Slow muon experiment by laser resonant ionization method at RIKEN-RAL muon facility

Abstract

We report first results of the slow muon generation from pulsed surface muon beam using a method of resonant laser ionization of muonium. The muonium is produced from the muon beam by electron capture near the surface of thin tungsten foil heated to 2100 K. The ionization is resonantly enhanced by using a vacuum-ultraviolet (VUV) radiation resonant with 2P state of muonium (λ 1S–2P=122.09 nm). A pulsed beam at 355 nm is used for an efficient ionization from the 2P state. The VUV beam with pulse duration of 4 ns and with bandwidth of over 100 GHz to match the Doppler broadening of muonium (200 GHz) is generated by resonant third-order sum-difference frequency conversion (ω VUV=2ω R−ω T) in krypton gas phase-matched with argon buffer gas. The ω R beam has been tuned to the two-photon resonance of 4p 55p[1/2,0] state in krypton to enhance the conversion process. Yield of the VUV radiation has been investigated for different energies and confocal parameters of the incident beams. The whole apparatus, including a beamline for transport and detection of ionized particles, has been tested by ionizing residual hydrogen atoms at an ultra-high vacuum level of 1.1×10 −9 hPa with efficiency better than 10 −3. The yield of slow muons obtained during the first beamtime was 0.03 μ +/s.