GDR dissipation and nuclear shape in hot fast-rotating Dy nuclei

doi:10.1016/0370-2693(93)91277-T

Physics Letters B

Volume 308, Issues 3–4, 1 July 1993, Pages 231-236

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Abstract

The statistical γ-ray decay of the GDR built on excited states in Dy nuclei has been investigated for selected domains of angular momentum up to about 70ħ and temperatures in the range 1–2 MeV. The GDR strength distribution extracted from the data indicate large average nuclear deformations (β ∼ 0.35) at high angular momentum and average temperatures T ⩾ 1.5 MeV. The experimental observation is supported by results from calculations in which thermal shape fluctuations are taken into account around an oblate equilibrium deformation β_eq. Although this equilibrium deformation increases with angular momentum, the calculations show rather large and constant average deformations 〈β ∼0.35.

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Study of giant dipole resonance in hot rotating light mass nucleus 31P
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An exclusive systematic study of the giant dipole resonance (GDR) parameters has been performed in very light mass nucleus ${}^{31}P$ in the temperature range of $\sim 0.8 – 2.1 MeV$ and average angular momentum of $\sim 11 – 16 ħ$ . The high-energy γ rays from the decay of the GDR, evaporated neutrons and γ-ray multiplicities have been measured. The angular distribution of high-energy γ rays has also been measured at $E_{beam} = 42 MeV$ . The GDR parameters, nuclear level density parameter and nuclear temperature were precisely determined by simultaneous statistical model analysis of high-energy γ ray and evaporated neutron spectra. It is observed that the measured width remains roughly constant up to a temperature of $\sim 1.6 MeV$ . Moreover, the thermal pairing plays no role in describing the GDR width in this open-shell light nucleus at the above-mentioned temperatures and angular momenta. The present measurements provide an excellent platform to extend the applicability of the existing theoretical models down to the very light mass nuclei.
Viscosity, fission time scale and deformation of 156Dy
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In the fusion-fission reaction $^{40} Ar +^{116} Cd →^{156} Dy^{∗} → fission$ , at E_b = 216 MeV and 238 MeV, γ-rays were measured in coincidence with fission fragments. The interpretation of the γ-ray spectra is done with the help of a modified version of the statistical-model code CASCADE. The spectra can be reproduced with nuclear viscosities in the range 0.01 < γ < 4. The extracted fission time scale is of the order of 10⁻¹⁹ s. The CASCADE analysis seems to favor a prolate deformation with β ≈ 0.45.
Thermal fluctuation and collisional damping effects in the hot giant dipole resonance
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The damping mechanisms of the giant dipole resonance at finite temperature T and rotational frequency ω are investigated by studying the GDR lineshape of ¹⁶⁷Er associated with a very narrow interval of excitation energy (of the order of 6 MeV) at T = 1.3 MeV. The strength function data can be reproduced either assuming collisional damping width equal to T = 0 and adiabatic thermal fluctuations or assuming a larger collisional damping without thermal fluctuations. The measurement of the angular distribution of the high energy gamma rays rules out the second possibility.
Viscosity and fission time scale of 156Dy at high excitation energies and large angular momenta
1996, Nuclear Physics A
The reaction $^{40} Ar +^{116} Cd →^{156} Dy^{∗} → fission$ was studied at beam energies E_b = 216 MeV and E_b = 238 MeV, wherein γ-rays were measured in coincidence with fission fragments. From these spectra the nuclear viscosity γ and fission time scale were deduced by comparing to statistical model calculations yielding viscosities in the range 0.01 < γ < 4 and fission time scales of the order of 10⁻¹⁹ s.
GDR γ-ray decay in 156Dy* from regions selected on temperature and angular momentum
1995, Physics Letters B
The strength distribution of the GDR built on highly excited states in a restricted temperature domain in ¹⁵⁶Dy and ¹⁵⁵Dy nuclei has been deduced by subtraction of γ-ray spectra obtained for the decay of ¹⁵⁴Dy^∗ and ¹⁵⁶Dy^∗ from regions selected on angular momentum. The resulting difference spectra have been analyzed within the statistical model. The results show a large deformation (|β| ∼ 0.51±0.29 and 0.35±0.14) for the angular-momentum regions with $〈J〉 ∼ 32 h ̵$ at T ≈ 1.8±0.2 MeV and $〈J〉 ∼ 46 h ̵$ at T ≈ 1.7±0.2 MeV, respectively, in satisfactory agreement with calculations performed in the framework of Landau theory of shape transitions and statistical fluctuations. The deduced centroid energies are in agreement with the systematics of the GDR built on the ground state. The width of the GDR shows a systematic increase with increasing temperature.
Isomer-selected decay of the GDR in 156Dy*
1995, Nuclear Physics, Section A
The strength distribution of high-energy γ-rays emitted in the statistical decay of the ¹⁵⁶Dy^∗ compound nucleus, with $E^{∗} = 92.5 MeV$ , has been measured inclusively and for selected decay pathways in which isomers in ¹⁴⁹Dy, ¹⁵¹Dy and ¹⁵²Dy were populated. For both spectra a deformation |β| = 0.3 is deduced and the centroid energies agree with those found from ground-state systematics. The high-energy γ-ray spectrum triggered on isomeric decay shows an increase of the width of the GDR in comparison with the inclusive spectrum.

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Physics Letters B

Abstract

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Cited by (11)

Study of giant dipole resonance in hot rotating light mass nucleus <sup>31</sup>P

Viscosity, fission time scale and deformation of <sup>156</sup>Dy

Thermal fluctuation and collisional damping effects in the hot giant dipole resonance

Viscosity and fission time scale of <sup>156</sup>Dy at high excitation energies and large angular momenta

GDR γ-ray decay in <sup>156</sup>Dy<sup>*</sup> from regions selected on temperature and angular momentum

Isomer-selected decay of the GDR in <sup>156</sup>Dy<sup>*</sup>