Search for Jacobi shape transition in A $\sim30$ nuclei

TL;DR

This study reports the first experimental evidence of the Jacobi shape transition in $^{31}$P using high-energy gamma-ray spectroscopy, revealing the influence of nuclear orbiting phenomena in alpha-cluster nuclei.

Contribution

It provides the first observation of Jacobi shape transition in $^{31}$P and compares it with $^{28}$Si, highlighting the role of nuclear orbiting in hindering the transition.

Findings

GDR spectrum shows a low energy component indicating Jacobi transition in $^{31}$P.

$^{28}$Si exhibits a different GDR line shape despite high angular momentum.

Nuclear orbiting phenomena may hinder the Jacobi shape transition in alpha-cluster nuclei.

Abstract

This paper reports the first observation of the Jacobi shape transition in $^{31}$P using high energy $\gamma$-rays from the decay of giant dipole resonance (GDR) as a probe. The measured GDR spectrum in the decay of $^{31}$P shows a distinct low energy component around 10 MeV, which is a clear signature of Corioli's splitting in a highly deformed rotating nucleus. Interestingly, a self-conjugate $\alpha$-cluster nucleus $^{28}$Si, populated at similar initial excitation energy and angular momentum, exhibits a vastly different GDR line shape. Even though the angular momentum of the compound nucleus $^{28}$Si is higher than the critical angular momentum required for the Jacobi shape transition, the GDR lineshape is akin to a prolate deformed nucleus. Considering the present results for $^{28}$Si and similar observation recently reported in $^{32}$S, it is proposed that the nuclear orbiting phenomenon exhibited by $\alpha$-cluster nuclei hinders the Jacobi shape transition. The present experimental results suggest a possibility to investigate the nuclear orbiting phenomenon using high energy $\gamma$-rays as a probe.