Design of a μSR spectrometer with ultrahigh arrays at China Spallation Neutron Source

Abstract

A new muon source has been proposed to conduct muon spin rotation/relaxation/ resonance (μSR) measurements at China Spallation Neutron Source (CSNS). Only 1 Hz of the CSNS proton beams (25 Hz in total) will be allocated for muon production. To make better use of muons in every pulse, an ultrahigh-array μSR spectrometer (UHAM) with thousands of detector channels is under design. Due to such a high granularity of detectors, multiple counting events generated from particle scattering or spiral motions of positrons in a strong longitudinal field should be carefully considered in the design. Six different structures were modeled and simulated based on two types of angular orientations (parallel arrangement and pointing arrangement) and three kinds of spectrometer geometries (cylinder, cone and sphere). A quality factor, Q, was proposed to evaluate the performance of these structures by integrating their impacts on the overall asymmetry, the counting rate and the percentage of multiple counts. According to the simulation, the conical structure with detectors pointing to the sample has the highest Q in both zero field and longitudinal field. The results also show that all kinds of structures cannot be operated under strong longitudinal fields with a strength over 2 T. The full simulation of a μSR spectrometer can provide good guidance for the construction of the UHAM in the upcoming upgrade of CSNS.