NEUTRON EMISSION FROM NUCLEAR EXCITATION BY RADIATIONLESS TRANSITIONS IN $mu$-ATOMIC $sup 209$Bi.

Neutron emission by radiationless $\ensuremath{\mu}$-atomic transitions in ${\mathrm{Bi}}^{209}$ has been observed. The analysis of the neutron time-of-flight spectrum shows that this process takes place in muon transitions from the $n=3$ or higher states to the ground state of the muonic atom while the nucleus is left either in the ground state or in an excited state, possibly one belonging to a particle-hole multiplet of ${\mathrm{Bi}}^{208}$. The process investigated here occurs in 7 \ifmmode\pm\else\textpm\fi{} 2% of all muon stops in bismuth.