Metastability of the Muonic Boron 2S State

The cascade of muonic boron and the two-photon decay 2S→1S have been investigated. The x-ray measurement with Ge and Si(Li) detectors was performed at low target gas pressures of pure B₂H₆ and admixtures with He. The 2S population of μB is found to be ε_2S  =  3.1(2)%. Cascade calculations indicate that more than half of the 2S states are populated with an empty electron shell. Two-photon decays have been observed in mixtures of B₂H₆ in He at partial diborane pressures of 67 Pa and below. The measured fraction of metastable 2S states per μB atom is 0.58(16)% in an admixture of 1% B₂H₆ in He at 6.7 kPa and is interpreted in terms of pressure dependent quenching.