Research And Application Of Low Background Thermal Neutron Detection Technology

Due to its excellent cosmic ray shielding ability,underground laboratory has become an ideal experimental site for many major frontier issues,such as the dark matter direct detection and the neutrinoless double beta decay.The signals to be detected by these issues have very low probability of occurrence,and the understanding and control of the background of the system become the key to such experiments.Neutrons are one of the most important backgrounds and the core research problem of this paper is the low background thermal neutron detection technology.The ³He proportional tube is suitable for low background thermal neutron detection,but the commercial available ³He proportional tubes are difficult to directly measure the thermal neutron flux lower than the level of 10^-7cm^-2s^-1.Two kinds of methods,the deposited energy and risetime method and the current waveform discrimination method,have been proposed in this paper to decrease the minimum detectable amount?MDA?of thermal neutrons.The deposited energy and risetime method combines the risetime method with the optimization of deposited energy spectrum to decrease the MDA.Firstly,the factors affecting the MDA of this method are analyzed,and the quantitative relationship between them is given.And the detector has been designed based on the optimization of deposited energy spectrum.In order to study the characteristics and the distribution of the system background,the combination of a position sensitive proportional tube with a ⁴He proportional tube has also been designed.The designed readout electronics system obtains the time and energy information at the same time to further reduce the background with the rise time method.The MDA of the low background thermal neutron detection system based on the deposited energy and risetime method is about ten times lower than that of the commercial one,and the ³He consumption is less than one quarter of the latter.A new current waveform discrimination method based on the double pulse phenomenon of neutron events in the ³He proportional tube is proposed.The principle of this method and the key detector indicators affecting the effect of the method are studied.Based on this analysis,a suitable commercial ³He proportional tube is selected to build a low background thermal neutron detection system based on the current waveform discrimination method.The effect of the method and its theoretical analysis are verified experimentally,and the limitation of the traditional risetime method has also been studied from the experimental and theoretical point of view.The comparison between the two methods proposed in this paper has also been made.Experiment results show that the current waveform discrimination method can reduce the MDA by more than an order of magnitude.The low background thermal neutron detection systems designed in this paper are applied to measure the thermal neutron background in China Jinping Underground Laboratory?CJPL?.The thermal neutron flux of CJPL Phase I,Phase II and inside the shield structures of China Dark Matter Experiment?CDEX?are given.These measurements not only verified the low background thermal neutron detection technology studied in this paper,but also have important positive significance for the CDEX experiment and the construction of CJPL.