A thermal neutron detection system based on boron-coated straw detector with prompt gamma coincidence

摘要

Accurate measurement of the neutron flux is of crucial importance for rare event search experiments in underground laboratories. The intrinsic radioactive background of the detector becomes the limiting factor for the detection of extremely low neutron flux. In this study, a thermal neutron detection system aimed at low flux measurement was developed based on boron-coated straw (BCS) neutron detectors. The neutron events can be distinguished from the detector background by coincidence measurement of neutron and the prompt gamma ray. With state-of-the-art BCS neutron detectors and NaI(Tl) gamma detector, a system sensitivity of ~120 cps/nv was achieved, comparable to that of the commonly used 3He counters. Based on the selected coincidence criteria, the background events were rejected to 0.1% with 45.3% of the neutrons preserved. The background accidental coincidence count rate of the system was measured as 2.3×10~(-5) cps, corresponding to a lower limit of measurable thermal neutron flux of 1.9×10~(-7) n/cm~2/s. The performance of the system can be further improved by using other gamma scintillator with lower neutron absorption (such as BGO) and adding extra shielding for ambient gamma rays.