B.A. Tulupov 1 , M.H. Urin 2 1 Institute for Nuclear Research, RAS, Moscow, Russia ;

1. DIRECT AND SEMIDIRECT NEUTRON RADIATIVE CAPTURE BY MEDIUM-HEAVY MASS NUCLEI: A NEW VERSION OF THE SEMIMICROSCOPIC DESCRIPTION B.A. Tulupov1, M.H. Urin2 1Institute for Nuclear Research, RAS, Moscow, Russia; 2National Research Nuclear University «MEPhI», Moscow, Russia

2. 1. Introduction The simplest photonuclear reactions (photoabsorption, direct+semidirect (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.

3. These relaxation modes are taken into account within thesemimicrosco- 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,usingthe cRPA, whilethe spreading effect is described in a phenomenological way. In the implementations ofthis approacha phenomenological mean field, theLandau-Migdal (momentum-independent) p-h interaction,connected by some self-consistency conditions, and momentum – dependent forcesare used, as the input quantities. Three specific parameters (α and k’L, L=1,2) are adjusted to describe the experimental photoabsorption cross section in the considered energy region for a given (spherical) nucleus. Themain innovation in the formulation of the newversion of the semimicro -scopic approach to study the simplest photonuclear reactions is the use of themodified expression for the energy-averaged 'free' p-h Green function which is different from that used within cRPA.

8. Here is the local limit of the energy-averaged free non-local p-h Green function satisfying to the equation: Innovation:

10. 3. Calculation results a). Excitation of IVGDR

22. Values of the fundamental parameters: S2(-)(m) : α=0.105 MeV –1 , k’1=0.35 , k’2 =0.1S2(-)(n) : α=0.1 MeV –1 , к’1 =0.4 , k’2 =0.1 Characteristics of the isovector giant quadrupole resonance: ω(m)max = 19,8 МэВ,FWHM(m) = 4,7 МэВ ω(n)max = 19,9 МэВ, FWHM(n) = 4,9 МэВ

23. 4. Conclusion In this work first implementations of the new semimicroscopic model,particle-holedispersive optical model, are realized. Satisfactory descriptionof 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.