Magnetic Moment of the Fragmentation-Aligned ${}^{61}\mathrm{F}\mathrm{e}(9/2^{+})$ Isomer

We report on the $g$ factor measurement of an isomer in the neutron-rich ${}_{26}{}^{61}\mathrm{F}\mathrm{e}$ ($E^{*}=861\mathrm{k}\mathrm{e}\mathrm{V}$ and $T_{1/2}=239(5)\mathrm{n}\mathrm{s}$). The isomer was produced and spin aligned via a projectile-fragmentation reaction at intermediate energy, the time dependent perturbed angular distribution method being used for the measurement of the $g$ factor. For the first time, due to significant improvements of the experimental technique, an appreciable residual alignment of the nuclear spin ensemble has been observed, allowing a precise determination of its $g$ factor, including the sign: $g=-0.229(2)$. In this way we open the possibility to study moments of very neutron-rich short-lived isomers, not accessible via other production and spin-orientation methods.

Figure 1

Schematic drawing of the TDPAD experimental setup. The beam passes through a $50\mu \mathrm{m}$ plastic scintillator before being stopped in a $500\mu \mathrm{m}$ Cu foil.

Figure 2

Top: the decay scheme of the 370(9) ns isomer in ${}^{54}\mathrm{F}\mathrm{e}$ and the $R(t)$ function of the summed $\gamma$ transitions. Bottom: comparison between the measured (with package suppressor) and the calculated alignment. The theoretical curve is divided by a factor of 1.8.

Figure 3

Top and right bottom: the decay scheme of the 239(5) ns isomer in ${}^{61}\mathrm{F}\mathrm{e}$ and the $R(t)$ functions for (a) ${}^{61}\mathrm{F}\mathrm{e}$ (center, 654 keV); (b) ${}^{61}\mathrm{F}\mathrm{e}$ (wing, 654 keV); (c) ${}^{61}\mathrm{F}\mathrm{e}$ (wing, 207 keV). The $R(t)$ function for 207 keV is consistent with that for the 654 keV but with opposite phase, as expected from the assumed multipolarities of the corresponding $\gamma$ transitions. Left bottom: comparison between the measured (with and without package suppressor) and the calculated alignment. The theoretical curve is divided by a factor of 1.8.

Figure 4

$g$ factor systematics around $N=40$ for known $9/2^{+}$ isomeric states. This measurement is the first one below $Z=28$, and supports a symmetric behavior with respect to the $Z=28$ shell closure. The unusual value for the $g$ factor of ${}^{67}\mathrm{N}\mathrm{i}$ could be explained by 2% proton excitations across $Z=28$ [2], but a remeasurement is highly recommended.