Muon spectrometers on China Spallation Neutron Source and its application prospects
Authors: Li Qiang , Li Yang , Lü You , Pan Zi-Wen , Bao Yu ,   Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China,   Spallation Neutron Source Science Center, Dongguan 523803, China,   State Key Laboratory of Particle Detection and Electronics, University of Science and Technology of China, Hefei 230026, China
Published: September 2024

<sec>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 will be completed as scheduled in 2029, making MELODY the first Chinese muon facility. This beam line mainly focuses on the application of muon spin relaxation/rotation/resonance (μSR) spectroscopy. The MELODY also reserves the tunnels for building a negative muon beam line and a decay muon beam line in the future, thereby further expanding the research field to 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.</sec><sec>The μSR spectroscopy is a mature technology for injecting highly spin polarized muon beams into various types of materials. The subsequent precession and relaxation of muon spin in its surrounding atomic environment reflect the static and dynamical properties of the material of interest, which are then measured by detecting the asymmetric emission of positrons from the decay of those muons, with an average lifetime of approximately 2.2 μs. This enables μSR to develop into a powerful quantum magnetic probe for investigating materials related to magnetism, superconductivity, and molecular dynamics. The combination of a positive muon and an electron is known as muonium, which is a unique and sensitive probe in studying semiconductors, new energy materials, free radical chemistry, etc. As the production of muon beams strongly relies on proton accelerator, only five muon facilities in the world are available for μSR experiments. This limits the large-scale application of muon related sciences. Especially, Chinese researchers face fierce competition and can only apply for precious and limited muon beam time from international muon sources to characterize the key properties of their materials.</sec><sec>The construction of the 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 high quality data in a low repetition rate operation mode. To achieve this goal, as shown in <xref ref-type="fig" rid="Figure1">Fig. 1</xref>, 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, and 3) extendable low temperature devices to cover most μSR applications and also fulfill experiments with extreme condition requirements.</sec><sec>The MIXE elemental analysis is a type of particle induced X-ray emission (PIXE) technology. Due to the heavier mass of negative muon, the energy of muonic X-ray is around 207 higher than that of X-ray or electron induced fluorescence X-ray. 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, the MIXE has been successfully applied to the elemental analysis of cultural heritages, meteorites, Li-ion batteries, etc. MELODY reserves tunnels for negative muon extractions and transport to a MIXE terminal. The MELODY research team is developing a new detection technology with high energy resolution and high counting capability to shorten the measuring time to an acceptable amount based on the 1-Hz repetition rate of muon pulses.</sec><sec>The 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 by constructing dedicated muon beam lines 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, discussions and expectations are made regarding the future upgrade of the CSNS-II muon source’s muon beamline and its broader applications.</sec>