Cross-shell excitations in ${}^{46}\mathrm{Ca}$ studied with fusion reactions induced by a reaccelerated rare isotope beam

Discovering unexplored high-spin states in neutron-rich nuclei can open up a new direction to study band structure and the associated shell structure in isospin-asymmetric many-body systems. However, experimental reach has so far been limited to neutron-deficient or stable nuclei which are preferentially produced in fusion reactions used in such studies. Here, we report the first $\gamma$-ray spectroscopy with fusion reactions using a reaccelerated rare-isotope beam of ${}^{45}\mathrm{K}$ performed at the ReA3 facility of the National Superconducting Cyclotron Laboratory. Using particle and $\gamma$-ray coincidence techniques, three new higher-lying states around 6 MeV and five new $\gamma$-ray transitions were identified for ${}^{46}\mathrm{Ca}$, suggesting three independent band structures formed from different particle-hole configurations. The rotational-like band built on the $0_2^{+}$ state is established up to the tentatively assigned $6_2^{+}$ state. New results are compared to large-scale shell model calculations, confirming the validity of the effective interaction describing particle-hole excitations across the $Z=20$ and $N=28$ shell gaps in the vicinity of doubly magic ${}^{48}\mathrm{Ca}$.

Figure 1

Particle identification using energy and angle information from the silicon detector. The experimental result (a) is compared with simulations (b) based on pace4 [34] and srim [36] calculations. The different behaviors of proton and $\alpha$ particles are well reproduced, including the maximum deposited energy of protons at around 12–14 MeV. The $\alpha$ gate used in Figs. 2 and 3 is defined for each angular bin by setting a minimum deposited energy to remove proton events.

Figure 2

Laboratory-frame particle-gamma coincidence spectrum. The plot compares the spectra with an open gate against the results when gating on $\alpha$ particles. $\gamma$-ray peaks from the reaction products ${}^{45,46}\mathrm{Ca}$ and ${}^{47,48}\mathrm{Sc}$ are labeled, as well as background from neutron-induced reactions.

Figure 3

Doppler-corrected particle-$\gamma$ coincidence spectrum, with a gate on $\alpha$ particles. A fast transition known from ${}^{46}\mathrm{Ca}$ is observed ($4_2^{+}\to 2_1^{+}$) [30], as well as newly identified peaks at 468 and 2715 keV and a peak structure at 309 keV, as seen in the inset. Laboratory-frame $\gamma$ rays are seen as doublets due to the different geometrical effects of the forward and backward SeGA detectors in the Doppler correction.

Figure 4

Background-subtracted particle-$\gamma -\gamma$ coincidence gated on 399-keV (a) and 1346-keV (b) $\gamma$ rays confirmed new ${}^{46}\mathrm{Ca}$ transitions. New transitions to previously (un)known states are labeled in blue (red). The counts around 900–1100 keV in (a) are remnants of the Compton edges of the $4_1^{+}$ and $2_1^{+}$ decays.

Figure 5

Experimental level scheme of ${}^{46}\mathrm{Ca}$ (a) compared to results of $sd\text{−}pf\text{−}sdg$ shell model calculations (b). Excited states ($\gamma$-ray transitions) newly identified from this work are shown in red (blue). The inset shows excitation energy vs. angular momentum for the yrast $J_1^{+}$ (black) and yrare $J_2^{+}$ (red) bands, where the lines are to guide the eyes.