Giant Dipole Resonance Strength Function Research Articles

Phenomenological Estimates of the Profiles of Giant Dipole Resonances Built on Ground- and Low-lying Exited States

Thermal Shape Fluctuation (TSF) Model describes the properties of the Giant Dipole Resonance (GDR) radiation emitted by hot rotating nu-clei. This approach turned out to be a very efficient method of extracting the information about the shapes of nuclei and their evolution with increasing angular momentum. The GDR can be built also on the ground-state band but this process needs advanced microscopic methods to be described adequately. We propose to investigate the GDR strength function with the developed for this purpose TSF model. The potential energy surface, which allows to calculate the shape probability using the so-called Boltz-mann factor, is obtained within macroscopic-microscopic method, where the Folded-Yukawa model is used for macroscopic part and Folded-Yukawa plus exponential mean-field potential to generate the single-particle energies. The Strutinsky shell-, and the particle number projected BCS pairing-correction energies give the microscopic contribution to the energy of a nucleus at each given deformation. The method has been tested for Se-lenium and Neodymium isotopes for which the photo-absorption spectra were measured and preliminary results will be presented.

Dipole Response Function of Nuclear Matter with Pairing Correlations

The dipole response function of nuclear matter at finite temperature is investigated by employing a linearized Landau–Vlasov equation with pairing correlations. We calculate the giant dipole resonance strength function with and without quasiparticle collisional effects. Calculations are carried out for nuclear dipole vibrations of finite nuclei by employing the Steinwedel–Jensen model, and their results are compared with experimental results for 120 Sn.

Highly selective studies of GDR inEr

Exclusive measurements of high energy γ-rays in the decay of164Er, formed in the reaction40Ar+124Sn, have been performed to study angular momentum dependence of the giant dipole resonance (GDR) strength function. The γ-ray spectra were measured in coincidence with A=160 evaporation residues (4n channel at Ebeam = 163 MeV and 187 MeV. A statistical model analysis, incorporating a two-component GDR strength function, shows increase in the GDR width as the average spin of the compound nucleus increases from 26ħ, (at 163 MeV to 54ħ (at 187 MeV). This is consistent with the predictions of a thermal shape fluctuation model.

Highly selective studies of GDR inEr

Exclusive measurements of high energy γ-rays in the decay of 164 Er, formed in the reaction 40 Ar+ 124 Sn, have been performed to study angular momentum dependence of the giant dipole resonance (GDR) strength function. The γ-ray spectra were measured in coincidence with A=160 evaporation residues (4n channel at E beam = 163 MeV and 187 MeV. A statistical model analysis, incorporating a two-component GDR strength function, shows increase in the GDR width as the average spin of the compound nucleus increases from 26ħ, (at 163 MeV to 54ħ (at 187 MeV). This is consistent with the predictions of a thermal shape fluctuation model.

Highly selective studies of GDR in 164Er

Exclusive measurements of high energy γ-rays in the decay of 164Er, formed in the reaction 40Ar+ 124Sn, have been performed to study angular momentum dependence of the giant dipole resonance (GDR) strength function. The γ-ray spectra were measured in coincidence with A=160 evaporation residues (4n channel at E beam = 163 MeV and 187 MeV. A statistical model analysis, incorporating a two-component GDR strength function, shows increase in the GDR width as the average spin of the compound nucleus increases from 26ħ, (at 163 MeV to 54ħ (at 187 MeV). This is consistent with the predictions of a thermal shape fluctuation model.

Gamma decay of the giant dipole resonance and feeding of superdeformed states in 143Eu

The γ decay of the giant dipole resonance (GDR) built on excited nuclear states has been measured in coincidence with the low-energy γ discrete transitions for the nucleus 143Eu. The reaction used was 110Pd(37Cl, 4n)143Eu at a beam energy of 165 MeV. The EUROBALL spectrometer (for the measurement of discrete γ transitions) coupled with the HECTOR array (for high-energy γ-ray detection) has been used. The high-energy γ-ray spectrum in coincidence with superdeformed (SD) discrete transitions of 143Eu shows an “excess” between 9–12 MeV if compared with the one associated to cascades which do not pass through the SD configurations. Such an “excess” is in the energy region where one expects the low-energy component of the GDR strength function built on a SD state. The measured intensity can be reproduced by the statistical model assuming that the superdeformation survives only few MeV above the yrast line. A similar and consistent scenario has also been obtained by comparing the high-energy γ-ray spectra of 143Eu in coincidence with its spherical (which is fed by the SD configuration) and its triaxial configuration (which is bypassed by the decay of the SD states).

The Giant Dipole Resonance in the superdeformed143Eu nucleus

The γ-decay of the Giant Dipole Resonance (GDR) built on excited nuclear states has been measured for the nucleus143Eu. The reaction110Pd(37Cl, 4n)143Eu at a beam energy of 165 MeV has been employed. This experiment aimed at searching the γ-decay of the GDR built on the superdeformed143Eu states, populated at high spins. High-energy γ-rays were detected in 8 large BaF2 scintillators in coincidence with discrete transitions measured with the NORDBALL array (in the configuration consisting of 17 HPGe detectors and a 2π multiplicity filter). The spectrum of high-energy γ-rays gated by low-energy transitions between states fed by the superdeformed states shows some excess of yield in the 7–10 MeV region with respect to that gated by transitions between states not populated by the superdeformed states. This excess should be due to the γ-decay of the the giant dipole oscillation along the superdeformed axis of the nucleus that is expected to have a frequency corresponding to ≈9–10 MeV (low-energy component of the GDR strength function). The measured excess, in spite of the large error bars, is found to be of the same order as predicted statistical model values.

Temperature Dependence of the Width of the Giant Dipole Resonance in 120Sn and 208Pb.

The giant-dipole resonance (GDR) in ${}^{120}\mathrm{Sn}$ and ${}^{208}\mathrm{Pb}$ is studied as a function of excitation energy, angular momentum, and intrinsic width. Theoretical evaluations of the full width at half maximum (FWHM) for the GDR strength function are compared with recent experimental data and are found to be in overall agreement. Differences observed between ${}^{120}\mathrm{Sn}$ and ${}^{208}\mathrm{Pb}$ are attributed to strong shell corrections in ${}^{208}\mathrm{Pb}$ favoring spherical shapes at low temperatures. At high temperature, the FWHM in ${}^{120}\mathrm{Sn}$ exhibits effects due to the evaporation width of the compound nucleus.

The temperature dependence of the width of the giant-dipole resonance

The giant-dipole resonance (GDR) in 120Sn and 208Pb is studied as a function of excitation energy, angular momentum, and intrinsic width within the context of the adiabatic model. Theoretical evaluations of the full-width-at-half-maximum (FWHM) for the GDR strength function are compared with recent experimental data and are found to be in good agreement.

Spin-induced shape changes in light-medium mass compound nuclei.

At very high spins an oblate-to-triaxial transition in the equilibrium shape of hot medium mass nuclei is expected, with superdeformed major to minor axis ratios of 2:1 and larger. To search for this shape transition, we measured \ensuremath{\gamma} ray production cross sections and angular distributions for the decay of $^{59,63}\mathrm{Cu}$ compound nuclei populated in the fusion of $^{32}\mathrm{S}$${+}^{27}$Al and $^{18}\mathrm{O}$${+}^{45}$Sc over a wide range of spin (J=0-47\ensuremath{\Elzxh}) and excitation energy (${\mathit{E}}^{\mathrm{*}}$=55\char21{}130 MeV). Very broad giant dipole resonance (GDR) strength functions are deduced at high bombarding energy (spin), implying the existence of large deformation in the ensemble of decaying states. Thermal shape and orientation fluctuation calculations based on the rotating liquid drop model provide a good description of the GDR strength functions for all cases. The calculations fail to reproduce the GDR strength functions at high bombarding energy (spin) cases when the oblate-to-triaxial shape transition and the associated softness in the PES are removed, indicating the observed broadening of the GDR strength function is due mostly to spin-driven deformation. Threshold bremsstrahlung production is inferred from measured angular distribution anisotropies in $^{18}\mathrm{O}$${+}^{45}$Sc collisions at 125 and 149 MeV.

Comparison of giant dipole resonance decay in stiff 92Mo and soft 100Mo excited nuclei.

Gamma-ray spectra from the decay of the giant dipole resonance (GDR) built on excited states of $^{92}\mathrm{Mo}$ and $^{100}\mathrm{Mo}$ nuclei at average spins (9--24)\ensuremath{\Elzxh} and temperatures of 1.35--1.50 MeV were measured, and the parameters of a two-component GDR strength function were extracted. Deformations determined from the fitted component energies are substantially larger than predicted by the rotating liquid drop model, suggesting large thermal shape fluctuations. Widths of the GDR in $^{92}\mathrm{Mo}$ increase from 7.6\ifmmode\pm\else\textpm\fi{}0.1 to 8.6\ifmmode\pm\else\textpm\fi{}0.2 MeV in the spin range of (9--19.5)\ensuremath{\Elzxh} and temperature range of 1.35--1.45 MeV, as compared with the ground-state GDR width of 5.4\ifmmode\pm\else\textpm\fi{}0.2 MeV. In $^{100}\mathrm{Mo}$ the GDR width is nearly constant and equals 9.8\ifmmode\pm\else\textpm\fi{}0.2, 9.9\ifmmode\pm\else\textpm\fi{}0.2, and 10.1\ifmmode\pm\else\textpm\fi{}0.2 MeV at average spins 9\ensuremath{\Elzxh}, 19.5\ensuremath{\Elzxh}, and 24\ensuremath{\Elzxh} and temperatures 1.35, 1.45, and 1.50 MeV, respectively, as compared with the ground-state width of 7.9\ifmmode\pm\else\textpm\fi{}0.2 MeV. The observed difference in the GDR widths between the two molybdenum isotopes at the same temperatures and spins indicates that shell effects are still present at the temperatures studied.

GDR decay in hot Hg isotopes. An approach to improve the sensitivity to GDR decay following fusion reactions

The giant dipole resonance (GDR) decay from the compound nucleus 190Hg ∗ at E ∗ ≈ 52 MeV is studied in coincidence with mass separated evaporation residues. It is demonstrated that the selection of decay channels with few-neutron emission, combined with a precise matching of the input excitation energy enhances the experimental sensitivity to the details of the GDR strength function by a factor of ten or more.

Statistical giant dipole resonance decay of highly excited states of 63Cu.

Continuum \ensuremath{\gamma}-ray spectra from decays of $^{63}\mathrm{Cu}$ formed at initial excitation energies of 22.5 to 77.4 MeV and maximum spin up to 40\ensuremath{\Elzxh}, using $^{4}\mathrm{Co}$, $^{6}$Li${+\mathrm{}}^{57}$Fe, $^{12}$C${+\mathrm{}}^{51}$V, and $^{18}$O${+\mathrm{}}^{45}$Sc entrance channels, have been measured and analyzed. The parameters of the giant dipole resonance strength function have been extracted using a statistical code in a nonlinear least-squares fitting routine. Except for the cases of $^{4}\mathrm{He}$ and $^{6}\mathrm{Li}$ at the highest bombarding energies, which show evidence for nonstatistical effects, spectra are well reproduced by statistical calculations. The mean energy and strength of the giant dipole resonance built on excited states of $^{63}\mathrm{Cu}$ are close to the ground-state values, while the width varies smoothly from \ensuremath{\sim}5 MeV for the ground-state giant dipole resonance up to 10.6\ifmmode\pm\else\textpm\fi{}0.6 MeV in the temperature range up to 1.9 MeV and mean spin in the range 0--23\ensuremath{\Elzxh}. The large range of energies studied permitted different level density formulations to be tested. Measured spectra from $^{6}$Li${+\mathrm{}}^{98}$Mo and $^{6}$Li${+\mathrm{}}^{181}$Ta at ${E}_{\mathrm{lab}{(\mathrm{}}^{6}\mathrm{Li})=36}$ MeV show a strong nonstatistical enhancement at high \ensuremath{\gamma}-ray energies.