Radiative Capture

The final state of radiative capture is described by either photon multiplicities, or photon production cross-sections, and the discrete and continuous contributions to the photon energy spectra, along with the angular distributions of the emitted photons.

For the description of the photon multiplicity there are two supported data representations. It can either be tabulated as a function of the energy of the incoming neutron for each discrete photon as well as the eventual continuum contribution, or the full transition probability array is known, and used to determine the photon yields. If photon production cross-sections are used, only a tabulated form is supported.

The photon energies \(E_\gamma\) are associated to the multiplicities or the cross-sections for all discrete photon emissions. For the continuum contribution, the normalised emission probability \(f\) is broken down into a weighted sum of normalised distributions \(g\).

\[f\left(E\rightarrow E_\gamma\right)~=~ \sum_{i}p_i(E)g_i(E\rightarrow E_\gamma)\]

The weights \(p_i\) are tabulated as a function of the energy \(E\) of the incoming neutron. For each neutron energy, the distributions \(g\) are tabulated as a function of the photon energy. As in the ENDF/B-VI data formats [Gro91], several interpolation laws are used to minimise the amount of data, and optimise the descriptive power. All data are derived from evaluated data libraries.

The techniques used to describe and sample the angular distributions are identical to the case of elastic scattering, with the difference that there is either a tabulation or a set of legendre coefficients for each photon energy and continuum distribution.

As an example of the results is shown in Fig. 118 the energy distribution of the emitted photons for the radiative capture of 15 MeV neutrons on Uranium (²³⁸U). Similar comparisons for photon yields, energy and angular distributions have been performed for capture on ²³⁸U, ²³⁵U, ²³Na, and ¹⁴N for a set of incoming neutron energies. In all cases investigated the agreement between evaluated data and Monte Carlo is very good.

Fig. 118 Comparison of data and Monte Carlo for photon energy distributions for radiative capture of 15 MeV neutrons on Uranium (²³⁸U). The points are evaluated data, the histogram is the Monte Carlo prediction.

Bibliography

Gro91

ENDF/B-VI: Cross Section Evaluation Working Group. Endf/b-vi summary document. Technical Report BNL-NCS-17541 (ENDF-201), Brookhaven National Laboratory, Upton, NY, USA, 1991. edited by P.F. Rose, National Nuclear Data Center.