Development of the customized radon detector and studies of the radon background onsite for the Daya Bay reactor neutrino experiment

Abstract

The goal of the Daya Bay Reactor Neutrino Experiment is to determine the last unknown neutrino mixing angle θ13, in sin22θ13 to sensitivity better than 0.01 at 90% confidential level in three years of running. In order to reach the goal, background from natural radiation should be controlled less than 50 Hz, accordingly radon, which is a naturally-occurring radioactive gas, is one of the factors need to be monitored. A High-sensitivity and High-reliability Hong Kong (H3) radon monitoring system is developed to support the slow control system of the experiment. It achieves the sensitivity requirement and fulfills the data acquisition specification by using an enlarged sample chamber, an improved electronic and a designed software, in addition to the dehumidifier system for long term use. Four final products of the radon detector were well calibrated and tested before application; the sensitivities of counted progenies Po-218 and Po-214 are 3.65 ± 0.37 x10-2 cpm/Bq·m3 (C.F.is 27.4±2.7 Bq/m3/cpm) and 3.94 ±0.40 x10-2 cpm/Bq·m3 (C.F. is 25.4±2.5 Bq/m3/cpm). Its performance is the highest among the same type of commercial available products. This radon monitoring system has been successfully taking data as the background measurement of the experiment for more than half a year, and will continue to provide useful information for the following several years. From the results, the radon levels in the air in Experimental Halls and in the water of water systems were higher than expected, but still at the acceptable levels. Further works have to be done to control them at preferred levels.