Thermal neutron background measurement in CJPL

This paper describes the measurement of thermal neutron flux in the CJPL underground laboratory in the proximity of the CDEX experiment. A low background thermal neutron detection system is designed which applies a combination of a 3He proportional tube and a 4He proportional tube as the detector. Thermal neutrons can be captured by the 3He proportional tube while the 4He proportional tube is for the purpose of background measurement. The tube wall is made up of oxygen-free copper to reduce the background due to radioactivity of the wall material. The electronics readout system has been developed to store triggered events׳ waveforms so as to get the amplitude spectrum and monitor the data quality. We observed an average thermal neutron flux of Φ=4.00±0.08×10−6/cm2s in the CJPL experiment hall in the proximity of CDEX experiment and the neutron and background events both distribute uniformly along the tube.