DIRECT AND SEMIDIRECT NEUTRON RADIATIVE CAPTURE BY MEDIUM-HEAVY MASS NUCLEI: A NEW VERSION OF THE SEMIMICROSCOPIC DESCRIPTION

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DIRECT AND SEMIDIRECT NEUTRON RADIATIVE
CAPTURE BY MEDIUM-HEAVY MASS NUCLEI A NEW
VERSION OF THE SEMIMICROSCOPIC DESCRIPTION 

• B.A. Tulupov1, M.H. Urin2
• 1 Institute for Nuclear Research, RAS, Moscow,
  Russia
• 2 National Research Nuclear University MEPhI,
  Moscow, Russia

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1. Introduction
The simplest photonuclear reactions
(photoabsorption, directsemidirect (DSD)
photonucleon and inverse reactions) many years
were a powerful tool of experimental studies of
the isovector giant dipole and quadrupole
resonances (IVGDR and IVGQR, respectively).
Theoretical description of above mentioned
reactions is most adequate within microscopic
and semimicroscopic approaches, allowing take
into account the main relaxation modes of
giant resonances (GRs). In case of GRs in
medium-heavy mass 'hard' spherical nuclei (in
particular, in singly- and doubly-closed-shell
nuclei) these modes are (i) particle-hole
(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 many-quasiparticle
configurations ( chaotic states), that leads to
the spreading effect.
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These relaxation modes are taken into account
within the semimicrosco- pic approach to the
description of GRs based on the versions of
the continuum-Random-Phase-Approximation
(cRPA). 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 the implementations of
this approach a phenomenological mean
field, the Landau - Migdal
(momentum-independent) p-h interaction,
connected by some self - consistency
conditions, and momentum dependent forces are
used, as the input quantities. Three
specific parameters (a and kL, L1,2) are
adjusted to describe the experimental
photoabsorption cross section in the
considered energy region for a given
(spherical) nucleus. The main innovation in the
formulation of the new version of the
semimicro -scopic approach to study the
simplest photonuclear reactions is the use of
the modified expression for the
energy-averaged 'free' p-h Green function
which is different from that used within cRPA.
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Innovation
Here is the local limit of the
energy-averaged free non-local p-h Green function
satisfying to the equation
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3. Calculation results

• a). Excitation of IVGDR

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Values of the fundamental parameters S2(-)(m)
a0.105 MeV 1 , k10.35 , k2 0.1
S2(-)(n) a0.1 MeV 1 , ?1 0.4 , k2
0.1

• Characteristics of the isovector giant
  
• quadrupole resonance
• ?(m)max 19,8 ???, FWHM(m) 4,7 ???
• ?(n)max 19,9 ???, FWHM(n) 4,9 ???

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4. Conclusion
In this work first implementations of the new
semimicroscopic model, particle-hole dispersive
optical model, are realized. Satisfactory
description of the neutron radiative
capture accompanied by excitation of the
isovector giant dipole and quadrupole resonances
in a few medium-heavy mass nuclei is obtained
without the use of specific adjustable
parameters. Within the mentioned model
verification and extension (at arbitrary,
but high enough excitation energies) of
the semimicroscopic approach to description
of giant resonances (including
direct-nucleon-decay properties) are achieved.
New implementations of the model to describe the
simplest photo- nuclear reactions are in
progress.