Design And Construction Of A ?SR Spectrometer Prototype

?SR(acronym for muon spin rotation/relaxation/resonance)technique uses highly spin-polarized muon beams(surface nuons are?100%polarized)to probe the structure and dynamics of condensed matter.Polarized muons which are generated by an accelerator are implanted into a sample and localized in their sites after the thermalization process.Thereafter,their polarization evolves in the local magnetic field(nuclear,atomic,or interatomic magnetic field).Finally,they decay into positrons and neutrinos with a lifetime of around 2.2 ?s.According to the non-conservation of parity in weak interactions,the angular distribution of emitted positrons is anisotropic(asymmetrical).By detecting the asymmetrical distribution of positrons,one can reconstruct the depolarization function which reflects the time-correlated variation in asymmetry.The inhomogeneities of the magnetic field in muons' sites can be achieved by this depolarization function.Thus,the magnetism and related properties of materials can be characterized.Owing to the high sensitivity of muons to magnetic fields(lower to 0.1 Gauss),?SR technique has been widely used to study magnetic systems,superconductors,semiconductors,particle transport in materials and so on.At China Spallation Neutron Source(CSNS),the first Chinese muon source EMuS(Experimental Muon Source)will be jointly built by the Institute of High Energy Physics(IHEP)and Particle Beam Application Laboratory.The laboratory is responsible for the design and construction of a 128-channel ?SR spectrometer prototype for EMuS.A two-ring structure ?SR spectrometer prototype was designed using Monte Carlo simulation programs.A single-channel ?SR detector prototype was built based on the design and tested twice at the ISIS Muon Facility.The beam tests confirmed that the performances of the spectrometer satisfy the expected goal.As the EMuS Baby-scheme beamline has characteristics of high beam intensity and low repetition rate,the two-ring spectrometer has been updated to a four-ring one.The newly designed four-ring spectrometer can fully utilize the Baby-scheme beamline's intensity to increase the detection efficiency of positrons and achieve relatively high intrinsic asymmetry to reduce the systematic errors from the spectrometer.The author studied at ISIS Muon Facility of Rutherford Appleton Laboratory in the UK and was responsible for prototyping detectors for the 1216-channel Super?MuSR spectrometer which will be built in the future.During the period,ISIS plans to cut the current 70-ns-long muon beam to 10 ns in FWHM.Thereafter,the counting rate of the spectrometer will have a 15?20-fold gain,and the frequency resolution will be increased from?8 to?100 MHz.To make the Super-MuSR into reality,a three-channel detector module was designed and constructed.The detector module had experienced two beam-tests.By analyzing the beam test results,we confirmed that SiPM(Silicon photomultiplier)+PZC(pole-zero cancellation circuit)can be used as detectors for the Super-MuSR,so as to make the highly segmented spectrometer compact and be operated under strong magnetic field.The main works of this thesis are listed as follows:(1)The first Chinese ?SR spectrometer prototype with a high asymmetry has been designed.The asymmetry distribution in the sample-centered coordinate frame and its dependence on the distribution of primary kinetic energy of detected positrons have been researched comprehensively.Accordingly,by optimizing the arrangement of detectors,and the structure of the beam pipe and degraders,the two-ring spectrometer has been updated to a four-ring one to improve the performances of the spectrometer prototype.The four-ring structure can make full use of the high intensity of Baby-schele beamline to increase the detection efficiency and to achieve a high asymmetry(?0.42,the value of current ?SR in the world is 0.2?0.25).A prototype detector,which was built according to the two-ring design,was tested twice at ISIS Muon Facility.The testing results confirm that the performance of the spectrometer prototype satisfies the expected goal.(2)The first SiPM-based prototype detector module was designed and built for the1216-channel Super-MuSR spectrometer of ISIS.It was confirmed that,by using the detector with scintillator+fiber structure,amplitudes of output signals are not affected by deposition locations of positrons on the scintillator,therefore the electronics can correctly discriminate positron signals with high energy or low energy.The scintillator should be buried with a single fiber so as to make sure that positron signals and tube noises can be distinguished significantly and reduce the difficulty of manufacturing and the unreliability of detection system.A PZC was optimized dedicatedly for the SiPM(signal width is?180 ns).Tested with muon beams twice,it has been confirmed that the SiPMs with PZCs can be run normally with a counting rate of 12 events/frame/detector(upper limit of spectrometers based on PMTs).This i.dicates that SiPMs can be used for the Super-MuSR to promote the construction of spectrometer.(3)The first program named MuSS(Musr Signal Simulation)was developed using Monte Carlo simulations combining with experimental data.The simulations exhibit that,both the time width of muon pulses and deadtime pile-ups of detectors can lead to distortions of ?SR spectra.If the beam time width is wide(70 ns in FWHM for ISIS),tlle intrinsic asymmetry of a spectrometer will decrease under high transverse fields,and eventually limits applications of pulsed ?SR technique.Due to deadtimes(shortest time interval between two signals)of detectors,?SR spectra will be distorted at early times(1?2muon decay lifetimes after a muon bunch enters the sample)because of the counting losses resulted from deadtime pileups.It has been confirmed that the influences of timing methods are not significant,but shortening the signal width with a PZC can reduce the deadtime to alleviate tlle distortions of spectra and increase the counting rate of detectors.There is no extra deadtime added to the detection system with a ratio(undershoots to the amplitude of signals)of 10%from PZCs.Even a ratio of 30%adds 1?2 ns so that influences of undershooting from PZCs on ?SR spectrometer can be ignored.This work can guide the optimization of signals based on PMTs or SiPMs.