Ⅱ-2. Nuclear Physics (Theory)

RIKEN Accel. Prog. Rep. 52 (2019)

Study of giant dipole resonance in hot rotating light mass nucleus 31 P†
D. Mondal,∗1 D. Pandit,∗1 S. Mukhopadhyay,∗1,2 S. Pal,∗1 S. Bhattacharya,∗3 A. De,∗4 N. D. Dang,∗5N. Q. Hung,∗6 S. Bhattacharya,∗1,∗2 S. Bhattacharyya,∗1,∗2 B. Dey,∗7 P. Roy,∗1 K. Banerjee,∗1,∗2,∗8 and
S. R. Banerjee∗9The Isovector Giant Dipole Resonance (GDR) is observed in all nuclei and is characterized by the resonanceenergy (EG ), width (ΓG ) and strength (SG ).1) Macroscopically, it is described as out-of-phase oscillation of
proton and neutron ﬂuids, while microscopically, it isa coherent excitations of one particle-one hole (1p-1h)
conﬁgurations across one major shell. It can be built
on the ground state as well as every excited states ofatomic nucleus. The GDR built on the nuclear excitedstates is experimentally studied by two complementary
methods, namely inelastic scattering reactions and fusion evaporation reactions.
In this report, a systematic study of the GDR parameters is presented in a very light mass nucleus 31 P
by using fusion evaporation reactions. The compoundnucleus (CN) 31 P was populated at three diﬀerent excitation energies by using the α beam (Ebeam = 28, 35,42 MeV) from K-130 cyclotron at the Variable Energy
Cyclotron Centre, Kolkata. The GDR and nuclear level
density (NLD) parameters have been determined by simultaneous statistical model analysis of the high-energy
γ ray and neutron spectra measured with the LAMBDAarray2) and neutron time of ﬂight detectors,3) respectively. The angular momentum (J) of the multiplicities
with a multiplicity ﬁlter.4) The angular distribution of
the high-energy γ rays has been performed to determine the bremsstrahlung component which is observed
for the beam energies above ∼10 MeV/nucleon. The estimation of the bremsstrahlung contribution is crucial
for precise determination of the GDR parameters.
It was observed that the GDR remains very much collective in this light mass nucleus and the EG remains
roughly constant at around 17.5 MeV as the nuclear
temperature (T ) changes. In Fig. 1, the measured GDR
widths are compared with the results of calculations
within diﬀerent models and plotted as a function of T .
Panels (a) and (b) show that the thermal shape ﬂuctuation model (TSFM)5) and the phenomenological thermal shape ﬂuctuation model (pTSFM)6) (phenomenological version of TSFM) overpredict the measured
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∗9Condensed from the article in Phys. Lett. B 784, 423 (2018)
Variable Energy Cyclotron Centre
Homi Bhabha National InstituteDepartment of Physics, Barasat Govt. College
Department of Physics, Raniganj Girls’ CollegeRIKEN Nishina Center
Institute of Fundamental and Applied Sciences, Duy TanUniversity
Saha Institute of Nuclear Physics
Department of Nuclear Physics, Australian National University(Ex) Variable Energy Cyclotron Centre- 78 -Fig. 1. Comparison of measured GDR width with the results of various model calculations as a function of T at
J = 11.5 h̄ (black dot-dashed line) and J = 15.5 h̄ (bluesolid line). The red long-dashed lines in panels (c) and(d) are the predictions at J = 0 h̄.

widths. In panel (c) the calculations within the critical temperature included ﬂuctuation model (CTFM)7)
is presented. Within this model, the GDR width remains constant at the ground-state value up to a criticaltemperature due to the GDR induced ﬂuctuation andincreases thereafter. Interestingly, this model nicely reproduced the measured widths. The widths were also
described quite well [panel (d)] by the results of calculations based on microscopic phonon damping model(PDM),8) according to which the GDR width arises owing to the coupling of the GDR state with the noncollective p-h, p-p and h-h conﬁgurations. The thermalparing gaps are found to vanish in the ranges of T andJ considered in this experiment, therefore, have no effect on the measured GDR width in this light masssystem. The present results establish the universalityof CTFM and PDM in describing the GDR width as afunction of temperature and angular momentum.
References

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