P.F. Bortignon

Escape and spreading properties of charge-exchange resonances in Bi 208

The properties of charge-exchange excitations of

Behavior of the giant-dipole resonance in 120Sn and 208Pb at high excitation energy

^{208}mathrm{Pb}

Neutrino capture cross sections for 40Ar and β-decay of 40Ti

with ensuremath{Delta}L=0, i.e., the isobaric analog and Gamow-Teller resonances, are studied within a self-consistent model making use of an effective force of the Skyrme type. The well-known isobaric analog case is used to assess the reliability of the model. The calculated properties of the Gamow-Teller resonance are compared with recent experimental measurements with the aim of better understanding the microscopic structure of this mode.

Effects of the tensor force on the multipole response in finite nuclei

Abstract The properties of the giant-dipole resonance (GDR) are calculated as a function of excitation energy, angular momentum, and the compound nucleus particle decay width in the nuclei 120 Sn and 208 Pb, and are compared with recent experimental data. Differences observed in the behavior of the full-width-at-half-maximum of the GDR for 120 Sn and 208 Pb are attributed to the fact that shell corrections in 208 Pb are stronger than in 120 Sn, and favor the spherical shape at low temperatures. The effects shell corrections have on both the free energy and the moments of inertia are discussed in detail. At high temperature, the FWHM in 120 Sn exhibits effects due to the evaporation width of the compound nucleus, while these effects are predicted for 208 Pb.

Test of dynamical effects in the decay of the 59Cu compound nucleus

Abstract Shell-model calculations of solar neutrino absorption cross sections for 40Ar, the proposed component of the ICARUS detector, are presented. It is found that low-lying Gamow-Teller transitions lead to a significant enhancement of the absorption rate over that expected from the Fermi transition between the isobaric analog states, leading to an overall absorption cross section for 8B neutrinos of (11.5 ± 0.7) × 10−43 cm2 or a total expected rate of 6.7 ± 2.5 SNU. We also note that the pertinent Gamow-Teller transitions in 40Ar are experimentally accessible from the β-decay of the mirror nucleus 40Ti. Predictions for the branching ratios to states in 40Sc are presented, and the theoretical halflife of 55 ± 5 ms is found to be in good agreement with the experimental value of 56−12+18 ms.

Restoration of isospin symmetry in highly excited nuclei

We present a thorough analysis of the effects of tensor interaction on the multipole response of magic nuclei, using the fully self-consistent random phase approximation model with Skyrme interactions. We disentangle the modifications to the static mean field induced by the tensor terms, and the specific features of the residual particle-hole (p-h) tensor interaction, for quadrupole (2(+)), octupole (3(-)), and also magnetic dipole (1(+)) responses. It is pointed out that the tensor force has a larger effect on the magnetic dipole states than on the natural parity states 2(+) and 3(-), especially at the mean-field level. Perspectives for a better assessment of the tensor force parameters are discussed.

Effects of angular momentum projection on the nuclear partition function and the observation of the giant-dipole resonance in hot nuclei

Abstract The energy spectra of α particles emitted in multiple-α decay chains from 59Cu compound nuclei formed in the reaction of 190 MeV 32S on 27Al have been measured. These spectra are sensitive to different regions of the phase space open to the decay of the compound nucleus. A direct test of dynamical effects related to the shape relaxation is obtained by comparing the experimental spectra with results from Monte Carlo statistical model calculations.

Excitations of 144Sm in (p,p').

Abstract Explicit relations between the isospin mixing probability, the spreading width Γ IAS ↓ of the Isobaric Analog State (IAS) and the statistical decay width Γ c of the compound nucleus at finite excitation energy, are derived by using the Feshbach projection formalism. The temperature dependence of the isospin mixing probability is discussed quantitatively for the first time by using the values of Γ IAS ↓ and of Γ c calculated by means of microscopic models. It is shown that the mixing probability remains essentially constant up to a temperature of the order of 1 MeV and then decreases to about 1/4 of its zero temperature value, at higher temperature than ≈3 MeV, due to the short decay time of the compound system.

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

Procedures for projecting the angular momentum in a model describing a hot nucleus that takes into account large-amplitude quadrupole fluctuations are discussed. Particular attention is paid to the effect angular momentum projection has on the observables associated with the γ-decay of the giant-dipole resonance (GDR). We also elaborate on which of the different projection methods provides the best overall description of the GDR, including angular distributions. The main consequence of angular momentum projection is the appearance of an effective volume element in the integrals associated with the thermal average of the physical observables. This effective volume element is controlled by the value of the moments of inertia of the system. In the limit of rigid-body moments of inertia, the effective volume element is found to differ only slightly from the volume element associated with the normalization of the five-dimensional quadrupole oscillator wavefunction in the β, γ, and Ω space, namely D[α] = β4 dβsin(3γ) dγdΩ. In the limit of irrotational flow moments of inertia, the leading behavior in the β degree of freedom is D[α] ∝ β dβ.

Giant dipole resonance studied with GASP

The reaction [sup 144]Sm([ital p],[ital p][prime]) has been studied with an energy resolution of 4--5 keV using 22 MeV protons. Angular distributions for states up to 4 MeV excitation energies are compared with distorted-wave and coupled-channels calculations including one-step and two-step excitations to study direct and sequential strength distributions. The results are compared with calculations in the nuclear field theory approach where quasiparticle-random-phase-approximation-like states and their mixing with two-phonon (quadrupole and octupole) states is taken into account. Reasonable agreement is obtained for both the positive and the negative parity states.