Neutron-neutron correlations in the photofission of U238

Abstract

Background: In the fission of actinides, the nearly back-to-back motion of the fission fragments has a strong effect on the kinematics of fission neutrons. This leads to a favoring of opening angles near ${0}^{\ensuremath{\circ}}$ and ${180}^{\ensuremath{\circ}}$ in the neutron-neutron $(n\text{\ensuremath{-}}n)$ opening angle distribution of correlated neutron pairs from the same fission event.Purpose: To measure the $n\text{\ensuremath{-}}n$ opening angle and energy correlations in the photofission of $^{238}\mathrm{U}$. As of this writing, measurements of correlated $n\text{\ensuremath{-}}n$ opening angle distributions have been reported only for the spontaneous and neutron-induced fission of actinides. This work is the first to report such a measurement using photofission and will provide useful experimental input for photofission models used in codes such as Monte Carlo N-Particle (MCNP) and the Fission Reaction Event Yield Algorithm (FREYA).Methods: Fission is induced using bremsstrahlung photons produced via a low-duty-factor, pulsed, linear electron accelerator. The bremsstrahlung photon beam impinges on a $^{238}\mathrm{U}$ target that is surrounded by a large neutron scintillation detection system capable of measuring particle position and time of flight, from which the $n\text{\ensuremath{-}}n$ opening angle and energy are measured. Neutron-neutron angular correlations are determined by taking the ratio between a correlated $n\text{\ensuremath{-}}n$ distribution and an uncorrelated $n\text{\ensuremath{-}}n$ distribution formed by the pairing of neutrons produced during different beam pulses. This analysis technique greatly diminishes effects due to detector efficiencies, acceptance, and experimental drifts.Results: The angular correlation of neutrons from the photofission of $^{238}\mathrm{U}$ shows a high dependence on neutron energy as well as a dependence on the angle of the emitted neutrons with respect to the incoming photon beam. Angular correlations were also measured using neutrons from the spontaneous fission of $^{252}\mathrm{Cf}$, showing good agreement with past measurements.Conclusions: The measured angular correlations reflect the underlying back-to-back nature of the fission fragments. An anomalous decline in $n\text{\ensuremath{-}}n$ yield was observed for opening angles near ${180}^{\ensuremath{\circ}}$ for $^{238}\mathrm{U}$.