Monte-Carlo Simulation Of Longitudinal Neutron Resonance Spin Echo Spectrometer

The neutron spin echo(NSE)technology uses neutron spin as a clock to encode neutron energy or phase information into the larmor precession of the neutron spin in the magnetic field.The energy resolution of NSE spectrometer which based on NSE technology,can reach the order of neV.The spin echo time of the NSE spectrometer in the same order as many materials dynamic time,and can be utilized to study dynamic characteristics of materials.In order tosupport the development of neutron scattering technology and meet the needs of materials research,the Chinese Academy of Engineering Physics is developing neutron spin echo spectrometer based on the China Mianyang Research Reactor.The simulation of the neutron scattering spectrometer can provide theory and data for the selection and performance improvement of the spectrometer components.In this thesis,the neutron scattering spectrometer simulation programs McStas version 2.4 and VITESS version 3.4,which are commonly used in the world,are selected as the simulation tools to simulate the neutron spin echo spectrometer of China Mianyang Research Reactor.McStas version 2.4 is used for simulation of neutron transport system,including neutron guides,velocity selector,polarizer and slits;VITESS version 3.4 is used for simulation of magnetic system,including first arm and second arm,aims to evaluate the influence of the main magnetic field inhomogeneity,main magnetic field strength and beam divergence on the neutron polarization.In order to meet the needs of spectrometer installation and shielding space,the effects of length,radius of curvature,number of channels and supermirror factor of neutron guides on neutron flux and spectrum at sample position were simulation with the distance is 1.0 m between the exit of neutron guide and reflectometer.According to simulation results,it is suggested that the length is 4.25 m,the radius of curvature is 40.0 m,the number of channels is 6 and the supermirror factor is 3.0.And the supermirror factor of straight guide is 2.5.The distance between the exit of the neutron transports system and the reflection spectrometer is about 1.0 m,and the total neutron flux at the sample position is around 1.29�109 n�s-1 under optimal parameters of the neutron guide.Simulation research selector selected neutron beam with the wavelength of 0.6 nm,0.8nm and 1.0nm.And analytically calculated selected wavelength range when rotor angle was-10�,0�,and +10�,respectively.According to the simulation and calculation results,the wavelength resolution is about 7.7%?23.5%,the selected wavelength range is about 0.6?2.9nm.The effects of cross-section,the radius of curvature,length,and supermirror factor of S-Bender polarizer on neutron flux and polarization at sample position were simulation.According to simulation results,it is suggested that cross-section is 35.0 mm�35.0 mm,the radius of curvature is 1.0 m,length is 7.0 cm and supermirror factor is 3.6.The neutron beam polarization in the range of 4.0 A?10.0 A is above 95.0%under these parameters.Compared the optimized and unoptimized results,such as neutron flux and polarization at sample position,and the effect of height and width of slits on neutron flux and beam divergence were simulation.After optimization,although neutron flux decreased by 16.5%,14.4%and 32.3%,for neutron wavelength of 0.6nm,0.8nm and 1.0nm,improved the neutron polarization to more than 95.0%when the cross-section of slits 30.0mm�30.0mm.The neutron beam divergence is 0.18�and 0.58� corresponding to the cross-section of 10.0mm�10.0mm and 30.0mm�30.0mm,respectively,which meets the physical design requirements of the spectrometer.The lamor precession uniformity of neutron spin in magnetic field can be affect and lead to decrease neutron polarization by inhomogeneity,Therefore the effects of main magnetic field inhomogeneity on neutron polarization were investigated by simulation.According to simulation results,when the main magnetic field is 530.0 Gs,if the polarization is higher than 20.0%,the main magnetic field inhomogeneity should not be more than 4.5�10-3,3.0�10-3,and 2.5�10-3,corresponding to the incident neutron wavelength are 6.0 A,8.0 A,and 10.0 A,respectively.The effects of main magnetic field strength on neutron polarization at analyser position were investigated by simulation.The main magnetic field inhomogeneity is 2.0�10-3,of the neutron beam polarization is higher than 20.0%,the main magnetic field strength should be lower than 1200.0 Gs,800.0 Gs and 700.0 Gs,corresponding to incident neutron wavelength are 6.0 A,8.0 A,and 10.0 A,respectively.The effects of neutron beam divergence on neutron beam polarization at analyser position were investigated by simulation.When the main magnetic field strength is 530.0 Gs,the main magnetic field inhomogeneity is 2.0�10-3,and the incident neutron wavelengths are 6.0 A,8.0 A,and 10.0 A,respectively,the neutron beam polarization changes within 20.0%.The research results in this thesis can provide theoretical and data support for the component selection and performance improvement of the longitudinal resonance neutron spin echo inelastic scattering spectrometer of the Mianyang Research Reactor in China.