First muon acceleration using a radio-frequency accelerator

Muons have been accelerated by using a radio-frequency accelerator for the first time. Negative muonium atoms (${\mathrm{Mu}}^{-}$), which are bound states of positive muons (${\mu }^{+}$) and two electrons, are generated from ${\mu }^{+}$’s through the electron capture process in an aluminum degrader. The generated ${\mathrm{Mu}}^{-}$’s are initially electrostatically accelerated and injected into a radio-frequency quadrupole linac (RFQ). In the RFQ, the ${\mathrm{Mu}}^{-}$’s are accelerated to 89 keV. The accelerated ${\mathrm{Mu}}^{-}$’s are identified by momentum measurement and time of flight. This compact muon linac opens the door to various muon accelerator applications including particle physics measurements and the construction of a transmission muon microscope.

Figure 1

Schematic drawing of the experimental setup.

Figure 2

Radio-frequency pulses input (Forward) into, reflected from, and filled in the RFQ. The digitizer window for the beam detector is also shown.

Figure 3

Distribution of the MCP pulse height and the TOF of the penetrating ${\mu }^{+}$. (a) Scatter plot of the pulse height vs TOF. (b) Pulse height of the MCP signal. The events above 100 mV were regarded as ${\mu }^{+}$. (c) TOF spectrum after the pulse-height cut was applied. The peak corresponds to the ${\mu }^{+}$’s injected into the RFQ with an energy of 89 keV.

Figure 4

TOF spectra of the negative-charge configuration with rf on and off. The clear peak of the rf on spectrum at 830 ns corresponds to the accelerated ${\mathrm{Mu}}^{-}$’s. The error bars are statistical. A simulated TOF spectrum of the accelerated ${\mathrm{Mu}}^{-}$’s is also plotted.