# Determining the average prompt-fission-neutron multiplicity for $^{239}$Pu($n$,$f$) via a $^{240}$Pu($\alpha$,$\alpha^{\prime}f$) surrogate reaction

## TL;DR

This study uses a surrogate reaction method to indirectly measure the average prompt-fission-neutron multiplicity for $^{239}$Pu($n$,$f$), providing results consistent with direct measurements across a broad energy range.

## Contribution

First application of the surrogate-reaction method to determine $ar{
u}$ for $^{239}$Pu($n$,$f$) using $^{240}$Pu($ extalpha$,$ extalpha^{ extprime}f$) reactions.

## Key findings

- Values of $ar{
u}$ agree with direct measurements.
- Measured $ar{
u}$ over 0.25--26.25 MeV energy range.
- Validated surrogate method for fission neutron multiplicity.

## Abstract

The average prompt-fission-neutron multiplicity $\bar{\nu}$ is of significance in the areas of nuclear theory, nuclear nonproliferation, and nuclear energy. In this work, the surrogate-reaction method has been used for the first time to indirectly determine $\bar{\nu}$ for $^{239}$Pu($n$,$f$) via $^{240}$Pu($\alpha$,$\alpha^{\prime}f$) reactions. A $^{240}$Pu target was bombarded with a beam of 53.9-MeV $\alpha$ particles. Scattered $\alpha$ particles, fission products, and neutrons were measured with the NeutronSTARS detector array. Values of $\bar{\nu}$ were obtained for a continuous range of equivalent incident neutron energies between 0.25--26.25~MeV, and the results agree well with direct neutron measurements.