Abstract
Measurement of the fission fragments in coincidence with the emitted prompt neutrons was undertaken recently, at JRC-Geel institute, for the 235U(n,f) reaction in the resolved resonance energy region, up to 160 eV incident neutron energy. From this experimental work, fluctuations of several fission observables (mass yields, average total kinetic energy T̅K̅E̅, average prompt neutron multiplicity v̅P) were clearly observed. In the present work, these experimental pre-neutron fission fragment mass and kinetic energy distributions were used as input data for the FIFRELIN Monte Carlo code. By adopting the Hauser-Feshbach statistical model, the code simulates the de-excitation of the fission fragments. Four free parameters are available in the code: two of them (called RTmin and RTmax) govern at the scission point the sharing of the total available excitation energy between the two nascent fission fragments, while the two others (called σL and σH) assign the initial fission fragment spins. In this way, fission observables (prompt particles energy spectra and multiplicities, delayed neutrons multiplicity,. . . ) and correlations between them can be predicted and investigated. Here, these four free parameters were tuned in order to reproduce the average prompt neutron multiplicity at the resonance En=19.23 eV, resonance for which the experimental statistical uncertainty on v̅P is the lowest one. Then, the calculations were perfomed for all resonances by keeping the same set of free parameters. We show that the calculated fluctuations of v̅P in the resonances can rather be well reproduced by considering only the fluctuations of the pre-neutron mass yields and kinetic energy. In addition, from our calculation procedure, other fission observables fluctuations can also be predicted.
Highlights
In the case of the 239Pu neutron induced fission reaction, fluctuations of the average prompt neutron multiplicity νP have been clearly established in the resolved resonance region
By selecting the most accurate Göök’s data, namely resonances for which the νP-value has a statistical uncertainty lower than 1%, correlations expected from physical grounds can be highlighted, as shown in Figs. 2d, 2e and 2f
When looking at the 12 resonances for which the statistical uncertainty on νP is lower than 1%, those correlations appear to be based on physical grounds
Summary
In the case of the 239Pu neutron induced fission reaction, fluctuations of the average prompt neutron multiplicity νP have been clearly established in the resolved resonance region (see for example [1]). Since the average energy carried away by the pre-fission photon is about 1 MeV [2], the excitation energy available for the fission fragments is lower than in the case of a direct fission, leading to a lower neutron multiplicity. According to this model, the νP-value is governed by the average νnP, f -value associated to the direct (n,f) contribution and the average νnP,γ f -value associated to the (n,γf) contribution. Since νnP, f >νnP,γ f , the average prompt neutron multiplicity for a given resonance can only be reduced compared to the resonances where the (n,γf) contribution is negligible
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