Optimization of detector placements in reduction of multiple counts for mu SR measurements at China Spallation Neutron Source

An Experimental Muon Source (EMuS) has been proposed to conduct muon spin rotation/relaxation/resonance (mu SR) measurements at China Spallation Neutron Source (CSNS). To make better use of muons in each pulse, a highly segmented mu SR spectrometer with more than 2000 detector channels is under design. Due to such high granularity of detectors, multiple counting events generated from particle scattering or spiral motion of positrons in a strong longitudinal field should be carefully considered in the design. According to the simulation, long scintillators have a good capability of angular discrimination. Detectors with cuboid geometries are better than those with frustum shapes. The cuboid detector with a length of 50 mm is longer enough to get the optimal range of discrimination angle. In a real mu SR spectrometer, detectors can be placed parallelly along the beam direction or pointing to the sample. A figure of merit (FoM) has been proposed to compare such two arrangements by integrating their impacts on multiple counts and total counting loss in zero and longitudinal fields. The outstanding performance of multiple counting rejection due to the angular discrimination capability makes the pointing arrangement achieve much higher FoM. The simulation results can provide good support for the design of the highly segmented mu SR spectrometer.