Abstract
In all cases where neutron fluctuations in a branching process (such as in multiplicity measurements) are treated in an energy dependent description, the energy correlations of the branching itself (energy correlations of the fission neutrons) need to be known. To date, these are not known from experiments. Such correlations can be theoretically and numerically derived by modelling the details of the fission process. It was suggested earlier that the fact that the prompt neutrons are emitted from the moving fission targets, will influence their energy and angular distributions in the lab system, which possibly induces correlations. In this paper the influence of the neutron emission process from the moving targets on the energy correlations is investigated analytically and via numerical simulations. It is shown that the correlations are generated by the random energy and direction distributions of the fission fragments. Analytical formulas are derived for the two-point energy distributions, and quantitative results are obtained by Monte-Carlo simulations. The results lend insight into the character of the two-point distributions, and give quantitative estimates of the energy correlations, which are generally small.
Highlights
The statistical properties of the detection counts of neutrons, generated in subcritical multiplying media by an external source, have long been used in reactor physics and nuclear safeguards to determine parameters of the system under investigation
It was argued that if the energies of the fission neutrons are independent of each other in any individual fission event, but the energy distributions of the neutrons depend on the actual number of the neutrons (i.e the energy spectra are different for different neutron multiplicities on fission), neutron energies become correlated when considering all possible fission events with different neutron numbers
The purpose of this paper is to explore the properties of the energy and angular correlations of fission neutrons in the lab system through calculating the covariances, and even the two-point distributions of the neutron energies first qualitatively through analytical considerations, and quantitatively through numerical simulations
Summary
The statistical properties of the detection counts of neutrons, generated in subcritical multiplying media by an external source, have long been used in reactor physics and nuclear safeguards to determine parameters of the system under investigation. The subcritical reactivity can be extracted from the variance to mean or from the temporal correlations of the detections; and in safeguards, the fissile mass can be determined from coincidence an multiplicity measurements [1]. The possibility of extracting such parameters is due to the fact that the branching, i.e. the neutron multiplication, generates temporal correlations between the neutrons born in the same chain. EPJ Web of Conferences case, one needs to know the two- and three-point energy distribution of the source (fission) neutrons, respectively, and so on. It was argued that if the energies of the fission neutrons are independent of each other in any individual fission event, but the (independent and identical) energy distributions of the neutrons depend on the actual number of the neutrons (i.e the energy spectra are different for different neutron multiplicities on fission), neutron energies become correlated when considering all possible fission events with different neutron numbers
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