Direct-decay properties of giant resonances

Abstract

A semi-microscopic approach based on the continuum-RPA method and a phenomenological treatment of the spreading effect is developed and applied to describe direct-decay properties of a few isovector giant resonances. Capabilities of the approach to describe giant-resonance gross properties are also checked.

On the direct and semidirect neutron radiative capture by medium-heavy mass nuclei

“…In case of GRs in medium-heavy mass 'hard' spherical nuclei (in particular, in singly-and doubly-closed-shell nuclei) these modes are: (i) particlehole (p-h) strength distribution, or Landau damping, which is a result of shell structure of nuclei; (ii) coupling of the (p-h)-type states, forming a given GR, to the single-particle (s. p.) continuum, that leads to GR direct-nucleon decay, or to the DSD reactions with one nucleon in the continuum; (iii) coupling of the mentioned (p-h)-type states to manyquasiparticle configurations (chaotic states), that leads to the spreading effect. These relaxation modes are taken into account within the semimicroscopic approach to the description of GRs (SMAGR) based on the versions of the continuum-Random-Phase-Approximation (cRPA) [2,3]. Within this approach Landau damping and coupling to the s. p. continuum are considered microscopically, using the cRPA, while the spreading effect is described in a phenomenological way in terms of the energy-dependent imaginary part W(ω) of the effective s. p. optical-model potential directly used in the cRPA equations.…”

“…The unique feature of the SMAGR is its ability to describe direct-nucleon-decay properties of various GRs without the use of specific adjustable parameters. (A number of implementations of the approach are briefly reviewed in [2,3]). As applied to the description of photoabsorption within the above described approach, it was found the necessity to take the isovector momentum-dependent forces into account to reproduce in partially self-consistent cRPA calculations the observed IVGDR energy and exceeding the experimental integral photoabsorption cross section over the Thomas-ReichKuhn sum rule.…”

“…Within the SMAGR phenomenological treatment of the spreading effect consists in the known substitution ω → ω + iW(ω) − P(ω) [5] in cRPA equations and, therefore, was expected to be valid in the 'pole' approximation (i.e., in the energy region of a given GR) [2,3]. In some cases it is necessary to know properties of (p-h)-type excitations at energies far from the corresponding GR energy.…”

On direct proton decay of the Gamow-Teller giant resonance

“…As applied to medium-heavy mass closed-shell nuclei, this aim can be achieved within the semi-microscopic approach to the description of giant resonances (SMAGR). The present formulation of this approach has been initially given in [1] and then extended to a number of implementations (see the reviews [2,3]). The SMAGR is a generalization of the standard and non-standard versions of the continuumRandom-Phase-Approximation (cRPA) developed to take into account a spreading effect.…”

“…The SMAGR is a generalization of the standard and non-standard versions of the continuumRandom-Phase-Approximation (cRPA) developed to take into account a spreading effect. The latter is described phenomenologically in terms of the energy-dependent imaginary part of an effective optical-model potential directly used in cRPA equations [2,3].…”