II-2. Nuclear Physics (Theory)

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Exotic nuclear shape due to cluster formation at high angularmomentum†Balaram Dey,∗1 Shan-Shan Wang,∗2 Deepak Pandit,∗3 Srijit Bhattacharya,∗4 Xi-Guang Cao,∗5 Wan-Bing He,∗6Yu-Gang Ma,∗7 N. Quang Hung,∗8 and N. Dinh Dang,∗9
It is now well established that clustering plays a very
important role in self-conjugate light nuclei and is also
associated with strongly deformed shapes of nuclei.1,2)
One of the probes to study this deformation experimentally at high temperature (T ) and angular momentum (J) is the γ-decay from the giant dipole resonance(GDR) built on excited states.1,2) The GDR lineshapegets fragmented in deformed nucleus providing crucial
information about the nuclear deformation. As a matterof fact, it has been successfully employed experimentallyto study the Jacobi shape transition, an abrupt changeof shape from non-collective oblate to collective triaxial
or prolate shape above a critical spin, in several light nuclei such as 31 P, 45 Sc, 46 Ti and 47 V.1,2) However, when
this shape transition is examined in self-conjugate nuclei 32 S1) and 28 Si2) through the reactions 20 Ne + 12 C
and 16 O + 12 C, respectively, the GDR lineshape fragments into two prominent peaks at high J(∼20h̄) providing a direct evidence of the large deformation but,intriguingly, the shapes found are completely diﬀerent
from those seen from Jacobi shape transition (signatureof which is a sharp peak at 10 MeV arises due to theCoriolis splitting of the GDR frequencies). Therefore,
these observations clearly highlight that the clustering
is not only important in mass A < 20 region but could
also play major role in A ∼ 30, which has not beenstudied enough.
Microscopic eﬀects such as shell structure, pairingand isospin eﬀects play important role in deciding thenuclear structures at low excitation energy. However,even after incorporating these eﬀects, the experimental GDR lineshapes (T ∼ 2.0 MeV and J ∼ 20h̄)
could not be explained for 28 Si and 32 S nuclei.3) In thepresent work, an extended quantum molecular dynamics (EQMD) model4) has been carried out to understandthe GDR lineshapes of 28 Si and 32 S nuclei, and to inves†
∗1∗2
∗3
∗4
∗5
∗6
∗7

∗8
∗9Condensed from the article in Phys. Rev. C 102, 031301(R)(2020)
Bankura University
Shanghai Institute of Applied Physics, Chinese Academy ofSciences
Variable Energy Cyclotron Centre
Dept. of Phys., Barasat Govt. CollegeShanghai Advanced Research Institute and Zhangjiang Laboratory
Key Laboratory of Modern Physics and Ion-Beam application(MOE), Institute of Modern Physics, Fudan University
Shanghai Institute of Applied Physics, Chinese Academy ofSciences, and Key Laboratory of Modern Physics and IonBeam application (MOE), Institute of Modern Physics, Fudan
University
Institute of Fundamental and Applied Sciences, Duy Tan University
RIKEN Nishina CenterFig. 1. EQMD calculations for both 32 S and 28 Si nuclei.

tigate how the exotic shapes due to cluster formation
at high T and J are manifested through GDR strengthfunction.
The results of EQMD calculation have been comparedwith the existing experimental data of 32 S and 28 Si as
shown in Fig. 1. It should be mentioned that the alphacorrelation is included in the EQMD model based on
the fact that the many body nucleon-nucleon correlation
is intrinsically embedded in all microscpic QMD typemodel and the details of EQMD results are discussed inRefs. 3, 4). It is found that the EQMD predicts the general trend of the experimental GDR strength functionsfor 32 S and 28 Si by considering the ring or toroidal conﬁguration. Nevertheless, the peak around 25 MeV canonly arrive due to cluster formation highlighting the existence of the α-clustering structure at such high T and J
since this peak could not be predicted within the meanﬁeld calculations. Interestingly, the rotation of these
exotic shapes will not lead to the Coriolis splitting of
the GDR strength function (due to larger moment of inertia leading to smaller angular frequency), which could
be the reason for the absence of the Jacobi shape transition in 32 S and 28 Si. The present result highlights therole of α cluster states above the decay threshold, whichis still an open ﬁeld of investigation.
References

1) D. Pandit et al., Phys. Rev. C 95, 034301 (2017).
2) B. Dey et al., Phys. Rev. C 97, 014317 (2018).
3) B. Dey et al., Phys. Rev. C 102, 031301R (2020).
4) W. B. He et al., Phys. Rev. Lett. 113, 032506 (2014).

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