Design And Research Of Fast Neutron Beam Loss Monitor In Low Energy Region Of High Intensity Proton Accelerator

The beam loss monitor is an indispensable part of the high-current proton accelerator.It can diagnose the performance and behavior of the beam which is passing through the accelerator,and discover the possible defects in the operation of the accelerator timely.It is of great significance to maintain the accelerator equipment and protect the field staff.However,during the operation of the high-current proton accelerator,even the beam with low energy loss will cause damage to the accelerator material,and this damage is difficult to be detected.Therefore,it is necessary to specifically detect the beam loss in the low energy region of the high-current proton accelerator,which is one of the focuses in the field of accelerator protection.A reliable monitor is a necessary condition for the success of beam loss monitor.The beam loss secondary products that can be detected in the low-energy region of the high-current proton accelerator are only neutrons and photons.According to the actual demand of the field monitor of the high-current proton accelerator,we need a fast neutron monitor that is only sensitive to fast neutrons and insensitive to photons and thermal neutrons.For this reason,we design a ¹⁰B+Si fast neutron monitor,which has the advantages of high detection efficiency,low sensitivity of gamma ray and thermal neutron,and fast detection response.It can detect beam loss in low energy region of accelerator timely and effectively.In this paper,a ¹⁰B+Si fast neutron monitor which based on ¹⁰B（n,α）~7Li nuclear reaction is designed by Monte Carlo simulation method.The Monte Carlo software MCNPX was used to optimize the material,size and geometry of the monitor components,and the optimal design scheme of the monitor was obtained by controlling the single variable method.The designed fast neutron monitor is cylindrical,and its structure consists of four parts:thermal neutron absorption layer（boron rubber,thickness of 5 mm）,neutron moderator layer（polyethylene,thickness of 50 mm）,reaction layer（boron film,thickness of 1.8μm）and silicon semiconductor monitor.The boron was deposited on the stainless steel substrate by magnetron sputtering.Two boron-coated stainless steel substrates were placed on both sides of the silicon monitor in a symmetrical manner,and the distance was 5 mm.The silicon monitor was placed in the geometric center of the whole fast neutron monitor.The simulation results of the monitor in the neutron energy range of 0.01～12 Me V show that the designed monitor is sensitive to fast neutron,insensitive to gamma ray and thermal neutron,and has a relatively flat sensitivity curve in the fast neutron energy range.After the design of the fast neutron monitor is completed,the detection efficiency of the monitor is tested by Monte Carlo simulation and radiation source test.The ²⁴¹Am-Be neutron source information in Monte Carlo simulation was provided by Institute of Modern Physics,Chinese Academy of Sciences.MCNPX software was used to simulate the monitor at different positions from the ²⁴¹Am-Be neutron source to obtain multiple sets of neutron counting rates.Radioactive source test is to place the fast neutron monitor on the ²⁴¹Am-Be neutron source test platform of Institute of Modern Physics,Chinese Academy of Sciences.The placement position is the same as the simulation position,and multiple rounds of irradiation tests are carried out to obtain multiple sets of measured neutron count rates.By comparing the simulated neutron counting rate with the measured neutron counting rate,the average detection efficiency of the fast neutron monitor is about0.02%.The test results show that the monitor can realize the rapid measurement of fast neutrons.The research results of ¹⁰B+Si fast neutron monitor in this paper provide material selection and structural reference for the design and development of beam loss monitor in the low energy region of high-current proton accelerator.The designed and assembled fast neutron monitor is expected to be applied to the beam loss detection of high-current proton accelerator in the low energy region,providing fast response signals for the machine protection system,and has a good application prospect.