Event-by-event study of prompt neutrons fromPu239(n,f)

Abstract

Employing a recently developed Monte-Carlo model, we study the fission of 240 Pu induced by neutrons with energies from thermal to just below the threshold for second chance fission. Current measurements of the mean number of prompt neutrons emitted in fission, together with less accurate measurements of the neutron energy spectra, place remarkably fine constraints on predictions of microscopic calculations. In particular, the total excitation energy of the nascent fragments must be specified to within 1 MeV to avoid disagreement with measurements of the mean neutron multiplicity. The combination of the Monte-Carlo fission model with a statistical likelihood analysis also presents a powerful tool for the evaluation of fission neutron data. Of particular importance is the the fission spectrum, which plays a key role in determining reactor criticality. We show that our approach can be used to develop an estimate of the fission spectrum with uncertainties several times smaller than current experimental uncertainties for outgoing neutron energies of less than 2 MeV.

Event-by-event evaluation of the prompt fission neutron spectrum from239Pu(n,f)

“…Indeed, for our primary result, we use only the evaluated ν to constrain our new evaluation of the PFNS, as in Ref. [6]. Thus, in this treatment, the spectra is an outcome rather than a comparative observable.…”

“…the uncertainties alone, ensures that well-measured observables carry more weight than poorly measured ones. This approach was used in the previous PFNS evaluation [6], which was restricted to lower energy (E n < 5.5 MeV).…”

“…Because the data are often given with an arbitrary normalization, we normalize them to unity while preserving their spectral shapes, following the prescription of Ref. [6]. We note that this procedure could introduce some bias into the result because a particular functional form has been assumed to obtain the integral normalization.…”

“…As discussed earlier [6], the experimental spectral data themselves are neither sufficiently accurate nor of sufficiently consistent quality to allow an improved PFNS evaluation. However, by combining measured information about the nuclear fragment yields and energies with the very precise evaluations of neutron multiplicities, it is possible to constrain the neutron spectrum rather tightly without having to rely on the spectral data themselves.…”

“…Our approach employs the fission model FREYA (Fission Reaction Event Yield Algorithm) which incorporates the relevant physics and contains a few key parameters that are determined by comparison to pertinent data through statistical analysis [6,7]. It simulates the entire fission process, starting from the possible emission of pre-equilibrium neutrons, and it generates a large sample of complete fission events with full kinematic information on the emerging fission products and the emitted neutrons and photons.…”