Ⅱ-2. Nuclear Physics (Theory)

RIKEN Accel. Prog. Rep. 50 (2017)

Giant dipole resonance in hot rotating nuclei†
D.R. Chakrabarty,∗1 N. Dinh Dang,∗2 and V.M. Datar∗3
Experimental observations on the dependence of thewidth of giant dipole resonance (GDR) on temperature (T ) and angular momentum (J), covering a wide
range of T , J and nuclear systems, can be summarized as follows. The GDR width is rather insensitive to the change of T up to ∼ 1 MeV and increases
rapidly up to a moderate T of ∼ 2.5 MeV. At higher
T , there are claims of observing a saturation of theGDR width. This observation has, however, been questioned in some works which have emphasized the needfor a better characterization of the excitation energy
EX (i.e. T and J) of the source in the data analysis.
The reanalysis of the data after taking into account thepre-equilibrium (PEQ) eﬀects in the mass asymmetricchannel, has shown a monotonic increase of the width
at least up to T ∼ 3.5 MeV.
On the other hand a recent experiment in a mass
symmetric channel, which shows negligible PEQ effects, has suggested the GDR width saturation at T ∼3 MeV. This highlights the need to resolve this important issue in future experiments. At still higher T ,the other observed phenomenon is the quenching ofthe GDR strength. In this regime again, various authors use a saturated GDR width in the data analysis.
The GDR quenching has been related to the competition between the equilibration time for the excitationof GDR and the lifetime of the nucleus or the transition of the hot nucleus to a chaotic regime. In thisregard, an interesting suggestion has been made on therelation of the vanishing of the collective vibration andthe liquid to gas phase transition in nuclear systems.
As for the J-dependence the GDR width, the disentangling of the J-eﬀect from the T -eﬀect is made mostly
in the moderate T -regime. The GDR width weakly
depends on J up to a certain value and increases athigher J. This value of J depends on the nuclear mass
number, being, for example, J ∼ 30h̄ for A ∼ 110. Inheavier nuclei (A ∼ 180) the width is roughly constant
up to the highest measured J ∼ 50h̄.
Among several theoretical approaches to calculating the GDR strength function, the microscopicmacroscopic thermal-shape ﬂuctuation model (TSFM)
has been most widely employed in comparisons withexperimental data. The J-dependence of the width isreasonably well explained by the model. Here the contribution to the width at low J comes from thermal
shape ﬂuctuations. The T dependence of the width is
not very well reproduced by this model, particularly†∗1∗2∗30.15

Yield (arb. units)

0.10

0.050.00(TSFM)(TSFM+ΓCN)

0.10

0.050.00

051015 20
E γ (MeV)

2505101520E γ (MeV)

2530Fig. 1. Experimental and theoretical GDR strength functions in 88 Mo at beam energies of 300 MeV (EX = 123.8MeV) [(a) and (c)] and 600 MeV (EX = 260.7 MeV)[(b) and (d)]. Results of Lublin-Strasbourg Drop model(LSD) based on TSFM (including the contribution ofthe evaporation width ΓCN ) and PDM calculations areshown by continuous lines. Dashed lines are LSD results without ΓCN contribution.

at low T . An improvement in this model, by including
pairing ﬂuctuations, is able to describe the GDR width
at low T . The macroscopic collisional damping model(CDM) predicts a monotonic increase of width with T
but does not address the J-dependence. The experimental data at high T do not agree with the CDM
predictions.
The microscopic models, which go beyond the ﬁnitetemperature random-phase approximation (FT-RPA),
have shown that the GDR spreading width Γ↓ , whicharises because of coupling to 2 particle - 2 hole conﬁgurations, is almost temperature-independent. Amongthe various microscopic models the phonon-damping
model (PDM)1) is the most successful one [Figs. 1 (c)and 1 (d), where Ex in (c) corresponds to the mean
temperature Tev = 2 MeV of nuclei in the evaporation process after the emission of GDR γ-ray, whereas
Tev = 3.1 MeV in (d)]. The results of the PDM calculations including pairing ﬂuctuations agree with theT -dependence of the observed widths at low and moderate T . It predicts the width’s saturation as well as
the quenching of the GDR γ-ray yield at very high T .
Reference

Condensed from the article in Eur. Phys. J. A 52, 143(2016).
Nuclear Physics Division, Bhabha Atomic Research Centre
RIKEN Nishina Center
INO Cell, Tata Institute of Fundamental Research

1) N. Dinh Dang, A. Arima, Phys. Rev. Lett. 80, 4145
(1998); N.D. Dang, A. Arima, Nucl. Phys. A 636, 427(1998).

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