Development of negative muonium ion source for muon acceleration

Abstract

A negative muonium ion (Mu^{-}) source using an aluminum foil target (Al target) was developed as a low-energy muon source. Mu^{-} ions are produced by irradiating the Al target with a 3-MeV positive muon (μ^{+}) beam and observed using a microchannel plate. An experiment to produce Mu^{-} ions was conducted to evaluate the performance of this Mu^{-} ion source. The measured event rate of Mu^{-} ions was (1.7±0.3)×10^{-3} Mu^{-}/s when the event rate of the incident μ^{+} beam was 1.3×10^{6}/s. The experiment was conducted at the Muon Science Establishment, D-line in the Materials and Life Science Experimental Facility within the Japan Proton Accelerator Research Complex. The formation probability, defined as the ratio of the Mu^{-} ions to the incident muons on the Al target, was (1.1±0.2(stat)_{+0.1}^{-0.0}(syst))×10^{-6}. This Mu^{-} ion source was first adopted in the commissioning of the muon accelerator at the D-line, and the event rate of the accelerated Mu^{-} ions was consistent with the expectation. This Mu^{-} ion source boosted the development of the muon accelerator, and the practicality of this low-energy muon source obtained using a relatively simple apparatus was demonstrated.