Structure Design And Performance Study Of Long Counter

As the nuclear industry enters a new stage of development in our country,the problem of non-traditional nuclear and radiation threat has been paid more attention.In order to cope with and deal with nuclear diversified events,requiring to improve the guarantee ability of radiation monitoring equipment,which means the conditions for the construction of neutron measurement standard is urgent.The long counter has been used as secondary standard instrument for neutron flux measurement in the world due to its features,including convenience,stable performance and low sensitivity to gamma rays,which was widely used in neutron standard laboratories and other radiation sites.In order to meet the military requirements for neutron dose monitoring in nuclear explosions and nuclear accidents,it is necessary to accurately measure neutron fluence in the range of 1eV to 15MeV.However,the commercial long counter has poor response at the low energy end,and the energy response decreases sharply when the incident neutron energy is higher than 5MeV.Therefore,it is necessary to design and develop a set of long counter with low energy limit and flat energy response.In this paper,a new type of long counter is designed and developed on the basis of a single BF₃thermal neutron proportional counter,and its performance was also tested and studied.Firstly,the general rules and methods of long counter design were summarized.The control variable method was applied in the process of affecting the energy response of the long counter,owing to complex relationship among the influencing factors.MCNP5 based on Monte Carlo method was used to calculate and analyze the influence of each structural parameter on the energy response,including the material of moderator,the pressure of thermal neutron detector,neutron absorber material,air hole,neutron proliferation material,shell and so on.The design method of the long counter and the main parameters affecting the energy response are mastered through comprehensive comparative analysis.Secondly,a long counter with good energy response in the range of 1eV¹5MeV was designed.Through reasonable selection of materials,optimization geometry and size,the energy response can be effectively improved.The design and processing of the long counter prototype is completed based on the influence mechanism analysis.The simulation results show that the combination of different hydrogen-containing moderator,neutron absorbing materials and adding compensation materials greatly broadens the flat response interval of long counters and the trend of sharp decline above5MeV is suppresses effectively.The final structural energy response of the long counter is 2.91cm²³.41cm² in the range of 1eV¹5MeV,the average energy response is3.10cm²,and the maximum relative deviation is about 10.1%,which is better than the commercial long counter,and the expected objective is achieved.Finally,a set of measurement system for neutron flux was built to test and verify the performance of the long counter developed in this paper.Specifically,it includes:（1）Performance parameter measurement of BF₃ proportional counter:to determine the performance of the proportional counter and the working parameters,so as to provide a basis for subsequent experiments.The proportional counter pipe plateau area adopted in this work is within the range of+1500V⁺1800V,the plateau length is about 300V,and the plateau slope is 0.17%/100V.（2）Energy response test experiment:the test was conducted at four standard energy points（144keV,1.2MeV,5MeV,14.8MeV）of the5SDH-2 tandem accelerator and the ²⁴¹Am-Be neutron source with five distances.The final results show that the optimized long counter in this paper has a consistent energy response within the energy range of 1eV¹5MeV,and the maximum relative deviation is less than 7%,which meets the requirements of long counter verification regulations.It can provide theoretical and technical support for the establishment of secondary standards for neutron laboratories,the transmission of radiation field values and the verification of neutron instruments.