Geant4 simulation of plastic scintillators for a prototype μSR spectrometer

The experimental muon source on China Spallation Neutron Source (CSNS) is expected to be a high intensity (10⁵ μ⁺/s) surface muon source with a small beam spot of 4-cm diameter. For a practical application of this muon source, we are devoting to develop the first pulsed μSR spectrometer in China. In this paper, the performance of plastic scintillators in the μSR spectrometer is studied by Monte Carlo simulation. The processes such as positron energy deposition, scintillation photons production, light propagation and photon-electron conversion are carefully considered. According to the results, an optimal dimension of the plastic scintillator is proposed using for our future spectrometer, which has a long-strip shape with the dimension variation range of 50-60 mm length, 5-8 mm height, and 10-12 mm width. Finally, we can build a spectrometer with a count rate up to 10⁴ e⁺/s by 100-120 forward and backward segmental detectors in total. The simulation could serve as an important guide for spectrometer construction. (authors)