Prospects for ultra-low-energy muon beam at J-PARC

Spin-polarized ultra-low-energy muons (LE-μ+) with energies in eV–keV range provide a sensitive magnetic microprobe for studying near-surface regions, thin-film samples, multi-layered materials, etc. with depth resolution on a nanometer scale. Yet, worldwide there is currently only one (continuous) source of low-energy muons that is routinely used for such measurements. A pulsed source with many unique parameters (such as low-energy resolution of ∼14 eV, time resolution of 7 ns, low background and spot size of just 4 mm) has been demonstrated at RIKEN-RAL muon facility at ISIS, but its use is limited by a rate of only 15 μ+/s. The method of low-energy muon generation is based on a resonant laser ionization of thermal energy muonium and is ideally suited for a pulsed muon source such as J-PARC MUSE, since the pulse structure of the generated LE-μ+ is then determined by the laser pulse duration. The double pulse structure of the surface muon beam can, therefore, be converted to a single LE-μ+ pulse with a pulse duration that can be as short as 1 ns and can also be externally triggered. J-PARC is designed to deliver surface muon beam with rates up to 4×108 μ+/s and direct transfer of the same laser technology from RIKEN-RAL to J-PARC would provide a LE-μ+ beam with rates comparable to the existing continuous LE-μ+ beam at PSI (∼104 LE-μ+/s). An improvement in the laser pulse energy could lead to a higher efficiency and higher rates up to 106 LE-μ+/s may be possible. Construction of an intense LE-μ+ beamline at J-PARC MUSE would open up the possibility to do routine depth-dependent μSR measurements with thin film samples, with the muon implantation depth as low as 1 nm. In addition, the unique capability to synchronize the muon implantation with the sample excitation (e.g. by another laser or rf pulse) would allow to carry out pump-probe-type experiments.