Spin polarization and production rate studies of surface muons in a novel solenoid capture system based on CSNS

A novel surface muon capture system with a large acceptance was proposed based on the China spallation neutron source (CSNS). This system was designed using a superconducting solenoid where a long graphite target was put inside it. Firstly, the spin polarization evolution was studied in a constant uniform magnetic field. As the magnetic field can interact with the spin of the surface muon, both the spin polarization and production rate of the surface muons collected by the new capture system were calculated by the G4beamline. Simulation results showed that the surface muons could still keep a high spin polarization (>90%) with different magnetic fields (0–10 T), and the larger magnetic field is, the more surface muons can be captured. Finally, the proton phase space, Courant–Snyder parameters, and intensities of surface muons of different beam fractions were given with magnetic fields of 0 and 5 T. The solenoid capture system can focus proton and surface muon beams and collect \(\pi ^{\pm }\) and \(\mu ^{\pm }\) particles. It can also provide an intense energetic positron source.