极低快中子本底探测技术研究

Due to it's ultra-low cosmic ray background, underground laboratories are the ideal places for experiments searching rare events, e.g. double-beta decay, neutrinos and weakly interacting massive particles (WIMPs), which are quite sensitive to (and should be protected from) many kinds of backgrounds. Neutrons are the most import one of those backgrounds. Neutrons, like WIMPs, induce nuclear recoils, so it is curial that the neutron flux is suppressed by shielding, active veto systems and the use of ultra-pure materials. The neutron flux from rock dominates over other sources. It originates in spontaneous fission (mainly U-238) and (α,n) reactions due to uranium and thorium traces. The neutron flux from rock and its effect on the detector sensitivity can be calculated using the measured .contamination levels of U and Th. However, there may be quite a large uncertainty in such a calculation. Thus, the neutron flux in each underground laboratory must be measured seperately. This project is aiming at the measurement method of ulta-low fast neutron background. This kind of study, which can serve underground laboratories (e.g. CJPL), is still new in China..Neutrons can be detected by its recoiling or nuclear reactions. We prefer to Gd-loaded .liquid scintillation detector in this project by using coincident measurement. We will record coincidently two kinds of pulses - - the first one assumed to be from proton recoils induced by neutron elastic scattering and the second one due to the Compton electrons produced by gammas from neutron capture on Gd, H or other elements. Simulation code has been developed and simulated results give the optimized detector sizes and shape, a Φ30×40cm cylinder. The two flat surfaces of the detector will be viewed by twoo photomultiplier tubes (PMTs) through light guides, attached with optical coupling grease. Signals from two PMTs will be fed into NIM low-threshold discriminator units set to trigger. Waveforms will be recorded by some kind of digitizers, a.k.a. FADCs. Further analysises, e.g. identification of events, discrimination of particles, will be performed based on the digitized waveforms. .The efficiency of the detector system will be calibrated by Co-60 source and Cf-252 chamber. Simulation codes will be developed to estimate the system efficiency, the corresponding spectrum, and the random coincidences of two gammas within a time window..In this project, a technique to measure low intensity neutron flux will be developed based on a detection of delayed coincidences between proton recoils and neutron capture gammas. The related simulation codes and DAQ codes will be developed also. The technique and codes will serve underground laboratories in China.

极深地下实验室由于其极低的宇宙射线本底，是开展相关的极低本底实验（如双β衰变测量、中微子测量及暗物质探测等）的理想场所。但深地实验室由于岩层等环境中存在的中子本底对这些实验的影响很关键，是实验中非常重要的因素。国际上目前运营中的深地实验室都对中子本底进行过相关的研究，构成了其它探索实验的重要参考，比如正在建设中的锦屏山极深地下实验室是目前世界上最深的地下实验室，其内的中子本底研究是实验室正式运营中非常关键的因素。本项目拟通过探测器制造、模拟计算及实验测量，发展基于载钆液体闪烁体快慢符合的极低快中子本底的探测技术，为相关极底本底实验的开展提供技术支持。