Development of negative muonium ion source for muon acceleration

A negative muonium ion (${\mathrm{Mu}}^{-}$) source using an aluminum foil target (Al target) was developed as a low-energy muon source. ${\mathrm{Mu}}^{-}$ ions are produced by irradiating the Al target with a 3-MeV positive muon (${\mu }^{+}$) beam and observed using a microchannel plate. An experiment to produce ${\mathrm{Mu}}^{-}$ ions was conducted to evaluate the performance of this ${\mathrm{Mu}}^{-}$ ion source. The measured event rate of ${\mathrm{Mu}}^{-}$ ions was $(1.7\pm 0.3)\times {10}^{-3}{\mathrm{Mu}}^{-}/\mathrm{s}$ when the event rate of the incident ${\mu }^{+}$ beam was $1.3\times {10}^6/\mathrm{s}$. 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 ${\mathrm{Mu}}^{-}$ ions to the incident muons on the Al target, was $(1.1\pm 0.2(\mathrm{stat}{)}_{+0.1}^{-0.0}(\mathrm{syst}))\times {10}^{-6}$. This ${\mathrm{Mu}}^{-}$ ion source was first adopted in the commissioning of the muon accelerator at the D-line, and the event rate of the accelerated ${\mathrm{Mu}}^{-}$ ions was consistent with the expectation. This ${\mathrm{Mu}}^{-}$ 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.

Figure 1

Schematic drawing of an experimental setup of the ${\mathrm{Mu}}^{-}$ ion production experiment. A detailed description of the diagnostic beamline is given in the text.

Figure 2

Schematic drawing of the Al target and the Soa lens.

Figure 3

Simulated beam-profile distribution of surface muons at the Al target. The dotted rectangle is a region of the Al target.

Figure 4

Generated two-dimensional distribution for the ${\mathrm{Mu}}^{-}$ ions in terms of the kinetic energy and the emission angle. The number of generated events is ${10}^6$ events. The emission angle obtained by the horizontal direction of the momentum vector is drawn. The distribution in the vertical direction is similar.

Figure 5

Simulated phase-space distributions of the ${\mathrm{Mu}}^{-}$ ion beam on the downstream surface of the Al target. The top and the bottom figures show the horizontal and vertical phase spaces, respectively. The number of generated events is ${10}^6$ events. The average kinetic energy of the ${\mathrm{Mu}}^{-}$ ions was selected: 1 keV($\beta \gamma =0.0044$).

Figure 6

Two-dimensional distribution of the ToF and the pulse height of the negative extraction. The number of incident ${\mu }^{+}$’s was $2.7\times {10}^{10}$.

Figure 7

Comparison of subtracted pulse-height distributions for decelerated ${\mu }^{+}$’s and ${\mathrm{Mu}}^{-}$ ions. The histograms were normalized based on the number of total events. The green arrow indicates the threshold level applied to obtain ToF spectra shown in Fig. 8.

Figure 8

The ToF spectra for the positive extraction (top) and negative extraction (bottom). The red dotted line in the negative extraction indicates the fitted function, and the green histograms indicate the simulated ToF distributions. Numbers of incident ${\mu }^{+}$’s for positive and negative extractions were $3.4\times {10}^{10}$ and $2.7\times {10}^{10}$, respectively. Peaks around $-300\mathrm{ns}$ were prompt positrons contaminated in the surface muon beam.