Delivering the world’s most intense muon beam

A new muon beam line, the muon science innovative channel, was set up at the Research Center for Nuclear Physics, Osaka University, in Osaka, Japan, using the 392 MeV proton beam impinging on a target. The production of an intense muon beam relies on the efficient capture of pions, which subsequently decay to muons, using a novel superconducting solenoid magnet system. After the pion-capture solenoid, the first 36° of the curved muon transport line was commissioned and the muon flux was measured. In order to detect muons, a target of either copper or magnesium was placed to stop muons at the end of the muon beam line. Two stations of plastic scintillators located upstream and downstream from the muon target were used to reconstruct the decay spectrum of muons. In a complementary method to detect negatively charged muons, the x-ray spectrum yielded by muonic atoms in the target was measured in a germanium detector. Measurements, at a proton beam current of 6 pA, yielded $(10.4\pm 2.7)\times 10^5\mathrm{muons}$ per watt of proton beam power (${\mu }^{+}$ and ${\mu }^{-}$), far in excess of other facilities. At full beam power (400 W), this implies a rate of muons of $(4.2\pm 1.1)\times 10^8\mathrm{muons}{\mathrm{s}}^{-1}$, among the highest in the world. The number of ${\mu }^{-}$ measured was about a factor of 10 lower, again by far the most efficient muon beam produced. The setup is a prototype for future experiments requiring a high-intensity muon beam, such as a muon collider or neutrino factory, or the search for rare muon decays which would be a signature for phenomena beyond the Standard Model of particle physics. Such a muon beam can also be used in other branches of physics, nuclear and condensed matter, as well as other areas of scientific research.

Figure 1

Layout of the MuSIC beam line showing the target surrounded by shielding and the PCS. To the right are the two modules of five and three element coils, respectively, of the MTS, covering an arc of 36°. Each coil in the MTS includes a correction dipole (CD). All superconducting coils are cooled with conduction cooling using Gifford-McMahon (GM) cryocoolers. The direction of the incoming proton beam is also indicated.

Figure 2

Schematic of the independent detection systems for measuring the muon flux. After a 36° arc of the muon beam line, the beam, including muons, exits the transport solenoid and optionally passes through an aluminum degrader D. The beam then passes through a plastic scintillator trigger counter S1 and a target T, where muons are stopped. X rays emitted by a captured muon dropping down orbital energy levels are detected in a germanium detector, which is also shielded. Electrons arising from the decay of a muon are registered in another plastic scintillator, S2.

Figure 3

Time distribution for an example data sample, with a degrader thickness of 5 mm, for each of the five S2 channels. The fit to the data with the function in Eq. (7) and the parameters extracted for each distribution are also shown.

Figure 4

Muonic x-ray spectra for different data sets.

Figure 5

Momentum distributions for all muons in a beam (white), the muons hitting the trigger counters (blue), and those stopped in the target (red).