基于FPGA的数字反符合γ谱仪系统设计

Design of Digital Anti-coincidence γ Spectrometer System Based on FPGA

本文设计了基于FPGA的数字反符合γ谱仪系统来降低天然本底和康普顿散射对低活度放射性测量的影响。该系统选用Φ145 mm×95 mm×80 mm的NaI(Tl)环形探测器与Φ75 mm×75 mm的NaI(Tl)主探测器构成反符合探测器,采用FPGA和高速ADC同步采样主探测器脉冲信号和反符合环形探测器输出信号。在FPGA中实现了核脉冲信号采集、缓存、反符合甄别、梯形成形等相关算法。在天然本底测量实验中,数字反符合γ谱仪系统的计数率为191.80 cps,本底抑制系数为2.69;对137Cs放射源的测量实验表明,在反符合探测器端面中心处,反符合测量峰总比为0.41,能量分辨率为6.99%;在反符合探测器侧面中间部位,反符合测量峰总比为0.30,能量分辨率为7.48%。实验结果表明,基于FPGA的数字反符合γ谱仪系统明显降低了天然环境本底和康普顿散射对测量的影响,适用于低活度放射性测量、现场就地放射性测量。

In this work,we designed an FPGA-based digital anti-coincidence gamma-ray spectrometer system to reduce the effect of the natural background radiation and Compton scattering on detection of the low-level radioactivity.In this system,the anti-coincidence detector consists of a NaI(Tl)detector with a size ofΦ75 mm×75 mm and an annular NaI(Tl)detector(Φ145 mm×95 mm×80 mm),the FPGA and the ADC were employed to sample the pulse signal from the main detector and the annular NaI(Tl)detector synchronously.We designed the related algorithms running in FPGA,such as signal acquisition and caching,anti-coincidence discrimination,and trapezoidal shaping.In the natural background measurement,the digital anti-coincidence gamma-ray spectrometer system reached a count rate of 191.80 cps and a suppression of background radiation coefficient of 2.69.In the 137Cs source irradiation experiment at the center of the front face of the anti-coincidence detector,the peak to total ratio reaches 0.41 and the energy resolution is 6.99%.The peak to total ratio reaches 0.30 and the energy resolution is 7.48%when the 137Cs source is placed in the middle part of the anti-coincidence detector.The experimental results show that the FPGA-based digital anti-coincidence gamma-ray spectrometer system in this paper can reduce the influence of natural background radiation and Compton scattering effectively,and can be used for on-site radioactivity detection and low-level radioactivity measurements.