Program - Brookhaven National Laboratory

Statistical description of the gamma-transitions in atomic nuclei using the radiative strength functions
(RSF) [1,2] is discussed. The practical semiphenomenological methods of the RSF calculations based on
excitation of the isovector giant dipole resonance (GDR) [1-3] are overviewed. New variant of modified
Lorentzian approach [1,2] for calculation of electric dipole RSF is proposed and tested. In this approach,
an RSF shape parameter (“energy-dependent width”) is given as a function of the first quadrupole state
energy in order to more properly taken into account nuclear structure peculiarities. New database and
systematics for the GDR parameters [2,3] are used for the calculations of the energy-dependent widths. The
excitation functions and gamma-ray spectrum from (n,xγ) reactions on middle-weight and heavy atomic
nuclei are calculated with different RSF shapes to choice optimal expression for the RSF.
[1] R.Capote, M.Herman, P.Oblozinsky et al., Nucl. Data Sheets 110, 3107 (2009),
http://www-nds.iaea.org/RIPL-3/ [2] V.A.Plujko, R.Capote, O.M.Gorbachenko, At.Data Nucl.Data
Tables 97, 567 (2011) [3] V.A.Plujko, O.M.Gorbachenko, V.M.Bondar, R.Capote, Journ. Korean Phys.
Society 59, No2, 1514 (2011)
PC 8 5:45 PM
On the Mechanism of Neutron Emission in Fission (2013)
N.V. Kornilov
Department of Physics & Astronomy, Ohio University, Athens, OH 45701
The Monte Carlo model for detailed calculation of the neutron emission in fission has been developed. The
model is based on traditional assumption: formation of compound nucleus, decay to fission fragments, and
neutron emission from exited FF after their total acceleration. Input parameters are: two dimensional
distribution of fission fragments (FF) yield versus masses and total kinetic energy (TKE), masses and
binding energies for all FF, absorption cross sections, level density (LD). The model is similar to well
known LANL model [1]. The main difference is the application of LD instead of Weisskopf assumption
( E*exp(-E/T)). This approach, and model for LD calculation were tested with neutron spectra in (p,n)
reactions [2]. Additional tests were applied for verification of MC code: - neutron multiple emission, -
energy-angular distribution with transformation from CMS to LS. Model outputs are: neutron spectra for
any selected mass and TKE in Centre of Mass and Laboratory systems, neutron multiplicity versus FF
masses and TKE, neutron energy-angular distribution in LS. The conclusion of [1] the present calculation
suggest that there is no need for the extra neutron source - is very optimistic. The bulk of experimental
data cannot be described in the frame of traditional model. The possible (most realistic) explanation is that
main assumption is wrong. More than 35% of fission events in neutron induced fission of 235U at thermal
point have another mechanism of emission than traditional one. This conclusion is rather interesting for
basic science. However, it is very difficult to understand, can this fact explain existing difference between
microscopic and macroscopic data? Old problem (more than 50 years) is still urgent now.
[1] P. Talou, et all, Proceeding of International Conference on Nuclear Data for Science and Technology
ND2010, Journal of the Korean Physical Society, 59, 797, (2011). [2] N. Kornilov, S. Grimes, A.Voinov, A
re-examination of the non-equi1ibrium mechanism of the (p,n) reaction, submitted for publication.
Session PD Fission Yields
Thursday March 7, 2013
Room: Empire West at 3:30 PM
PD 1 3:30 PM
232