Metastability of the Muonic Boron<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mn>2</mml:mn><mml:mi mathvariant="italic">S</mml:mi></mml:math>State

The cascade of muonic boron and the two-photon decay $2S\ensuremath{\rightarrow}1S$ have been investigated. The x-ray measurement with Ge and Si(Li) detectors was performed at low target gas pressures of pure ${\mathrm{B}}_{2}{\mathrm{H}}_{6}$ and admixtures with He. The $2S$ population of $\ensuremath{\mu}\mathrm{B}$ is found to be ${\ensuremath{\epsilon}}_{2S}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3.1(2)%$. 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 ${\mathrm{B}}_{2}{\mathrm{H}}_{6}$ in He at partial diborane pressures of 67 Pa and below. The measured fraction of metastable $2S$ states per $\ensuremath{\mu}\mathrm{B}$ atom is 0.58(16)% in an admixture of 1% ${\mathrm{B}}_{2}{\mathrm{H}}_{6}$ in He at 6.7 kPa and is interpreted in terms of pressure dependent quenching.