattop time with 150ns to 450ns additional flattop time. Full width at half maximum, position, symmetry, and area of Mn-Kα peak are evaluated and compared. Results at similar influence of ballistic deficit show increased signal throughput for adaptive flattop time by 17% using the 50ns peaking time, and by 15% using the 100ns peaking time. Comparison at similar signal throughput, on the other hand, shows superior spectra quality achieved by adaptive flattop time, indicating a significantly lower impact of ballistic deficit.