Researches On Key Techniques In Measurements Of Neutron Fluxes And Energy Spetra

Currently,the neutron detection has been widely used infields of military,well logging,contraband detection,environmental radiation detection,medicine and space particles environment exploration,which has great scientific significance and broad application prospects.However,both in neutron nuclear physics experiments and neutron detection in many fields including industry,life science,materials science,environmental science and space science,there are alwaysseveral factors hinderingneutron flux and energy spectrum measurements,which are neutron-gamma discrimination,neutron signal pulse pile-up in strong radiation environments,andlow precision and instability of neutron energy spectrum unfolding.This leads to the fundamental question of this study that is how to improve the efficiency and accuracy of neutron flux and spectrum measurements.Regarding this problem,the thesis focuses on the following key technical issues:1.The neutron-gamma discrimination issue.Since almost all neutrons are accompanied by a strong ?-ray background due to the neutron inelastic scattering with the surrounding environment and other reasons,while commonly used neutron detector materials such as liquid scintillatorsare also sensitive to ?rays,the neutron-gamma discriminationhas become a key issuein neutron flux and energy spectrummeasurements.However,existing studies onneutron-gamma discrimination only focus on improving the performance of neutron-gamma discrimination methods and exploring new neutron detector materials,while neglecting the influencing factor of Photomultiplier Tube(PMT),which is the key component of a neutron detector that converts the weak light signal to a corresponding electrical signal.Therefore,the focus of this study is to investigate the impacts of PMT on the neutron-gamma discrimination performance in order to provide reference for neutrondetection systems based on liquid scintillators in terms of choosing PMTs and improving the performance of neutron-gamma discrimination.2.The pulse pile-up issue in intense radiation environments.The neutron count rate in strong radiation environments is quite high,thus the detector output adjacent pulsesare often partiallyor even completely overlap,called pulse pile-up.Pulse pile-up will give rise to the distortions of pulse waveforms and neutron energy spectra and reduce the neutron detection efficiency.Therefore,it is a key issue to deal with the pile-up pulses in neutron flux and energy spectrummeasurements of strong radiation environments.The conventionalway to resolve the pulse pile-up problem is using the pile-up reject circuit or the double exponential fitting.The former directlydiscardspile-up pulses at the expense of reducing the pulse passing rate and increasing the dead time of the system.The latter can reconstructparts of the original signal pulses,but is computationally complex and difficult to converge to the optimal value.This study aims to indentify pile-up pulses correctly and independentlyat high count rates and restore the distorted pulse waveforms and neutron energy spectra,which is capable of reducing the losses of energy resolution due to pulse pile-up and improving the neutron detection efficiency as well.3.The neutron energy spectrum measurement issue.The differential pulse height spectrum measured by a neutron detectoris the first kind Fredholm integral of its inherent response function and the energy spectrum of theincident neutrons,so the neutron energy spectrum measurementis actually converted tosolving a higher dimensional Morbid Linear Equations problem making use of the differential pulse height spectrum and the detector response function(the number of equations and variables are as many as tens or hundreds).Such equations are difficult to solve,and the result is unstable and evencould be negative,leading to its inconformity with the physical meaning of the neutron energy spectrum.Therefore,how to accurately unfold neutron energy spectrum is another key issuein neutron flux and energy spectrummeasurements.Conventional methods such as theTruncated Singular Value Decomposition method and the Regularization method tend to get trapped in local optimal solutions and have other limitations including low precision and instability of the results.Therefore,the goal of this studyis to firstly calculate the inherent response function of the detector employing software and then acquire the differential pulse height spectrum through experiments and signal processing,and eventually unfold the neutron energy spectrumaccurately based on a novel method.To solve the above key technical issues,this thesis evaluated the impacts of PMT on the neutron-gamma discrimination performance for the first timeat home and abroad,proposeda neutron-gamma discrimination method based on optimal coupling liquid scintillator BC501 A to various PMTs,put forward an independent method of pulse pile-up identification and reconstruction and presented a neutron energy spectrum unfolding method based on Particle Swarm Optimization.These methods and the conclusions providedcorresponding solutions for the above key technical issues.The validity of the methods was verified through theoretical analysis and simulation,and their technical feasibility was demonstrated by physical experiments.Thus,the thesis has initially developed a unique neutron detection signal processing method,which will promote the application and development of neutron detection technology.