Study of the Jacobi shape transition in A≈30 nuclei

Abstract

This paper reports the first observation of the Jacobi shape transition in 31 P using high energy γ-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 α-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 α-cluster nuclei hinders the Jacobi shape transition. The present experimental results suggest a possibility to investigate the nuclear orbiting phenomenon using high energy γ-rays as a probe.Many body quantum systems like atomic nuclei provide a unique opportunity to explore a variety of phenomena arising due to interplay of different physical processes, particularly at high excitation energy (E * ) and angular momentum (J). One such interesting phenomenon is the Jacobi shape transition, where beyond a critical angular momentum (J C ), an abrupt shape change from non-collective oblate shape to collective triaxial or prolate shape takes place [1]. The study of exotic Jacobi shapes in nuclei has been a topic of considerable interest [2, 3]. The Jacobi shape transition is expected to occur in light and medium mass nuclei, where high rotational frequencies are achieved before the excited nucleus can undergo fission. Further, it is expected that the Jacobi shape transition should be a common feature over a wide range of nuclei. Experimentally, the Jacobi shape transition has been observed in a few light mass nuclei A ∼ 45 [4-7] via the γ-decay of giant dipole resonance (GDR). It is known that the GDR is the cleanest, and hence most extensively used, probe to study the properties of nuclei at high temperature (T ) and J [8]. The GDR can be understood macroscopically as an out-of-phase oscillation between protons and neutrons, and microscopically in terms of coherent particle-hole excitations. The GDR γ-emission occurs at the early stage of compound nucleus (CN) decay and can probe the nuclear shape. The GDR components corresponding to vibration along and perpendicular to the axis of rotation are differently affected by the Corioli's force. As a result the GDR strength function splits into multiple components with a narrow well separated peak around 8-10 MeV [4], which is an unambiguous signature of the Jacobi shape transition. It should be mentioned that the search for Jacobi shapes has also been made through studies of quasi-continuum gamma radiation [9]. However, indications of highly deformed shapes could not be uniquely ascribed to the Jacobi ...

AGATA: Nuclear structure advancements with high-energy $$\gamma $$ rays

“…Such a phenomenon is evidenced in the Giant Dipole Resonance strength function as large splitting of the Lorentzian shape, with a low energy component whose centroid energy is close or below the particle threshold. Such phenomenon was already observed in a number of light nuclei (around A = 45) [113][114][115][116], but it is expected to be even more pronounced in medium mass nuclei. It was also observed that this low energy component is correlated with the cold structures of high deformations [117].…”