First high-resolution γ -ray spectroscopy of Si41

The first high-resolution in-beam γ-ray spectroscopy is reported for the neutron-rich nucleus ⁴¹Si, a tenant of the $N=28$ island of inversion. Excited states were populated in the direct one-proton removal reaction from ⁴²P projectiles and $pn$ removal from ⁴³P. Seven γ-ray transitions were observed, only one of which had been reported previously in the literature. This makes ⁴¹Si the most neutron-rich odd-even $N=27$ isotone with high-resolution excited-state information. For the one-proton removal, the measured partial cross-section distribution to all observed bound final states is contrasted with results from direct one-proton removal calculations that combine eikonal reaction dynamics with SDPF-MU shell-model spectroscopic factors and assume various possible initial states for the poorly known ⁴²P projectile. Rather distinct calculated cross-section distributions emerge that, in comparison to the new data, imply that the initial state in ⁴²P is most likely 3^- or 2^- rather than 1^- or 0^-, the predicted shell-model ground state of ⁴²P. It is further shown that the level scheme from the novel VS-IMSRG calculation closely agrees with the one of SDPF-MU, the most successful phenomenological shell-model effective interaction in describing the much discussed neighboring isotope ⁴²Si, perhaps cross-validating these complementary approaches on the quest to model rapid shell evolution away from the valley of β stability.