Neutron Multiplicity Correlations with Fission Fragment Mass and Energy from ²³⁹Pu(n,f)

Abstract

There exists experimental evidence for strong fluctuations of the average neutron multiplicity from resonance to resonance in 239Pu(n,f). These fluctuations have been shown to impact nuclear reactor benchmarks by reducing the criticality. The fluctuating neutron multiplicity can be explained as a consequence of the competition between direct fission and the (n,γf) process. However, there is also evidence for fluctuations of the fission fragment mass yields from resonance to resonance. The mass yield fluctuations may also contribute to fluctuations of the neutron multiplicity averaged over all fission fragment masses. In order to model the contribution to the neutron multiplicity fluctuations by the fission fragment mass yield fluctuations new data on the correlations between fission fragment properties and neutron multiplicities are in need. We present experiments carried out to determine prompt neutron multiplicity correlations with fission fragment masses and total kinetic energies in the reaction 239Pu(n,f). The experiment has been performed at the GELINA facility at JRC-Geel. A twin position-sensitive Frisch-grid ionization chamber is used for fission fragment identification via the double kinetic energy technique. An array of scintillation detectors is employed for neutron counting. Correlations between average neutron multiplicities and fission fragment properties have been measured with improved resolution in both mass and TKE, compared to data from the literature.

Monte Carlo simulation of the measurement by the 2E technique of the average prompt neutron multiplicity as a function of the mass of fragments from thermal neutron-induced fission of 239Pu

“…The average neutron multiplicity (measured by the 2E technique), as a function of the mass of fragments from the 239 Pu(nth, f) reaction. We show the simulated average prompt neutron multiplicity as a function of the primary fragment mass (ν s , diamonds) and the corresponding to the fragment pseudo mass output of simulation (νµ, squares), compared with the corresponding to the experimental data of Tsuchiya et al [13] and with Göök et al [18], respectively. et al [13] and Göök et al [18], respectively.…”

“…Added to this is the resolution of the energy measurement, which depends on the detector. As an example, the total resolution of the mass of the Göök et al experiment is 4 u [18].…”