基于质子加速器的muon束流设计和慢muon装置的研究

发布时间：2018-06-10 16:25

本文选题：质子加速器 + 表面μ~+　；参考：《中国科学技术大学》2016年博士论文

【摘要】：自从在宇宙射线中发现μ子后,对μ子的研究和应用逐渐发展起来,但是宇宙射线中的μ子强度太低、能量太高且不可控制,这些都限制了对μ子科学的研究。随着质子加速器的发展和μ子基本物理性质的发现,高强度的μ子束在粒子物理、材料科学、能源科学、生命科学等领域都有重要作用。其中利用自旋极化的μ子束作为磁探针来研究凝聚态的方法称为(muon spin rotation/relaxation/resonance)技术。μSR技术的基本原理是极化μ子束注入材料后,它的自旋与材料中磁场相互作用,自旋方向会发生变化,之后衰变产生的正电子倾向于沿着μ子极化方向出射,通过探测正电子的空间和时间信息可以获得材料中磁场的相关信息。基于质子加器的高强度极化μ子源是通过高能质子轰击石墨靶得到的。质子与靶核反应产生π介子,由静止在靶表面附近的π介子衰变产生的μ子,称为表面μ子,极化率接近100%；飞行中的π介子产生的μ子,称为衰变μ子,经过某一动量筛选可得到极化率约70%的较高能量的μ子束。这两种类型的μ源能量都在MeV量级,在实验中测量得到的是体材料性质。通过慢化表面μ子得到的慢μ可研究纳米材料、薄膜材料、样品表面等的性质。由于μ子慢化效率较低,高强度μ子源是得到可用于实验的慢μ源的前提；高强度μ子源同时也是通过准直等方法获得较小束斑或微束μ源的前提。本文主要利用蒙卡模拟软件和束流矩阵计算软件开展μ子束流方面的研究。中国散裂中子源(China Spallation Neutron Source, CSNS)经过直线和环形质子加速器的加速,最终可得到1.6 GeV的高能质子,一期功率100 kW。本文基于CSNS上的高能质子束,利用超导螺线管收集和传输技术,设计出我国的高强度脉冲型μ子束线。瑞士PSI (Paul ScherrerInstitute)拥有目前国际上强度最高的连续型表面μ源和唯一一条能用来做μSR实验的慢μ束线。微米量级的μ束可以研究小于1 mm2的材料,可以使μSR发展成为位置灵敏的技术,因此PSI考虑基于高强度表面μ源建设一个微米束斑的μ子束线。慢μ实验端外加的电磁场在实验中对束斑影响很大,本论文研究了这些电磁场对束斑的影响,并给出了减小样品处束斑大小的办法。极化率是表面μ子非常重要的特点,本文对超导螺线管磁场对表面μ+极化率的影响进行了详细计算。本论文主要的研究成果如下：1)基于中国散裂中子源设计出利用超导螺线管收集系统来获得高强度脉冲型μ源的束线。首先通过Geant4和Fluka两种蒙卡模拟软件计算出低Z和偏高Z靶材的μ子和π介子的能动量分布。使用Geant4计算得到表面μ子和不同能最π介子的产率,以及这些粒子动量方向与初始质子束夹角的分布。用Fluka计算了质子轰击四种靶材的能最沉积,综合分析得到石墨靶是产生μ子和π介子的理想靶材。根据散裂中子源上高能质子应用区的布局,规划出超导螺线管收集和传输系统的布局。使用G4beamline软件计算在衰变螺线管之后的表而和衰变μ子相空间分布,将这个相空间分布的参数作为初始源,应用TRANSPORT得到衰变螺线管之后聚焦元件的束流包络,应用TURTLE计算了在实验端的表而和衰变μ子相空间分布和产率。2)对PSI的πE3束线上考虑建造国际上第一个微米量级连续型μ子束线的可行性进行了计算。用TRANSPORT和TURTLE两种模拟软件首先考虑了束流后端的两组四极磁铁组(triplet)对初始束流的聚焦情况,计算结果表明大角散的束流会明显减小微米束流的传输效率,大的束斑也能减少传输效率,但其影响小于角散的影响。计算结果结合刘维尔定理表明在两组triplet之前的束流尽量调成大束斑的平行束,更利于后端μ子束的聚焦。通过束流光学模拟优化,最终整体束线在200×200μm2范围内的传输效率为10-4和10-5最级,因此估算最终经过准直后到达微米范围内的μ子强度达到103/s和104/s量级,可以用来进行微束实验。3)研究了PSI上的低能μ子束在样品端外加的横向/纵向磁场和电场对束斑的影响。样品端外加的磁场用来进行纵向和横向μSR实验。外加的平行于μ子动量方向的电场用来将μ子加速到不同能量(0.5～30 keV)来研究不同厚度的样品。它们会使束斑或偏移或发散,通过比较实验测试结果和使用基于Geant4的musrSim软件模拟的结果,发现可以通过调整锥形透镜RA的设置来极大地减少外加电磁场对束斑的影响,模拟结果与实验吻合。当前慢μ上的束斑大小σx和σy约为6 mm,使慢μ装置只能用来测量大于1 cm2的样品。在慢μ束上考虑了移除触发探测器(10 nm碳膜)和缩小慢化体处提取束斑大小来减小实验端样品尺寸。移除触发探测器时束斑大小可以减小到3.5～4.0 mm。在移除这个探测器的情况下,使用二次慢化体也可以提供μSR实验测量的时间信号。此外,也模拟研究了在触发探测器前和在慢化体前的束流准直方法来减小束斑。4)研究螺线管中表面μ+自旋极化率和产率。μ子在磁场中进行Larmor进动,螺线管中磁场在与μ子极化方向垂直方向上的分量会对极化方向产生影响。使用G4beamline计算了不同靶长、不同螺线管磁场强度、不同靶偏转角度时的表面μ子的自旋极化率和产率的变化。计算结果表明,螺线管的磁场会对极化率产生一定的影响,但影响并不大,可以忽略。获得较高产率的最佳靶长在350mm以上,螺线管磁场大于4T,偏转角大于20°。考虑了超导螺线管传输系统对表面μ+的聚焦和对极化率的影响,由丁螺线管的纵向分量较小,对表而μ+极化率影响可以忽略；磁场越大,聚焦效果越好,传输螺线管的磁场最好大于2T。
[Abstract]:Since the discovery of muon in cosmic ray, the research and application of muon have been developed gradually, but the intensity of muon in cosmic rays is too low, the energy is too high and it is too high, which limits the study of the muon science. With the development of the proton accelerator and the discovery of the basic physical properties of the muon, the high-intensity muon beams are in particle physics. In the fields of material science, energy science, life science and other fields, the method of using the spin polarized muon beam as a magnetic probe to study the condensed state is called (muon spin rotation/relaxation/resonance) technology. The basic principle of the micron SR technique is that the spin of the polarized muon beam is interacted with the magnetic field in the material. The spin direction will change, and the positron produced by the decay tends to go out in the direction of the muon polarization, and the related information of the magnetic field in the material can be obtained by detecting the space and time information of the positron. The high intensity polarized muon source based on proton addition is obtained by bombarding the graphite target through high energy protons. The muon, produced by the decay of the pion decay near the surface of the target, is called the muon, called the surface muon, and the polarizability is close to 100%. The muon produced by the pion in the flight is called the decay muon. After a certain momentum screening, the muon beam of high energy of about 70% of the polarizability can be obtained. These two types of Mu source energy are all in the order of MeV, and are measured in the experiment. The slow micron obtained through slow surface muon can study the properties of nanomaterials, film materials, and sample surfaces. Because of the low efficiency of the muon slows, the high intensity muon source is the prerequisite for the slow Mu source used in the experiment, and the high intensity muon source also obtains small beam spots or microbeams through the collimation and so on. This paper mainly uses the Monte Carlo simulation software and the beam matrix calculation software to carry out the study of the muon beam. The China Spallation Neutron Source (CSNS) is accelerated by a straight line and a ring proton accelerator, and can eventually get 1.6 GeV high energy protons, and the first phase of the power of 100 kW. is based on the height of CSNS. The proton beam, using the superconducting solenoid collection and transmission technology, designs the high intensity pulse muon beam line of our country. The Swiss PSI (Paul ScherrerInstitute) has the highest intensity continuous surface Mu source in the world and the only slow micro beam line used to do the micron SR experiment. The micron magnitude Mu beam can study materials less than 1 mm2. In this paper, the influence of the electromagnetic field on the beam spot is studied in this paper, and the method of reducing the beam spot size of the sample is given. The method of reducing the size of the beam spots at the sample SR is given. The influence of the superconducting solenoid magnetic field on the surface Mu + polarizability is calculated in detail in this paper. The main research results in this paper are as follows: 1) based on the Chinese spallation neutron source, a superconducting solenoid collection system is designed to obtain the beam lines of the high-strength pulse type muon source. First, Geant4 and Fl are used. UKA two Monte Carlo simulation software is used to calculate the energy distribution of the muon and pion of low Z and high Z targets. The yield of the surface muon and the most pions of different energies, and the distribution of the angle between the momentum direction and the initial proton beam are calculated using Geant4. The energy deposition of the proton bombardment targets is calculated by Fluka, and the comprehensive analysis is obtained. To the graphite target is the ideal target for producing muon and pion. According to the layout of the high energy proton application area on the spallation neutron source, the layout of the collection and transmission system of the superconducting solenoid is planned. The G4beamline software is used to calculate the space distribution of the decay solenoid and the decay muon phase, and the parameters of this phase space distribution are taken as the initial. Source, the beam envelope of the focusing element after the decay solenoid is obtained by using TRANSPORT. TURTLE is used to calculate the feasibility of constructing the first micron continuous muon beam line on the PSI PI E3 beam line by using TURTLE to calculate the spatial distribution of the decay muon phase and the yield.2. Two modes of TRANSPORT and TURTLE are used. The proposed software first considers the focusing of the initial beam by the two group of quadrupole magnet groups (triplet) in the back end of the beam. The calculation results show that the large angular beam will obviously reduce the transmission efficiency of the micron beam flow. The large beam spot can also reduce the transmission efficiency, but its influence is less than the influence of the dispersion. The calculation results show that the two groups of T are combined with the Liu Ville theorem. The beam flow before riplet is adjusted to the parallel beam of the large beam spot, which is more conducive to the focus of the muon beam in the back end. Through the beam optics simulation, the transmission efficiency of the whole beam in the range of 200 * 200 mu M2 is 10-4 and the 10-5 level. Therefore, the estimation of the micron intensity within the micron range after the collimation is up to 103/s and 104/s. The effect of the transverse / longitudinal magnetic field and electric field on the beam spot on the sample end of the low energy muon beam on the sample is studied for the micro beam experiment.3. The external magnetic field applied to the sample end is used to carry out the longitudinal and transverse micron SR experiments. The applied electric field parallel to the direction of the muon momentum is used to accelerate the muon to different energy (0.5 to 30 keV) to study the difference. The thickness of the samples. They make beam spots or offset or diverge. By comparing experimental results and using the results of Geant4 based musrSim software simulation, it is found that the effect of the applied electromagnetic field on the beam spot can be greatly reduced by adjusting the configuration of the conical lens RA. The simulation results are in agreement with the experiment. The sigma y is about 6 mm so that the slow unit can only be used to measure the sample larger than 1 cm2. In the slow beam, the removal of the trigger detector (10 nm carbon film) and the reduction of the size of the beam spot at the slower reduce the sample size of the experimental end. The size of the beam spot can be reduced to 3.5 to 4 mm. when the detector is removed, so that the detector is removed, so that the detector is removed. The two slowers can also provide time signals measured by the experimental measurement of SR. In addition, the study of the beam collimation method before the trigger detector and the beam collimation before the slow body to reduce the beam spot.4) studies the micron spin polarization and yield of the surface of the solenoid. The Larmor precession in the magnetic field is carried out in the magnetic field, and the magnetic field in the solenoid is in the direction of the muon polarization in the solenoid. The components in the vertical direction will affect the polarization direction. Using G4beamline, the spin polarization and yield of the muons on the surface of the different targets, different solenoids and different target deflection angles are calculated. The results show that the magnetic field of the solenoid will have a certain influence on the polarizability, but the influence is negligible. The best target length of high yield is above 350mm, the magnetic field of the solenoid is greater than 4T and the deflection angle is more than 20 degrees. The effect of the superconducting solenoid transmission system on the focus of surface Mu + and the influence on the polarizability is considered. The longitudinal component of the solenoid is smaller and the influence of the meter and the polarization rate can be ignored. The greater the magnetic field, the better the focusing effect, the transmission screw is better. The magnetic field of a line tube is better than 2T.
【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O571.53

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