Structure of 42-silicon: challenge for nuclear theory at the limits

Predictive modeling throughout the nuclear landscape, including the most exotic nuclei near the limits of nuclear existence, is an overarching driver of nuclear science. This quest thrives through the interplay of experiment and theory, whereby observables measured in nuclei with large neutron-to-proton imbalance reveal isospin-dependent aspects of nuclear models. They also identify benchmark nuclei, critical for understanding and for quantitative extrapolations toward the shortest-lived rare isotopes – many outside the reach of present-day experiments but whose properties underpin the modeling of nucleosynthesis processes, for example. Over the few decades of rare-isotope research, certain nuclei defying textbook expectations have emerged as pivotal – they are typically located in regions of rapid structural changes or at the extremes of weak binding where universal phenomena characteristic of open quantum systems are present. The Z = 14 isotope ⁴²Si₂₈ is one of such nuclei.

The two leading shell-model effective nuclear interactions, both originally optimized to reproduce the low first-excited state energy, but whose predictions for other observables differ significantly, are interrogated by the population of states in neutron-rich ⁴²Si with a one-proton removal reaction from ⁴³P projectiles, using the GRETINA gamma-ray detection system at NSCL. The differences in the two theoretical descriptions are examined and linked to predicted low-lying excited 0⁺ states and shape coexistence phenomena. The new data underscore the difficulty in extrapolating shell-model calculations towards the neutron dripline based on very limited data. The results are published in Phys. Rev. Lett. 122, 222501 (2019)

Figure 1: A network of E2 transitions connecting various states of ⁴²Si that is used to examine the differences in the shell model implementations used. The line thicknesses represent the transition strength.