This paper reports the first observation of the Jacobi shape transition in ${}^{31}\mathrm{P}$ using high energy $\gamma$ rays from the decay of giant dipole resonance (GDR) as a probe. The measured GDR spectrum in the decay of ${}^{31}\mathrm{P}$ shows a distinct low energy component around 10 MeV, which is a clear signature of Coriolis splitting in a highly deformed rotating nucleus. Interestingly, a self-conjugate $\alpha$-cluster nucleus ${}^{28}\mathrm{Si}$, populated at similar initial excitation energy and angular momentum, exhibits a vastly different GDR lineshape. Even though the angular momentum of the compound nucleus ${}^{28}\mathrm{Si}$ 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 ${}^{28}\mathrm{Si}$ and similar observation recently reported in ${}^{32}\mathrm{S}$, 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.

• Received 11 October 2017

DOI:https://doi.org/10.1103/PhysRevC.97.014317