Development of a new positron counting system with SiPM readout for muon spin spectrometers

Abstract

Muon spin spectroscopy has been utilized for both particle physics and material science. At JPARC (Japan Proton Accelerator Research Complex), the world's highest-intensity pulsed muon beam has been realized. With the increase of the beam intensity, the requirement for the detector's high-rate capability becomes essential. Spectroscopy of muon spin is performed by measurement of positron angular asymmetry from muon decays. A new muon spin spectrometer with silicon photomultiplier (SiPM) readout is under development for precision measurement of muonium hyperfine splitting. The positron detector consists of tiled plastic scintillators, SiPMs, and fast readout-circuits with ASIC based ASD and FPGA implemented multi-hit TDC. A prototype of the detector was developed and its performance was evaluated with a high intensity pulsed muon beam at J-PARC. Based on an understanding of SiPM's characteristics and analog circuit response, a realistic event generator was developed for evaluation of systematic uncertainties of the measurement. Several methods of pileup correction were studied to minimize the systematic uncertainty. In this paper, overview of the project and details of the detector development are discussed.