Development Of A Polarization ~3He Neutron Polarization System And Its Application In Neutron Scattering

In comparison with the X-Ray and the electron beam,the neutron is a very unique probe.It carries no charge and it has a tiny magnetic moment.Once the neutron is polarized,its advantage and strength can be more fully made use of as a material probe.On one hand,the neutral polarized neutron can penetrate deep into the bulk material.Through its interaction with the magnetic material,the structure information which is hard to get by other methods can be obtained.On the other hand,through encoding the energy change or scattering vector information on the precessions of the neutron polarization in the magnetic field,better resolutions on both the energy change of the nonelastic neutron scattering and the scattering angle of the elastic small angle neutron scattering can be achieved,on which the neutron spin echo techniques are based.All the techniques and instruments based on polarized neutrons require effective method to polarized and analyze the neutron polarization.The neutron polarizing and analyzing methods based on polarized 3 He,is probably the best known technique to do the job.World recognized neutron facilities as NIST of U.S.,ORNL of U.S.and ILL of France etc,all applied the polarized 3He technique.The technique is also chosen by the three main neutron sources in China,including China Mianyang Research Reactor(CMRR).The polarized 3He system consists four subsystems,the optical pumping system,the 3He cell making system,the magnetic field environment and the polarization monitoring system.The available space and details of the spectrometer have to be taken into account for integrating the 3He systems.Based on CMRR,in order to design,optimize and integrate the 3He systerms to the spectronmeters,series theoretical analyses and experimental realizations are made.In this work,we mainly conducted studies as follows.1.We theoretically calculated the small angle scattering cross section of the polarized neutron by the magnetic nano particle.The results clearly indicate the sensitivity advantage of the polarized neutrons on probing magnetic structures.This study provides the theoretical base for applying the polarized 3He systems on the SANS(Small Angle Neutron Scattering)spectrometer.For the magnetic nano sphere made of Fe304,detailed analyses and simulations show that the contrast of the magnetic scattering signal can be significantly enhanced even for this weak magnetic scattering material.2.We studied the rotational effects of the nano particles when scattering with the UCN(Ultra Cold Neutron).Due to the quantum coherence originated from the similar value of the neutron wavelength and particle size,cold neutrons and UCNs can interact intensively with the nano particles.Taking of this advantage,UCN moderators can be realized by adding nano particles to superfluid liquid Helium.Only translational degree of freedom were considered in the previous work.In this work,we further studied effects caused by rotational freedoms of the nano particles.We found that slowing down efficiency of the moderator can be increased by as much as~40%when including the rotational effects.Considering that usually hundreds even thousands collisions will happen during the UCNs' moderating process,a~40%increase in efficiency for a single collision could have a significant cumulative effect.3.We optimized the design for the main magnetic field of the polarized 3He system.Usually Helmholtz coils are used to provide the holding field for the polarized 3He system.The Helmholtz coils have a simple structure and it is easy to realize in laboratories.However,to achieve the necessary field uni-formity,the Helmholtz coils are too large to move around and integrate to the neutron spectrometers.Through theoretical analyses and numerical simulation-s,we throughly studied the magnetic fields in the central areas of a number of possible magnetic coil systems,including the two coil system of the Helmholtz coils,the four coil systems of the Garret coils,Lee-Whitting coils and the Mer-rit coils.Considering the ease of machining,we finally chose the Merrit coils to provide the main field for the polarized 1He system.We built a prototype of the Merrit coil and mapped its magnetic field using the triple axis Fluxgate magnetometer moved by the three dimensional translational stages controlled by a computer.Though the Merrit coils have a much smaller size in comparison with the Helmholtz coils(The occupied volume of the former is only?1/4 of the latter),they provide a more uniform magnetic field.4.We optimized the pickup coils for the NMR system of the polarized 3He system.Through theoretical analyzes and calculations for pickup coils with dif-ferent configurations and positions,taking into account the 3He cell shape and dimension,we optimize the coil design by the SNR(Signal to Noise Ratio),We obtained the optimized dimension and winding method for the pickup coils.Our analyses indicates that SNR can be improved by almost an order if the optimized design is applied.In summary,we conducted a theoretical study and preparation for the future possibility of integrating polarized 3He system to the SANS spectrometer.We performed optimization and design for the main field of the polarized 3He system,including the whole process of theoretical analyses,numerical simulations and the experimental realization.We discovered that rotational effects could play an important role on moderating UCNs during our study on polarized SANS.We expect our research results can be used both theoretically and experimentally in the future studies.