Direct Observation of Long-Lived Isomers in ${}^{212}\mathrm{Bi}$

Long-lived isomers in ${}^{212}\mathrm{Bi}$ have been studied following ${}^{238}\mathrm{U}$ projectile fragmentation at 670 MeV per nucleon. The fragmentation products were injected as highly charged ions into a storage ring, giving access to masses and half-lives. While the excitation energy of the first isomer of ${}^{212}\mathrm{Bi}$ was confirmed, the second isomer was observed at 1478(30) keV, in contrast to the previously accepted value of $>1910\mathrm{keV}$. It was also found to have an extended Lorentz-corrected in-ring half-life $>30\min$, compared to 7.0(3) min for the neutral atom. Both the energy and half-life differences can be understood as being due a substantial, though previously unrecognized, internal decay branch for neutral atoms. Earlier shell-model calculations are now found to give good agreement with the isomer excitation energy. Furthermore, these and new calculations predict the existence of states at slightly higher energy that could facilitate isomer deexcitation studies.

Figure 1

${}^{212}\mathrm{Bi}$ data, illustrating the ground and two isomeric states as a function of time. The revolution frequency is inversely proportional to the mass-to-charge ratio. During the second half of the observation period, the ground-state and first-isomer ions merge (see text).

Figure 2

${}^{212}\mathrm{Bi}$ events, illustrating the ground and two isomeric states, together with unresolved events for ${}^{212}\mathrm{Pb}$ and ${}^{212m}\mathrm{Po}$. Each count corresponds to 20 s of observation time. Note that the revolution frequency has had 59 440 kHz subtracted. The peaks are labeled with spin and parity values and neutral-atom half-lives, and excitation energies from the present work are included for the ${}^{212}\mathrm{Bi}$ isomers.

Figure 3

Partial level scheme for ${}^{212}\mathrm{Bi}$, showing the calculated energies of the yrast states on the left, together with a few non-yrast states ($8^{-}$, ${16}^{-}$, and ${17}^{-}$) that are discussed in the text. On the right are the observed isomers with their energies measured in the present work.