Muon spin spectroscopy

The experimental muon source (EMuS) project foreseen for muon science and neutrino physics is utilizing a superconducting solenoid for muon and pion capture, and is proposed at the China Spallation Neutron Source (CSNS). For the optimal particles collection efficiencies, an adiabatic magnetic field from 5 to 2.2 T along the central axis of the solenoid is generated by the solenoid. The superconducting solenoid is composed of four windings and an iron yoke. The iron yoke is arranged for flux returning and magnetic field shielding. Two conductor options for the windings, the aluminum stabilized NbTi Rutherford cable and the NbTi monolith wire are compared from the mechanical structure and radiation performance. The results of the mechanical analysis from ANSYS show that the excitation stress is suitable. But the results of the radiation analysis from FLUKA Monte-Carlo show that the recovery of the Residual Resistivity Ratios (RRRs) is necessary for the stabilizers of the superconductor. The recovery cycle of the aluminum stabilizer and the monolith wire solenoids are three months and one year, respectively. However, the RRR of copper for the monolith wire solenoid will be less than 50, after 12 years continuous running, which is shorter than the EMuS lifetime of 30 years. Therefore, the aluminum stabilized NbTi Rutherford cables are chosen to fabricate the windings.

Conceptual Design of the Capture Superconducting Solenoid for Experimental Muon Source

Conceptual design and update of the 128-channel μSR prototype spectrometer based on musrSim

EMuS (Experimental Muon Source) as the first muon source in China will be built at China Spallation Neutron Source (CSNS). A 128-channel μSR spectrometer prototype is under development for EMuS. As the core part of μSR spectrometer prototype, the detection system (detectors and readout electronics) has been constructed. Beam test (at ISIS Muon Source in U.K.) and magnetic shielding test have been carried out to test the performances of this detection system. In this paper, a new test method has been developed to test the magnetic shielding capability by using the energy spectrum of 22Na source. A magnetic field up to at least 30 Gauss can be eliminated by the proposed shielding. This satisfies the actual requirement of 8 Gauss. The dead time of the prototype detector was measured with a dedicated setup and measured to be 1.4 ns.

Design and test of μSR detection system of EMuS at CSNS

A 128-channel positron detection system will be constructed at Experimental Muon Source (EMuS) of China Spallation Neutron Source (CSNS) to conduct muon spin rotation/relaxation/resonance ( $\mu $ SR) measurements. Each detector channel consists of a scintillator, a light guide, and a photomultiplier tube (PMT). The long light guide is bent to an “L” shape to propagate optical photons from the scintillator to the PMT. A series of Geant4 simulations and experiments has been performed to optimize the light collection performance of the L-bent detector. Geometry deformation induced by the bending process has been well modeled. Simulation results agree with the experimental tests. Accordingly, a novel hybrid wrapping method (scintillator with polytetrafluoroethylene (PTFE) and light guide with aluminum tape) has been developed to improve the amplitude of detector signals greatly. Compared to wrapping the detector with merely PTFE tapes or aluminum tapes, this method leads to an enhancement in light collection efficiency of 73% or 14%, respectively. Two optimally manufactured prototype detectors using the novel wrapping method have been tested at the ISIS Muon Facility. The beam tests demonstrated that the L-bent detectors can precisely measure the behavior of muon spins inside samples. Therefore, the L-bent detector design is competent for $\mu $ SR applications at CSNS/EMuS.

Development of L-Bent Positron Detectors for μ SR Applications at China Spallation Neutron Source

A muon facility—EMuS (Experimental Muon Source)—at China Spallation Neutron Source (CSNS) has been studied since 2007. CSNS, which is designed to deliver a proton beam power of 100 kW at Phase-I, and will serve multidisciplinary research based on neutron scattering techniques, has just completed construction, and is ready to open to general users from September 2018. As an additional platform to CSNS, EMuS aims to provide different muon beams for multiple applications, among which, magnetism study by μSR techniques is a core part. By using innovative designs, such as a long target in conical shape situating in superconducting capture solenoids and forward collection method, EMuS can provide very intense muon beams with a proton beam of 5 kW and 1.6 GeV, from surface muons, decay muons, and high momentum muons to slow muons. In this article, the design aspects of EMuS, including general design, target station, muon beamlines, and μSR spectrometer, as well as prospects for applications on magnetism studies, will be reviewed.

https://www.mdpi.com/2412-382X/2/4/23/pdf

EMuS Muon Facility and Its Application in the Study of Magnetism

Muon Spin Rotation, Relaxation and Resonance ({\mu}SR) technology has an irreplaceable role in studying the microstructure and properties of materials, especially micro-magnetic properties. An experimental muon source is being built in China Spallation Neutron Source (CSNS) now. At the same time, a 128-channel {\mu}SR spectrometer as China's first {\mu}SR spectrometer is being developed. The time spectrum of {\mu}SR can be obtained by fitting the curve of positron count rate with time. This paper presents a 32-channel Time-to-Digital Converter (TDC) implemented in a Xilinx Virtex-6 Field Programmable Gate Array (FPGA) for measuring the positron's flight time of {\mu}SR Spectrometer. Signal of each channel is sampled by 16 equidistant shifted-phase 200 MHz sampling clocks, so the TDC bin size is 312.5ps. The measuring range is up to 327us. This TDC has the ability to store multiple hit signals in a short time with a deep hit-buffer up to 512. Time tag is added to each data to record the moment when the data was detected. Programmable time window and channel shielding give the flexibility to choose the time range and channels of interest. The delay of each channel can be calibrated. The data is transmitted to data acquisition system (DAQ) through Gigabit Ethernet. TDC and control logic are configured in real time by DAQ. The results of test show that the Full Width at Half Maximum (FWHM) precision of single channel is better than 273 ps with a low sensitivity to temperature and the linearity is pretty well.

Design of 32-channel TDC Based on Single FPGA for {\mu}SR Spectrometer at CSNS

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EMuS Muon Facility and Its Application in the Study of Magnetism

Design of 32-channel TDC Based on Single FPGA for {\mu}SR Spectrometer at CSNS