一种高效紧凑反康普顿谱仪的初步研制

[Background] The existing anti-Compton high-purity germanium spectrometors mainly use analog circuits to realize multi-channel anti-coincidence and suppression of the Compton scattering background. Due to the high complexity of analog circuits, the debugging is not easy, and the temperature stability and flexibility are insufficient. Conditional changing caused by the delay in the long-term operation leads to the degradation of anti-Campton effect, and even failure of normal work in severe cases. [Purpose] This study aims to develop a digital anti-Compton high-purity germanium spectrometer based on time-stamped list mode. [Methods] The spectrometer combines high-purity germanium detector, ring-shaped detector and plug coincidence detector. Every incident event information of the detector (arrival time, energy, detector ID, etc.) is sent to the computer as an independent data packet, and the resolution curve is drawn automatically by the host computer software to determine the optimal resolution window τ value, and the original high-purity germanium energy spectrum is obtained in real time, and the anti-Compton high purity germanium energy spectrum, anti-Compton spectrum, coincidence resolution curve. A 200 MHz synchronous clock is employed to time-stamp each event data packet to ensure the time synchronization accuracy of 5 ns and meet the anti-Compton requirements of the high-purity germanium spectrometer. Instead of the traditional trapezoidal forming, the symmetrical zero-area trapezoidal forming method is used to realize fast and slow forming dual channels inside the FPGA (Field Programmable Gate Array) chip to achieve excellent low-frequency noise suppression and baseline estimation. For the large-volume high-purity germanium detector, a correction algorithms of rise time and multi-point energy deposition correction are developed, which effectively improve the system energy resolution. The SQLite fast database is employed to store the event data packets of all the detectors to realize the effect of one measurement and repeated use. [Results] The Compton reduction factor of this system reaches 9.8, the peak-to-Compton ratio can reach 1 158:1, and the all-energy peak full width at half maximum (FWHM) of ⁶⁰Co's 1 332.5 keV is 1.70 keV (relative detection efficiency is 60%). The integral background was reduce from 1.674 s^-1 (before anti-Compton) to 0.483 s^-1 (after anti-Compton). The minimum detection activity (MDA) of ¹³⁷Cs at 1.2 h was reduced from 20.5 mBq before anti-Compton to 11.3 mBq after anti-Compton. [Conclusion] This proposed scheme of anti-Compton spectrometer avoids the waste of repeated measurement time caused by parameter adjustment, and embodies the advantages of list-mode digital measurement design.