Discovery of Highly Excited Long-Lived Isomers in Neutron-Rich Hafnium and Tantalum Isotopes through Direct Mass Measurements

A study of cooled ${}^{197}\mathrm{Au}$ projectile-fragmentation products has been performed with a storage ring. This has enabled metastable nuclear excitations with energies up to 3 MeV, and half-lives extending to minutes or longer, to be identified in the neutron-rich nuclides ${}^{183,184,186}\mathrm{Hf}$ and ${}^{186,187}\mathrm{Ta}$. The results support the prediction of a strongly favored isomer region near neutron number 116.

Figure 1

Examples of 30th harmonic Schottky spectra for $A=184$ (left) and 187 (right) isobars and isomers, each representing 10 s of data, with different distributions of ions from different injections in the ESR. The ${}^{184}\mathrm{Hf}$ and ${}^{187}\mathrm{Ta}$ isomers (m1 and m2) and ground states ($g$) are identified as individual bare ions. The ground states of ${}^{184}\mathrm{Ta}^{72+}$, ${}^{187}\mathrm{W}^{73+}$, and ${}^{187}\mathrm{Re}^{73+}$ are also indicated. The statistical significance of single-ion identifications is $5\sigma$ for 10 s of observation time (as illustrated) and greater than $5\sigma$ for the typical 40 s of observation per injection. The total numbers of observed ions are given in Table .

Figure 2

Time evolution of $A=187$, $q=73+$ Schottky spectra for a single injection in the ESR with 4.4 s binning. During the early times there is cooling. The left-hand vertical trace (m) is for a single bare ion of the first isomer of ${}^{187}\mathrm{Ta}$. After 48 s it $\gamma$ decays, represented by a sudden increase in frequency, to the ${}^{187}\mathrm{Ta}$ ground state (g). The two uninterrupted vertical traces are for the ${}^{187}\mathrm{W}$ and ${}^{187}\mathrm{Re}$ ground states (cf. Fig. 1).