Japanese physicists have accelerated positive muons to 100 kiloelectronvolts. To do this, they created ultraslow muons by multiphoton ionization of muonium atoms and accelerated them in a high-frequency quadrupole. A report on the work is available on the preprint portal arXiv.org.
Muon accelerators can become important tools for both fundamental science (for example, for precise measurement of the anomalous magnetic moment of the muon) and applied problems. In particular, it will be possible to create a muon microscope with a much higher penetrating ability than an electron microscope for studying thick materials.
Building an efficient muon accelerator is no easy task. An intense muon flux can be produced by decaying pi mesons, which in turn are produced by irradiating stationary targets with proton beams. The muons produced in this way occupy a large volume of phase space, so they need to be cooled to form muon bunches, and then accelerated. The problem is that muons have a short lifetime (about two microseconds), so traditional particle cooling methods are not suitable. Scientists have already solved this problem by cooling with cryogenic helium gas, but no successful muon acceleration has been reported so far after cooling.
Physicists from the MUSE experiment at the Japanese proton accelerator J-PARC solved both problems: they were able to cool muons after their generation and then accelerate them to 100 kiloelectronvolts. To do this, the scientists directed a stream of muons produced by the decay of pi mesons at a target made of silicon aerogel (SiO2) 8 millimeters thick, both sides of which were irradiated with a pulsed laser. Some of the muons slowed down in the target, forming muonium atoms (𝜇+e-). Then the muonium atoms decayed under the action of photons from the laser, and the muons cooled in this way were directed by an electric field into the accelerator. As an accelerator, the physicists used a high-frequency quadrupole about two meters long, with a peak power of 2.6 kilowatts and a frequency of about 324 megahertz. The scientists analyzed the beam characteristics using a horizontal bending magnet, a microchannel anode, and a beam profile monitor installed after the quadrupole in the diagnostic line.
As a result, the scientists managed to accelerate beams of positive muons to 100 kiloelectronvolts, which corresponds to approximately four percent of the speed of light in a vacuum. The scientists estimated the efficiency of cooling and extraction of muons at 19 percent, and the loss of muons in the beams at three percent due to muon decays. The transverse normalized emittance of accelerated muons in the horizontal and vertical planes was 0.85𝝅 and 0.25𝝅 millimeters⋅milliradians, respectively, which, according to the scientists, corresponds to a decrease in phase space by two orders of magnitude and demonstrates good accelerator efficiency.
According to physicists, the results obtained demonstrate the possibility of creating a muon accelerator for studying muons directly, as well as for other physics problems.
Read about how muons are already being studied in our article “Fallen from the Sky”.