FPGA Implementation of Digital Filters for China Dark Matter Experiment

Abstract

The direct detection of light dark matter is a prominent topic in international physics research. Weakly interacting massive particles (WIMPs) are the leading dark matter candidates. The scientific goal of the China Dark matter EXperiment (CDEX), located in the China Jinping Underground Laboratory (CJPL), is to detect light dark matter candidates using a high-purity germanium (HPGe) detector array. In 2013, CDEX-1B achieved the most sensitive experimental results for spin-independent WIMPs below $\mathrm {4\ GeV/}c^{2}$ , with a data collection of $783~\mathrm {kg\cdot day}$ . In 2018, CDEX-10 obtained the most sensitive experimental results for spin-independent WIMPs in the 4–5- $\mathrm {GeV/}c^{2}$ range, using $102.8~\mathrm {kg\cdot day}$ of data. The CDEX-50, which is currently under construction, is expected to achieve an energy threshold of 100 eV, which poses a huge challenge to the waveform digitization system. However, the reduction of the threshold is constrained by the analog shaping system’s ability to suppress preamplifier noise. Therefore, this article studies the implementation of a CR-(RC)4 filter, a trapezoidal shaping filter, and a digital penalized least mean square (DPLMS) filter on a field programmable gate array (FPGA). The DPLMS filter consumes the most resources, followed by the CR-(RC)4, while the trapezoidal filter consumes the least amount of resources. An experimental platform based on a Broad Energy Germanium (BEGe) detector was built to compare different digital filters and analog shaping systems. The results show that the DPLMS filter is the most effective at improving the signal-to-noise ratio of the preamplifier output. Compared to the analog shaping systems used in CDEX-1B and CDEX-10, the standard deviation of the baseline noise was reduced from 216.3 to 170.0 eV. The energy corresponding to a 99% trigger efficiency was reduced from 862 to 712 eV, a decrease of about 17.4%. These results can provide a reference for upgrading the CDEX-50 waveform digitization system and further improve the sensitivity of dark matter detection.