Study of the Jacobi shape transition in A ≈ 30 nuclei

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