Calculation of fission observables through event-by-event simulation

The increased interest in more exclusive fission observables has demanded more detailed models. We present here a new computational model, $\mathrm{FREYA}$, that aims to meet this need by producing large samples of complete fission events from which any observable of interest can then be extracted consistently, including arbitrary correlations. The various model assumptions are described and the potential utility of the model is illustrated by means of several novel correlation observables.

Figure 1

The mean total kinetic energy, ${\overline{K}}_{\mathrm{tot}}$, of the two fission fragments, and the associated dispersion (bars), as a function of the mass number of the heavy fragment, $A_H$, for $0.53\hspace{0.5em}\mathrm{MeV}\hspace{0.5em}\mathrm{n}$ on ${}^{235}\mathrm{U}$ (bottom) and ${}^{239}\mathrm{Pu}$ (top). The data from Nishio [14] (with a few representative dispersions), Tsuchiya [15], and Wagemans [16] are shown. The dispersions reflect the width of the kinetic energy distribution and are not (experimental or theoretical) uncertainties.

Figure 2

The mean kinetic energy of a single fragment and the associated dispersion (bars) as a function of its mass number $A_f$, for $0.53\hspace{0.5em}\mathrm{MeV}\hspace{0.5em}\mathrm{n}$ on ${}^{235}\mathrm{U}$ (bottom) and ${}^{239}\mathrm{Pu}$ (top). Pu data from Nishio [14] and Tsuchiya [15] are shown.

Figure 3

The mean excitation energy ${\overline{E}}^{*}$ (curves) of a fission fragment and the associated dispersion (bars), as a function of the mass number $A_f$ for $0.53\hspace{0.5em}\mathrm{MeV}\hspace{0.5em}\mathrm{n}$ on ${}^{235}\mathrm{U}$ (bottom) and ${}^{239}\mathrm{Pu}$ (top).

Figure 4

The mean number of neutrons (curve) resulting from fission fragment of a particular mass number $A_f$, with the associated dispersion indicated (bars), for $0.53\hspace{0.5em}\mathrm{MeV}\hspace{0.5em}\mathrm{n}$ on ${}^{235}\mathrm{U}$ (bottom) and ${}^{239}\mathrm{Pu}$ (top). Data from Apalin [17], Maslin [18], Nishio [14, 19], and Tsuchiya [15] are shown.

Figure 5

Multiplicity distributions for neutrons emitted by the light (top), the heavy (middle), or either (bottom) fragment resulting from thermal-neutron induced fission of ${}^{235}\mathrm{U}$ (circles, blue) and ${}^{239}\mathrm{Pu}$ (squares, red).

Figure 6

The mean total kinetic of energy the fission products together with the associated dispersions (bars), as a function of the neutron multiplicity in the event, for $0.53\hspace{0.5em}\mathrm{MeV}\hspace{0.5em}\mathrm{n}$ on ${}^{235}\mathrm{U}$ (bottom) and ${}^{239}\mathrm{Pu}$ (top).

Figure 7

The mean neutron energy ${\overline{\epsilon }}_{\mathrm{n}}$ (curves) together with its dispersion (bars) as a function of fragment mass $A_f$ for $0.53\hspace{0.5em}\mathrm{MeV}\hspace{0.5em}\mathrm{n}$ on ${}^{235}\mathrm{U}$ (bottom) and ${}^{239}\mathrm{Pu}$ (top).

Figure 8

The mean kinetic energy and the associated dispersions of all the neutrons emitted in fission events with a specified total neutron multiplicity ν, induced by thermal neutrons on ${}^{239}\mathrm{Pu}$ and ${}^{235}\mathrm{U}$ (solid curve, black dots), as well as the mean kinetic energy and the associated dispersions of all the neutrons emitted from the light fragment as a function of the corresponding multiplicity ${\nu }_L$ (dashed curve, green squares) and the mean kinetic energy and the associated dispersions of all the neutrons emitted from the heavy fragment as a function of the corresponding multiplicity ${\nu }_H$ (dashed curve, red diamonds).

Figure 9

The spectral shape of neutrons evaporated from the light (top), the heavy (middle), or either (bottom) fragment for specified values of the respective multiplicity ${\nu }_L,{\nu }_H$, or ν, in fission induced by thermal neutrons on ${}^{235}\mathrm{U}$.

Figure 10

Similar to Fig. 9 but for $\mathrm{n}+{}^{239}\mathrm{Pu}$.

Figure 11

The angular correlation between two evaporated neutrons for $0.53\hspace{0.5em}\mathrm{MeV}\hspace{0.5em}\mathrm{n}$ on ${}^{235}\mathrm{U}$ and ${}^{239}\mathrm{Pu}$, considering only neutrons with a kinetic energy above 1 MeV.

Figure 12

The mean total excitation energy of the two fission products, together with the associated dispersions (bars), as a function of the neutron multiplicity in the event, for $0.53\hspace{0.5em}\mathrm{MeV}\hspace{0.5em}\mathrm{n}$ on ${}^{235}\mathrm{U}$ (bottom) and ${}^{239}\mathrm{Pu}$ (top).