Muon spectrometers on the China Spallation Neutron Source and its application prospects

Abstract

The China Spallation Neutron Source Phase-II Project (CSNS-II) includes the construction of a muon source, namely “Muon station for sciEnce technoLOgy and inDustrY” (MELODY). A muon target station and a surface muon beam line are on the schedule to be completed in 2029, making MELODY the first Chinese muon facility. This beam line is mainly focused on the application of muon spin relaxation/rotation/resonance (μSR) spectroscopy. MELODY also reserves tunnels to build a negative muon beam line and a decay muon beam line in the future to further extend the research field into muon-induced X-ray emission (MIXE) elemental analysis and μSR measurements in thick cells, respectively. The two types of material characterization technologies keep their uniqueness in multi-disciplinary researches, and also provide complementary insights for other techniques, such as neutron scattering, nuclear magnetic resonance, and X-ray fluorescence analysis, etc.<br>The μSR spectroscopy is a well-established technology that implants highly spin polarized muon beams into various types of materials. The subsequent precession and relaxation of muon spins in their surrounding atomic environment reflect the static and dynamical properties of the material of interest, and then are measured by detecting asymmetrically emitted positrons decaying from those muons with an average lifetime of around 2.2 μs. This enables μSR to develop into a powerful quantum magnetic probe to investigate materials concerning magnetism, superconductivity, and molecular dynamics. The bounding of a positive muon and an electron, knowns as muonium, making it a unique and sensitive probe in the study of semiconductors, new energy materials, free radical chemistry, etc. As the production of muon beams strongly rely on proton accelerators, only five muon facilities worldwide are available for μSR experiments. This limits the application of muon related sciences in a large scale. Particularly, Chinese researchers face fierce competence to apply for precious and limited muon beam time only from international muon sources to characterize key properties of their materials.<br>The construction of MELODY muon facility at CSNS-II aims to provide intense and pulsed muon beams for Chinese and international users to conduct their μSR measurements with good quality data in a low repetition rate operation mode. To achieve this goal, as shown in Figure 1, the μSR spectrometer is designed with: 1) over 3000 detector units to obtain a sufficient counting rate of 80 Million/h to significantly suppress statistical fluctuations in a short measuring time; 2) a high asymmetry of 0.3 to greatly amplify μSR signals so as to further reduce statistical fluctuations; 3) extendable low temperature devices to cover most μSR applications and also fulfill experiments with extreme condition requirements.<br>The MIXE elemental analysis is a type of particle induced X-ray emission (PIXE) technologies. Due to the heavier mass of negative muons, the energy of muonic X-rays is around 207 higher than that of X-ray or electron induced fluorescence X-rays. Thus, the MIXE technology is more sensitive to materials with low atomic numbers, and thick samples can be effectively studied without scratching their surfaces. Due to these advantages, MIXE has been successfully applied in the elemental analysis of cultural heritages, meteorites, and Li-ion batteries, etc. MELODY reserves tunnels for negative muon extractions and transport to a MIXE terminal. The research group for MELODY is under the development of new detection technology with high energy resolution and high counting capability to reduce the measuring time into acceptable amount based on the 1-Hz repetition rate of muon pulses.<br>μSR spectroscopy and MIXE are the two most important application fields of accelerator muon beams. The MELODY muon facility aims to develop and promote these technologies in China with dedicated muon beam lines to be constructed in CSNS-II and in the future. In this overview, we introduce the principles and advantages of the μSR and MIXE technologies, as well as the physical design and application prospects of the μSR and MIXE spectrometers based on the CSNS-II muon source. Finally, the future muon beamline upgrade and more wider applications on the CSNS-II muon source are discussed and desired.