Problem of Level Densities in Compound
Nuclear Reactions
Alexander Voinov
1 The Problem Overview
The purpose of this contribution is to give a brief overview of experimental basis for
level density models currently used in modern nuclear reaction codes such as Empire
[1] and Talys [2]. The goal is to understand the source of uncertainties associated
with those models which result in uncertainties in cross section calculations. The
discussion of the possible ways of their improvements will follow.
Modern reaction codes including those mentioned above are used in variety of
applications including astrophysics calculations and evaluation of reaction cross
sections for the ENDF data base [3]. Therefore, the robustness of model inputs in
general and level density models in particular is crucial for accuracy of calculations.
Each code has an option to input different level density models. This is certainly
helpful when codes are used for analysis of available experimental data but it results
in ambiguities and uncertainties when they are used for predictions of unknown
cross sections either in energy or nuclear chart regions where experimental data are
not available. It is appropriate to mention here specific studies on uncertainties of
Hauser-Feshbach (HF) calculations which reveal a factor of up to 3 uncertainty for
stable nuclei and much greater for nuclei off the stability line. These uncertainties
are mainly due to the difference in HF inputs and specifically due to different level
density models [4].
From physics point of view, the reaction cross section is generally due to
different reaction mechanisms, however, the compound mechanism [5] constitutes
a substantial fraction or even dominates at low energies (<∼5 MeV/A) which are
relevant to most of applications. The theory of this model was developed in Ref. [6]
A. Voinov ()
Department of Physics and Astronomy, Ohio University, Athens, OH, USA
e-mail: voinov@ohio.edu
© This is a U.S. government work and not under copyright protection
in the U.S.; foreign copyright protection may apply 2021
J. Escher et al. (eds.), Compound-Nuclear Reactions, Springer Proceedings in
Physics 254, https://doi.org/10.1007/978-3-030-58082-7_13
113
Nuclear Reactions
Alexander Voinov
1 The Problem Overview
The purpose of this contribution is to give a brief overview of experimental basis for
level density models currently used in modern nuclear reaction codes such as Empire
[1] and Talys [2]. The goal is to understand the source of uncertainties associated
with those models which result in uncertainties in cross section calculations. The
discussion of the possible ways of their improvements will follow.
Modern reaction codes including those mentioned above are used in variety of
applications including astrophysics calculations and evaluation of reaction cross
sections for the ENDF data base [3]. Therefore, the robustness of model inputs in
general and level density models in particular is crucial for accuracy of calculations.
Each code has an option to input different level density models. This is certainly
helpful when codes are used for analysis of available experimental data but it results
in ambiguities and uncertainties when they are used for predictions of unknown
cross sections either in energy or nuclear chart regions where experimental data are
not available. It is appropriate to mention here specific studies on uncertainties of
Hauser-Feshbach (HF) calculations which reveal a factor of up to 3 uncertainty for
stable nuclei and much greater for nuclei off the stability line. These uncertainties
are mainly due to the difference in HF inputs and specifically due to different level
density models [4].
From physics point of view, the reaction cross section is generally due to
different reaction mechanisms, however, the compound mechanism [5] constitutes
a substantial fraction or even dominates at low energies (<∼5 MeV/A) which are
relevant to most of applications. The theory of this model was developed in Ref. [6]
A. Voinov ()
Department of Physics and Astronomy, Ohio University, Athens, OH, USA
e-mail: voinov@ohio.edu
© This is a U.S. government work and not under copyright protection
in the U.S.; foreign copyright protection may apply 2021
J. Escher et al. (eds.), Compound-Nuclear Reactions, Springer Proceedings in
Physics 254, https://doi.org/10.1007/978-3-030-58082-7_13
113