Design Of Surface Muon Source Based On Spallation Neutron Source And Study Of The Related Simulation Techniques

Since the discovery of muon tracks in the cosmic rays research in1936, muon has been serviced as a popular research object in particle physics and application physics. Especially, the muon source produced by the process of high energy and high power proton beam bombarding solid materials plays an important role in many disciplines, such as particle physics, materials science, biomedical, energy, geography and so on. Surface muon beam, which is produced by the pion decay at the production target surface with energy of4.1MeV, is one of the basic types of muon source and has a high polarization (～100%) after a beamline collection and transport. Material research in microstructure using surface muons depends on the microscopic (atomic-level) interactions of the polarized muon beam with the surrounding particles and magnetic environment in materials, and studies the internal information of materials by detecting the positrons decayed by stopping muons in the experiments. This popular probe technique in the condensed materials research is named as μSR spectroscopy, which stands for Muon Spin Rotation, Relaxation, Resonance and related Research.In this thesis, a detailed physical design of surface muon source based on spallation neutron source facility has been performed. An intensity simulation has been studied for three main sub-systems, which includes Monte Carlo calculation for muon production target, beam optics calculation for surface muon beam and Geant4simulation for the detector system of μSR spectrometer. All these design and simulation in this work are dedicated to two facilities:J-PARC MUSE (under construction) and CSNS EMuS (planning). Additionally, some simulation methods and simulation tools are extensively studied in this work. Especially, the beam optics simulation tool of G4Beamline was used to study the space and temporal focusing of the pulsed slow positron beamline.The main achievements in the thesis are shown as follows:(1) Physical design and simulation research for two surface muon beamline (S-Line/D-Line) at MUSE are given. Firstly, Geant4and Fluka were used to calculate the production rate of secondly particles on the J-PARC muon production target. This calculation gives a good verification for the production mechanism of surface muon. The difference of production rate between upper and lower target surface explains the special layout of the four beamline at MUSE. Comparison study shows that the high fringing magnetic field (>0.1T), which is induced by H-Line in the target station area, has only a little affection in the surface muon collection by SQ1, where the beam loss ratio is less than10percent. Secondly, many tools for beam optics calculation have been used to study the optimization issues, such as beam elements parameters configuration, beamline layout option, beam optics optimization and so on. Method of partitioning a2D phase space into beam core and-beam hole has been used to calculate the beam emittance and Courant-Snyder parameters of surface muon beam collected at the entrance of SQ1, and then the beam core ellipse zooming method was used to calculate the beam emittance for any given beam fraction. After these calculations, an optimal matching between the beam emittance and acceptance of collection system has to be considered and then a beam with a reasonable emittance has to been served as an initial beam for beam optics optimization calculation. Finally, a surface muon beam intensity of8×106μ+/s and1.5×107μ+/s with a beam spot Φ=4cm and beam divergence less than50mrad could be obtained for S-Line and D-Line, respectively. These simulation results achieve our expected goals for MUSE project. Additionally, comparison between the simulation results and the experimental results by D-Line beam commissioning has also been given in this thesis. It shows that the optimal current configuration for decay solenoid and final beam characteristics obtained in the simulation have a good consistency with the experimental results. This consistency certainly demonstrates the validity and reliability of the simulation methods used in this thesis, and concludes that the results for S-Line simulation are accurate and reliable.(2) Preliminary design for EMuS muon target and beamline is presented in this thesis. In order to obtain a high-intensity surface muon source, structural parameters of target (length, thickness and material) and azimuth angle of beam collection system have been carefully studied. Conclusions can be made as follows:The collection system with a central axis at an angle of90degree with respect to initial proton beamline will be most effective for surface muons collection. The optimal surface muon yield may reach10-5μ+/(p·GeV) when targets with thickness of80mm and radius of17mm (C),15mm (Be),7mm (Ta) and5mm (W) are used. Considering the similar methods of phase space analysis and beam optics optimization used in the MUSE simulation, we got an impressive result that using a thick target (length in proton beam direction) and collecting muons at side right of target surface could be more than one order of magnitude in beam collection efficiency. Finally, it expects a surface muon beam intensity of5×105μ+/s with a beam spot Φ=4cm and beam divergence less than100mrad to be obtained at EMuS. Its performance meets the demands of μSR spectroscopy studies in China for now.(3) Geant4was used to study the design of detector system for the prototype μSR spectrometer based on EMuS surface muon source. A simulation method for a single detector unit was first proposed in this thesis. Based on this simulation, it provides much valuable guidance for our future design. The nonlinearity relation between energy loss ratio and incident positron energy has a good explanation for the interaction of positrons with materials. Light yield of low energy positrons (0-5MeV) in plastic scintillator is in proportion to incident energy, whereas the light yield of medium energy positron is only dependent on the size of height, so a reasonable energy range has to be selected while μSR experimenting. Long plastic scintillator has a large solid angle for particle detection but a disadvantage of bad energy resolution (>40%) for positrons stopping at different position along the scintillator strip, which would be bad for signal discrimination and energy spectrum selection. The count rate of spectrometer is so directly dependent on the length and width of scintillator that an optimal structural scintillator strip has to be chosen to ensure no pipe up in a high-rate detecting. Finally, a plastic scintillator strip with length50-60mm, width10-12mm and height5mm was testified as an optimal positron detector unit for our μSR spectrometer. The high light yield and the light transmission efficiency in this scintillator meet the requirements for signal discrimination and μSR spectroscopy measurement. A μSR spectrometer with100-120segmental detection channels and detection rate of～1.0×104events/s is sufficient for the applications of EMuS surface muon.(4) Space and temporal focusing has been studied for the updated Pulsed Slow Positron Beam (PSPB) by G4Beamline. According the results, it knows that the updated coil system could provide a magnetic strength up to200～300G and has a so good homogeneity along the beamline that the beam loss by some unexpected factors is eliminated. With the solid Rare Gas (Neon) Moderator system, PSPB has100%transport efficiency and an energy-selective beam with spot less than5mm. After an optimization calculation for the temporal focusing study, a time resolution of pulsed positron beam less than200ps could be obtained for this beamline system. And it found that the high accelerator voltage has a significant affection to the temporal focusing, different parameters configurations have been given for the cases of high and low accelerator voltage, which are used to realize variable-energy positron beam. Many significant parameters related to deteriorate the performance of temporal focusing has also been carefully studied. For example, the low extraction electric voltage induces bad chopper efficiency and extends the pulse width of positron beam after chopping, which requires a better performance bunch system to realize our expected goal. In this thesis, G4Beamline has been testified as a suitable tool for PSPB beamline design and beam commissioning. Undoubtedly, it has more advantages over traditional simulation tools or algorithm-methods in the beam optics calculation.