Lowest Excitations in ${}^{56}\mathrm{T}\mathrm{i}$ and the Predicted $N=34$ Shell Closure

Recent experimental characterization of the subshell closure at $N=32$ in the Ca, Ti, and Cr isotones has stimulated shell-model calculations that indicated the possibility that the $N=34$ isotones of these same elements could exhibit characteristics of a shell closure, namely, a high energy for the first excited $2^{+}$ level. To that end, we have studied the decay of ${}^{56}\mathrm{S}\mathrm{c}$ produced in fragmentation reactions and identified new $\gamma$ rays in the daughter $N=34$ isotone ${}^{56}\mathrm{T}\mathrm{i}$. The first $2^{+}$ level is found at an energy of 1127 keV, well below the expected position that would indicate the presence of an $N=34$ shell closure in ${}^{56}\mathrm{T}\mathrm{i}$.

Figure 1

Part of the $\beta$-delayed $\gamma$-ray spectrum in the range 0–2000 keV for events occurring within the first second after a ${}^{56}\mathrm{S}\mathrm{c}$ implant.

Figure 2

Proposed level scheme for ${}^{56}\mathrm{T}\mathrm{i}$. The absolute intensity of a $\gamma$ ray is shown in brackets following the $\gamma$-ray decay energy. The $Q_{\beta }$ value was determined from mass excess data [20]. The filled circle represents known coincidences.

Figure 3

$E(2_1^{+})$ values versus neutron number for the even-even ${}_{24}\mathrm{C}\mathrm{r}$, ${}_{22}\mathrm{T}\mathrm{i}$, and ${}_{20}\mathrm{C}\mathrm{a}$ isotopes. Experimental values are denoted by dashes. Shell model calculations using the GXPF1 [14] and KB3G [22] interactions are shown as filled circles and crosses, respectively.