Core excited states in the $A=51$ mirror nuclei

Three previously unknown high-energy $\gamma$-ray transitions between 4.2 and 5.4 MeV were identified in the $T_z=-1∕2$ nucleus ${}^{51}\mathrm{Fe}$ following the fusion-evaporation reaction ${}^{32}\mathrm{S}{({}^{28}\mathrm{Si},2\alpha 1n)}^{51}\mathrm{Fe}$. These transitions represent decays of core excited states. The $\gamma$ rays were detected in the Ge detector array Gammasphere combined with the neutron detector system Neutron Shell and the charged-particle array Microball. The three transitions are related to the mirror transitions in the $T_z=+1∕2$ nucleus ${}^{51}\mathrm{Mn}$, and the resulting mirror-energy difference diagram is discussed with predictions from large-scale shell-model calculations.

Figure 1

$\gamma$-ray spectra obtained from the fusion-evaporation reaction ${}^{32}\mathrm{S}{+}^{28}\mathrm{Si}$. Panel (a) shows a $2\alpha 1p$-gated spectrum in coincidence with the $27∕2^{-}\to 23∕2^{-}704\mathrm{keV}$ transition in ${}^{51}\mathrm{Mn}$. Panel (b) is the sum of $2\alpha 1n$-gated spectra in coincidence with clean and intense transitions in ${}^{51}\mathrm{Fe}$, namely at 253, 314, 370, 508, 636, 777, 884, 893, 1263, 1437, 1510, and 1807 keV. In addition, the sum energy of the two evaporated $\alpha$ particles was restricted to a maximum of 27 MeV. The insets of panels (a) and (b) highlight the high-energy portion of the two spectra. Single-escape peaks (SE) of transitions are also indicated. Panels (c) and (d) show spectra in coincidence with the new 4199 and 5381 keV transitions in ${}^{51}\mathrm{Fe}$, respectively, with the same overall restrictions applied as in panel (b).

Figure 2

(a) Level scheme of ${}^{51}\mathrm{Fe}$ [3, 4] and (b) the relevant part of the level scheme of ${}^{51}\mathrm{Mn}$ [11]. Energy labels are in keV, tentative transitions and levels are dashed lines, and the widths of the arrows correspond to the relative intensities of the transitions. (c) MED diagram of yrast states in $A=51$ mirror nuclei (see text). Open squares indicate experimental data, filled circles indicate calculated MED values according to Ref. 9, and filled diamonds indicate MED values excluding the standard $V_{\mathit{Cm}}$ term and taking instead into account contributions from different single-particle energies for protons and neutrons.