Optimal crossed overlap of coherent vacuum ultraviolet radiation and thermal muonium emission for μSR with the Ultra Slow Muon

For μSR with ultra slow muon, we are constructing U line in materials and life science facility (MLF), J-PARC at present. Generation of ultra slow muon requires thermal muonium generation and laser resonant ionization process with vacuum ultraviolet radiation (1S→2P) and 355-nm radiation (2P→unbound). For laser resonant ionization, the coherent radiations and the thermal muonium emission must be coincident in time and space. The radiations can be steered in a chamber for reasonable overlap in space, and they can be easily overlapped in time because they are generated from one laser source. The trigger signal of the accelerator is useful for stable overlap in time.