Abstract
| Cross sections and strength functions in neutron induced reactions on 147 Sm
nucleus from slow neutrons up to 15-20 MeV's were evaluated. The neutron
resonance parameters, transmission coefficients and the Hauser – Feshbach
formalism were included in the calculations. In the MeV's region theoretical
evaluations are performed by using Talys free software and author's computer
programs. The obtained cross sections and strength functions are compared with
experimental data in order to explain possible non-statistical effects reported
previously by some authors on the alpha widths distributions. 1 Introduction
Cross sections, asymmetry effects and strength functions at the EG-5 and IREN
basic facilities of FLNP-JINR by using a double gridded ionization chamber were
regularly measured in the last decade. By recent measurements cross sections for
5 and 6 MeV of 147 Sm (n,α) reaction were obtained. Because the values of the
cross sections are very low (hundreds of microbarns) their measurements are very
difficult. The cross sections experimental data are very well described by the
theoretical model evaluations performed in this study [1,2]. Capture processes
of neutrons with emission of charged particles, starting from thermal region up
to 14 MeV, on 147 Sm, were analyzed. Cross sections for (n,α) reactions, from
slow neutrons up to some MeV's, in the frame of Hauser – Feshbach formalism
(HFF), were evaluated using computer codes realized by authors [3]. The main
element of HFF is represented by the transmission coefficients for incident and
emergent channels. Transmission coefficients were calculated by applying a
quantum-mechanical approach based on reflection factor [3,4]. Starting from 0.5
MeV up to 14 MeV, a separation in the contribution of different nuclear reaction
mechanisms related to discrete and continuum states were realized with the help
of Talys computer codes. It was demonstrated that the main contribution to the
cross sections is given by compound nucleus processes followed by direct
processes. Also, nuclear data as parameters of optical potential, nuclear states
densities and other were extracted. The computed cross sections and strength
functions are compared with experimental data in order to explain possible
nonstatistical effects reported previously by some authors on the distributions
of alpha widths. 2 Theory and codes Compound processes can be described in the
frame of the statistical model of nuclear reactions. Main assumptions of
statistical approach are: a) by interaction of incident particle with target
nucleus a compound nucleus (CN) is formed; b) CN time of life is much larger
than the time necessary to incident particle to pass the target nucleus; c) CN
decays on one possible channels and " forget " how it was formed (Bohr
hypothesis); d) CN and residual nucleus are characterized by a great number of |