Program - Brookhaven National Laboratory

A significant breakthrough in our theoretical understanding of the fission process was made in 1956 when
A.Bohr introduced the concept of ”fission channels” that determine the outcome of many fission observables
[1]. A decade later, a new picture of the fission barrier was achieved by Strutinsky [2] who superimposed the
oscillatory frame of the shell correction term onto the liquid drop energy term. On both the experimental
and theoretical evidence of the existence of class-II compound states (so called λII), Lynn used a formal
R-matrix formalism for interactions in the deformation channels [3] to reproduce physically and more
accurately the magnitude of the average sub-threshold fission cross sections. In particular, Lynn stated
several possible coupling modes of class-I and class-II states and derived the corresponding class-II width
fluctuation factor, WII, expressions superimposed on the classic class-I reaction width fluctuation factor,
Wnr, of standard Hauser-Fesbach theory. Ruled by Porter-Thomas statistics as other reaction channel
widths, fluctuations of class-II fission, ΓλII↑, and coupling, ΓλII↓, widths can be handled similarly to other
width fluctuations such that WII = GammaλII↓ΓλII↑/ΓλIIλII . One purpose of this presentation will be to
emphasize the existence of this WII fluctuation factor, which is commonly disregarded in standard Hauser-
Feshbach reaction codes. An additional consequence of the existence of an intermediate structure is the
reduction of the number of degrees of freedom, νeff , of the classic class-I overall fission width fluctuation
factor, Wnf , traditionally assumed to be equal to 1 per each fully open outer barrier Bohr channel. The right
assessment of νeff is dependent of the underlying intermediate structure coupling strength and selected
mode, and of the fissioning system. This presentation will show how the intermediates structures and
the additional impact of the WII factor affect the average cross-sections. We use, as illustration, our full
calculations for the fissile and fertile isotopes of the plutonium series [4]. These are based on Monte Carlo
averaging over the intermediate and fine structure calculated with formal R-matrix theory.
[1] Bohr, A. In Peaceful Uses of Atomic Energy, Proc. of the conf. of Geneva, 1955 (United Nations, New
York), 2, (1956) 220. [2] Strutinsky, V.M. Nuclear Physics A 95, (1967) 420. [3] Lynn, J.E. J. Phys. A:
Math. Nucl. Gen., 6, (1973) 542. [4] Bouland O., Lynn J.E. and Talou P., EPJ Web of Conferences DOI:
10.1051, (2012) 2108004.
JC 3 11:20 AM
Unitary Account of Nuclear Reaction Mechanisms for Deuteron-Induced Activation at Low
and Medium Energies
M. Avrigeanu, V. Avrigeanu
Horia Hulubei National Institute for Physics and Nuclear Engineering, P.O. Box MG-6, 077125
Bucharest-Magurele, Romania
An extended analysis of the reaction mechanisms involved within deuteron interaction with target nuclei
from 27 Al to 231 Pa, i.e. the breakup (BU), stripping, pick-up, pre-equilibrium emission (PE) and evaporation
from fully equilibrated compound nucleus (CN), is presented. An increased attention is given to the
BU mechanism [1-3] including all its components, namely the elastic (BE), inelastic (fusion) (BF), and total
breakup (BU). An extension of the empirical parameterization of the BE cross sections beyond the energies
considered in this respect, checked by microscopical calculations in the frame of Continuum-Discretized
Coupled-Channels (CDCC) formalism [4], should be completed. Concerning the deuteron BU importance,
regardless the differences between various parametrizations [1,5], both of them predicts its enhanced role
with the target-nucleus mass/charge increase, as well as the BU dominance around the Coulomb barrier
for heavy nuclei as, e.g., 231 Pa. Furthermore, the consideration of the deuteron BU contribution to the
activation cross section has to take into account two opposite effects, namely the important BU leakage
of initial flux as well as the BF enhancement brought by the BU–nucleon interactions with the target
nucleus [1-3]. On the other hand, the stripping (d,p) and (d,n), as well as the pick-up (d,t) reactions,
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