A muon spin spectroscopy has been utilized for both of particle physics and material science. At J-PARC (Japan Proton Accelerator Research Complex), the world's highest-intensity pulsed muon beam has been realized. With the increase of the beam intensity, 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 the silicon photomultiplier (SiPM) readout is under development for precision measurement of muonium hyperfine splitting [1]. The positron detector consists of tiled plastic scintillators, SiPMs (MPPC manufactured by Hamamatsu), and fast readout-circuits with ASIC based ASD and FPGA multi-hit TDC [2]

A prototype of the detector was developed and its performance was evaluated with the high intensity pulsed muon beam at J-PARC [3]. 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. Characteristics of new types of SiPM were evaluated for further upgrade of the detector.

In this presentation, overview of the project and detail of the detector R&D will be discussed. As a major application of the detector, precision measurement of muonium hyperfine splitting will be introduced.

[1] : S. Kanda et al., Proceedings of the 2nd International Symposium on Science at J-PARC (to be published). [2] : K. M. Kojima et al., J. Phys. Conf. Ser. 551 012063 (2014). [3] : S. Kanda et al., JPS Conf. Proc. 2, 010404 (2014).

The University of Tokyo