Study On D-T Neutron Energy Spectra Measurements Using Recoil Proton Method

D-T neutron generator is important monoenergetic fast neutron source, and neutron spectra is one of the significant parameters need to be measured in research of neutron physics and neutron application technology using D-T neutron generator.According to application demand of the intense neutron generator(ZF-400I) based on T(d,n)4He (D-T) reactions with a rotating target being developed in Lanzhou University, a scheme used to measure neutron energy spectrum using recoil proton method is proposed, which means letting D-T neutron beams bombard polyethylene film to produce protons, then using a thick depletion layer Si(Au) surface barrier detector to measure the recoil proton energy spectrum, and the measured recoil proton energy spectrum can be unfolded to obtain the fast neutron energy spectrum.Using MCNPX code to simulate the integral yield, average energy spectrum, differential yield and angular distribution of recoil proton, the recoil proton energy spectra at different angles and so on. Then according to simulation results and taking the Si(Au) detector damage caused by incidence of neutron and the influence of (n, a),(n, p) reactions between fast neutrons and Si to recoil proton energy spectrum measurement into consideration, the detector should be placed at the direction of 45 degree to the direction of neutron beam, and the thickness of polyethylene film should be chosen from the range between 0.2mm and 0.4mm at this present.According to the basic problem about how to obtain neutron energy spectrum from measured recoil proton energy spectrum, the least square method and the iteration algorithm these two methods are investigated, and algorithm equations to these methods are given to calculate neutron energy spectrum, then computer programs used to calculate neutron energy spectrum are developed corresponding to the least square method and the iteration algorithm based on Matlab and C++software platform.A series of monoenergetic neutron beams are set to bombard polyethylene film, recoil proton energy spectra have been simulated by MCNPX code, then the response matrix of detector R is determined. According to the structure of target of ZF-400I neutron generator in Lanzhou University and the preliminary conception of the scheme of neutron energy spectrum measurement system using recoil proton method, a incident neutron energy spectrum is assumed through MCNPX simulation, and the recoil proton energy spectra at 0 and 45 degree which can be regarded as the measured proton energy spectra are also simulated. Based on these, simulation tests on two methods mentioned above are carried out using computer programs developed previously. Test result corresponding to the least square method shows that the concussion and negative value are existed in calculated neutron energy spectrum, so the least square is unsuited to calculate neutron energy spectrum based on recoil proton method, and should be ignored. Test result corresponding to the iteration algorithm shows that the calculated neutron energy spectra as detector placed at 0 and 45 degree have good agreement with the assumed incident neutron energy spectrum, which prove that the iteration algorithm is a more ideal method which can be used to calculate neutron energy spectrum.Preliminary simulation and design of the D-T fast neutron energy spectrum measurement based on recoil proton method are carried out. Range of proton in Si is calculated by SRIM code to make sure that the depletion thickness of Si(Au) should be larger than 1.61 mm, then the type of detector is determined. The influence of neutron collimation shield on neutron energy spectrum, the detection efficiency of measurement system, the shielding effect of collimation shield are all investigated by simulating the transportation of particles in D-T fast neutron energy spectrum measurement system. Preliminary evaluations of detector damage caused by neutron and the influence of neutron on recoil proton energy measurement are finished by simulating proton and α particle produced from reactions between neutron and Si(Au) detector. The simulation results show that placing the Si(Au) detector at 45 degree is a reasonable choice and the measurement system designed before meet basic demands of neutron energy spectrum measurement.