Determination of ${}^{238}\mathrm{Pu}(n,f)$ and ${}^{236}\mathrm{Np}(n,f)$ cross sections using surrogate reactions

The cross sections for ${}^{238}\mathrm{Pu}\left(n,f\right)$ reaction for equivalent neutron energy of 13.0–22.0 MeV have been determined by the “surrogate ratio” method by measuring ${}^{235}\mathrm{U}\left({}^6\mathrm{Li},df\right)$ and ${}^{232}\mathrm{Th}\left({}^6\mathrm{Li},df\right)$ transfer induced fission reactions proceeding through the excited fissioning nuclei ${}^{239}\mathrm{Pu}$ and ${}^{236}\mathrm{U}$, respectively, and using ${}^{235}\mathrm{U}\left(n,f\right)$ cross-section data as the reference. Similarly, the cross sections for the ${}^{236}\mathrm{Np}\left(n,f\right)$ reaction in the equivalent neutron energy range 9.9–22.0 MeV have been determined by the “hybrid surrogate ratio” method via the measurements of ${}^{235}\mathrm{U}\left({}^6\mathrm{Li},\alpha f\right)$ and ${}^{235}\mathrm{U}\left({}^6\mathrm{Li},df\right)$ transfer induced fission reactions, using ${}^{238}\mathrm{Pu}\left(n,f\right)$ cross-section data as the reference. The empire-3.1 calculations for ${}^{238}\mathrm{Pu}\left(n,f\right)$ and ${}^{236}\mathrm{Np}\left(n,f\right)$ cross sections agree well with the present data, however, they are slightly underestimated by the endf/b-vii.1 evaluations.

Figure 1

Fission decay probability in ${}^{236}\mathrm{Np}(n,f)$ (upper panel) and ${}^{238}\mathrm{Pu}(n,f)$ (lower panel) reactions calculated using the empire code as a function of neutron energy with different compound-nucleus $J$ values.

Figure 2

Typical alpha and deuteron spectra from ${}^{235}\mathrm{U}$ + Ni-Cu backing (pink line) and only Ni-Cu backing (blue line) are shown in (a) and (c). Corresponding spectra only from ${}^{235}\mathrm{U}$ target (green line) obtained from the difference of the above two contributions are shown in (b) and (d), respectively.

Figure 3

Typical fission spectrum obtained in coincidence with light charged particles in ${}^6\mathrm{Li}$+${}^{235}\mathrm{U}$ reaction.

Figure 4

Deuteron spectra for (a) and (b) ${}^{232}\mathrm{Th}({}^6\mathrm{Li},\mathrm{d}$)${}^{236}\mathrm{U}$, and (c) and (d) ${}^{235}\mathrm{U}\left({}^6\mathrm{Li},d\right){}^{239}\mathrm{Pu}$ transfer reactions, respectively. Deuterons measured in coincidence with fission fragments for the respective reactions are shown in (a) and (c) and those in singles are shown in (b) and (d). Background from the Ni-Cu backing is subtracted.

Figure 5

Determined ${}^{238}\mathrm{Pu}\left(n,f\right)$ cross sections (solid circle) along with the data (hollow circle) measured by Ressler et al. [8]. Solid and dashed lines correspond to the results of empire calculations and endf/b-vii.1 evaluations, respectively.

Figure 6

Coincident and inclusive spectra for (a) and (b) alpha, and (c) and (d) deuteron, respectively, in the ${}^6\mathrm{Li}$+${}^{235}\mathrm{U}$ reaction.

Figure 7

${}^{236}\mathrm{Np}\left(n,f\right)$ cross section as a function of equivalent neutron energy. Open squares are the existing data measured by Britt et al. [9]. Dotted and solid lines represent the empire-3.1 calculations.

Figure 8

empire predictions for ${}^{236}\mathrm{Np}\left(n,f\right)$ cross section as a function of neutron energy using four different sets of parameters of nuclear level density and fission barriers. Open squares are the existing data measured by Britt et al. [9]. Dotted and solid lines represent the empire-3.1 calculations.