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Register a new userGiant Dipole Resonance Width in near-Sn Nuclei at Low Temperature and High Angular Momentum
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High energy gamma-rays in coincidence with low energy yrast gamma-rays have been measured from 113Sb, at excitation energies of 109 and 122 MeV, formed by bombarding 20Ne on 93Nb at projectile energies of 145 and 160 MeV respectively to study the role of angular momentum (J) and temperature (T) over Giant Dipole Resonance (GDR) width. The maximum populated angular momenta for fusion were 67hbar and 73hbar respectively for the above-mentioned beam energies. The high energy photons were detected using a Large Area Modular BaF2 Detector Array (LAMBDA) along with a 24-element multiplicity filter. After pre-equilibrium corrections, the excitation energy E* was averaged over the decay steps of the compound nucleus (CN). The average values of temperature, angular momentum, CN mass etc. have been calculated by the statistical model code CASCADE. Using those average values, results show the systematic increase of GDR width with T which is consistent with Kusnezov parametrization and the Thermal Shape Fluctuation Model. The rise of GDR width with temperature also supports the assumptions of adiabatic coupling in the Thermal Shape Fluctuation Model. But the GDR widths and corresponding reduced plots with J are not consistent with the theoretical model at high spins.
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Using a unique two-arm detector system for heavy ions (the BRS, binary reaction spectrometer) coincident fission events have been measured from the decay of $^{60}$Zn compound nuclei formed at 88MeV excitation energy in the reactions with $^{36}$Ar beams on a $^{24}$Mg target at $E_{lab}(^{36}$Ar) = 195 MeV. The detectors consisted of two large area position sensitive (x,y) gas telescopes with Bragg-ionization chambers. From the binary coincidences in the two detectors inclusive and exclusive cross sections for fission channels with differing losses of charge were obtained. Narrow out-of-plane correlations corresponding to coplanar decay are observed for two fragments emitted in binary events, and in the data for ternary decay with missing charges from 4 up to 8. After subtraction of broad components these narrow correlations are interpreted as a ternary fission process at high angular momentum through an elongated shape. The lighter mass in the neck region consists dominantly of two or three-particles. Differential cross sections for the different mass splits for binary and ternary fission are presented. The relative yields of the binary and ternary events are explained using the statistical model based on the extended Hauser-Feshbach formalism for compound nucleus decay. The ternary fission process can be described by the decay of hyper-deformed states with angular momentum around 45-52 $hbar$.
A set of high resolution zero-degree inelastic proton scattering data on 24Mg, 28Si, 32S, and 40Ca provides new insight into the long-standing puzzle of the origin of fragmentation of the Giant Dipole Resonance (GDR) in sd-shell nuclei. Understanding is provided by state-of-the-art theoretical Random Phase Approximation (RPA) calculatios for deformed nuclei using for the first time a realistic nucleon-nucleon interaction derived from the Argonne V18 potential with the unitary correlation operator method and supplemented by a phenomenological three-nucleon contact interaction. A wavelet analysis allows to extract significant scales both in the data and calculations characterizing the fine structure of the GDR. The fair agreement supports that the fine structure arises from ground-state deformation driven by alpha clustering.
The E1(T=1) isovector dipole giant resonance (GDR) in heavy and super-heavy deformed nuclei is analyzed over a sample of 18 rare-earth nuclei, 4 actinides and three chains of super-heavy elements (Z=102, 114 and 120). Basis of the description is self-consistent separable RPA (SRPA) using the Skyrme force SLy6. The self-consistent model well reproduces the experimental data (energies and widths) in the rare-earth and actinide region. The trend of the resonance peak energies follows the estimates from collective models, showing a bias to the volume mode for the rare-earths isotopes and a mix of volume and surface modes for actinides and super-heavy elements. The widths of the GDR are mainly determined by the Landau fragmentation which in turn is found to be strongly influenced by deformation. A deformation splitting of the GDR can contribute about one third to the width and about 1 MeV further broadening can be associated to mechanism beyond the mean-field description (escape, coupling with complex configurations).
The isoscalar giant dipole resonance (ISGDR) has been investigated in 208Pb using inelastic scattering of 400 MeV alpha particles at forward angles, including 0deg. Using the superior capabilities of the Grand Raiden spectrometer, it has been possible to obtain spectra devoid of any instrumental background. The ISGDR strength distribution has been extracted from a multipole-composition of the observed spectra. The implication of these results on the experimental value of nuclear incompressibility are discussed.
Exclusive measurements of high energy $gamma$-rays are performed in $rm ^{124}Ba$ and $rm ^{136}Ba$ at the same excitation energy ($sim$ 49 MeV), to study properties of the giant dipole resonance (GDR) over a wider $N/Z$ range. The high energy $gamma$-rays are measured in coincidence with the multiplicity of low energy $gamma$-rays to disentangle the effect of temperature ($T$) and angular momentum ($J$). The GDR parameters are extracted employing a simulated Monte Carlo statistical model analysis. The observed $gamma$-ray spectra of $rm ^{124}Ba$ can be explained with prolate deformation, whereas a single component Lorentzian function which corresponds to a spherical shape could explain the $gamma$-ray spectra from $rm ^{136}Ba$. The observed GDR width in $rm ^{136}Ba$ is narrower compared to that of $rm ^{124}Ba$. The statistical model best fit GDR cross sections are found to be in good agreement with the thermal shape fluctuation model (TSFM) calculations. Further, it is shown that the variation of GDR width with $T$ is well reproduced by the TSFM calculations over the temperature range of 1.1--1.7MeV.
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