A Grand Tour of Nuclear Fission Physics
269
4 Data Evaluation
Fission theory and experiment are brought together in data evaluations. Evaluations
are used to fill gaps where data are missing and resolve discrepancies between
data sets, with the help of theory and modeling. We will focus on models used
in evaluations of: (1) fission cross sections, and (2) properties of fission fragments
and of the neutrons and gamma rays they emit. Cross-section evaluations typically
rely on Hauser-Feshbach theory [100] and transition-state theory, first introduced
by Eyring to describe chemical reaction rates [101] and later adapted to nuclear
reactions by Wigner [102]. In the transition-state model, passage over a barrier is
mediated by a set of states on top of the barrier. The probability of transmission
through these states was formulated by Hill and Wheeler in 1953 [85]. States in
the first well of the potential energy surface are called “class-I” states. States in
the well separating the two fission barriers are called “class-II” states. Both classI and class-II states can enter into calculations of the fission cross-section [103].
This basic model has been improved by using barrier properties and level densities
calculated within a consistent microscopic framework [104], and through the use of
R-matrix theory with Monte-Carlo simulations of class-I and class-II state properties
and of their coupling matrix elements [105]. Various models of fission-fragment
properties have been incorporated into event-by-event codes such as GEF [106],
FIFRELIN [107], CGMF [108], and FREYA [109, 110]. These codes can be used
to evaluate fragment yields, TKE, and the spectra and multiplicities of the neutrons
and gamma rays they emit. The FREYA code, for example, provides an event-byevent simulation of post-scission physics with full kinematic information for the
products and emitted particles [109, 111]. Additional information on event-by-event
simulation codes for the evaluation of fission data can be found in the recent review
papers by Capote et al. [112] and Talou et al. [113].
5 Conclusion
Nuclear fission remains an active area of study more than 80 years after its discovery.
The field has made tremendous strides since the first papers on fission were
published in 1939, yet we still lack a predictive theory of this complex phenomenon
that follows the fissioning system from its formation to the last decay of the final
products, within a single consistent framework. Fission provides a rich choice
of measurable quantities to test and refine models with, and very sophisticated
descriptions of fission have been developed. Yet, there remain many fundamental
questions for experimentalists and theorists, such as: what happens at scission?
How do we describe the transition from one to two nuclei within a microscopic
framework? what are the appropriate degrees of freedom throughout the fission
process? How is the initial energy of the parent nucleus distributed among the
final products? These questions can be addressed by theory as its predictive power
269
4 Data Evaluation
Fission theory and experiment are brought together in data evaluations. Evaluations
are used to fill gaps where data are missing and resolve discrepancies between
data sets, with the help of theory and modeling. We will focus on models used
in evaluations of: (1) fission cross sections, and (2) properties of fission fragments
and of the neutrons and gamma rays they emit. Cross-section evaluations typically
rely on Hauser-Feshbach theory [100] and transition-state theory, first introduced
by Eyring to describe chemical reaction rates [101] and later adapted to nuclear
reactions by Wigner [102]. In the transition-state model, passage over a barrier is
mediated by a set of states on top of the barrier. The probability of transmission
through these states was formulated by Hill and Wheeler in 1953 [85]. States in
the first well of the potential energy surface are called “class-I” states. States in
the well separating the two fission barriers are called “class-II” states. Both classI and class-II states can enter into calculations of the fission cross-section [103].
This basic model has been improved by using barrier properties and level densities
calculated within a consistent microscopic framework [104], and through the use of
R-matrix theory with Monte-Carlo simulations of class-I and class-II state properties
and of their coupling matrix elements [105]. Various models of fission-fragment
properties have been incorporated into event-by-event codes such as GEF [106],
FIFRELIN [107], CGMF [108], and FREYA [109, 110]. These codes can be used
to evaluate fragment yields, TKE, and the spectra and multiplicities of the neutrons
and gamma rays they emit. The FREYA code, for example, provides an event-byevent simulation of post-scission physics with full kinematic information for the
products and emitted particles [109, 111]. Additional information on event-by-event
simulation codes for the evaluation of fission data can be found in the recent review
papers by Capote et al. [112] and Talou et al. [113].
5 Conclusion
Nuclear fission remains an active area of study more than 80 years after its discovery.
The field has made tremendous strides since the first papers on fission were
published in 1939, yet we still lack a predictive theory of this complex phenomenon
that follows the fissioning system from its formation to the last decay of the final
products, within a single consistent framework. Fission provides a rich choice
of measurable quantities to test and refine models with, and very sophisticated
descriptions of fission have been developed. Yet, there remain many fundamental
questions for experimentalists and theorists, such as: what happens at scission?
How do we describe the transition from one to two nuclei within a microscopic
framework? what are the appropriate degrees of freedom throughout the fission
process? How is the initial energy of the parent nucleus distributed among the
final products? These questions can be addressed by theory as its predictive power