Research On Measurement Method Of ²⁵²Cf Neutron Radiation Field Energy Spectrum And Fluence Based On Multi-Sphere Spectrometer

Neutron metrology is one of the important elements and challenges of ionizing radiation metrology.In recent years,with a complex nuclear safety situation and the rapid development of nuclear technology in China,there is an urgent need for neutron metrology,and the technical requirements for neutron metrology accuracy are also increasing.To achieve this,our institute has constructed a neutron reference radiation field based on a high-emission ²⁵²Cf neutron source.This research,as a part of this construction task,focuses on neutron energy spectrum and fluence measurement methods.The multi-sphere spectrometer,with is long-term development,has become a common instrument for neutron measurement.However,due to the lack of specificity of its system response,improving the accuracy in neutron energy spectrum and fluence measurements of the particular neutron radiation fields is still an important project to be researched.Therefore,a profound research on the key technologies of neutron energy spectrum measurement,scattering background correction,and Bonner sphere optimization for multi-sphere spectrometers has been carried out based on the combination of Monte Carlo（MC）simulations and experimental measurements.This research provides key technical support for accurate measurement of the energy spectrum,fluence and calibration parameters of the ²⁵²Cf and D₂O moderated ²⁵²Cf neutron reference radiation fields.The main research contents and conclusions are as follows:（1）The methods have been studied for neutron fluence and energy spectrum measurement.A multi-sphere neutron spectrometer was established consisting of a ³He proportional counter,a sphere moderator,and an electronic and control terminal.The effects of its components on the neutron detection were discussed.The response matrix of the multi-sphere spectrometer system was calculated using an MC software--MCNP 5and verified in a mono-energetic neutron reference radiation field with a high agreement between the experiment and the simulation.The ²⁵²Cf and D₂O moderated ²⁵²Cf neutron radiation fields were measured using the multi-sphere neutron spectrometer.The method for inversion of incident fluence and energy spectrum by spectral analysis has been established.（2）The methods for the correction of neutron scattering effect were studied.To overcome the difficulty of reducing the scattering background using the shadow cone method due to the large size of D₂O moderated ²⁵²Cf neutron source,a method for calculating neutron scattering rates through MC simulation was explored.The simulation results were compared with the experimental results based on the shadow cone method in the ²⁵²Cf neutron field,with a relative error within±6%,which proved the feasibility of the MC simulation method.As regards the D₂O moderated ²⁵²Cf neutron source,the scattering corrections for the multi-sphere spectrometer measurements were completed by MC simulation,and the obtained neutron energy spectrum,average energy and fluence-to-dose conversion coefficient were in agreement with the recommended values in the standard（ISO 8529-1:2021）.（3）Optimized design of the spherical moderator was implemented.In order to improve the energy resolution of the multi-sphere spectrometer,a method for the design of the spherical moderator is proposed.Response function curves were obtained by MC simulations for the neutrons with energy less than 20 Me V in the spherical moderator with different materials and structures.The abilities of the moderating sphere in slowing down and absorbing neutrons were adjusted by changing the material combination,size,and boron content.Five optimized moderating spheres were chosen,and the new response matrixes were established for the standard nine spheres and the optimized ones.The theoretical calculation and energy spectrum measurement of D₂O moderated ²⁵²Cf neutron radiation field prove that this method is effective and improves the neutron spectrum resolution performance in medium energy range.Through the above research,a method for measuring the energy spectrum and fluence of ²⁵²Cf and D₂O moderated ²⁵²Cf neutron reference radiation field based on the multi-sphere spectrometer was established.It could be proven that the MC simulation method would accurately construct the response matrix of the multi-sphere spectrometer and could be used for scattering neutron correction instead of the shadow cone method.In addition,optimizing the moderator can help improve the spectral response of the multi-sphere spectrometer.This research has laid a solid foundation for ²⁵²Cf neutron reference radiation field calibration and instrument calibration.