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Heavy nuclei toe a delicate balance between the attractive strong nuclear force and the repulsive Coulomb force. When this balance is disrupted in favor of the Coulomb force, heavy nuclei may elongate and fission into two fragments. Fission was first observed over eighty years ago and theoretically explained shortly thereafter. We now know that good macroscopic and microscopic fission models play a vital role in modeling the r-process of heavy element synthesis, which has sparked renewed interest in a more accurate understanding of fission. Much has been published in the past year about the mechanism of angular momentum generation of the nascent fission fragments, with several groups reaching conclusions at variance with one another. Systematic biases in past experiments make results more difficult to interpret. Fragments shed much of their angular momentum, post-neutron evaporation, via gamma-ray emission. Thus, the number of emitted gamma rays is linked to the angular momentum of a fragment after neutron evaporation.
Scientists at the Los Alamos Neutron Science Center measured the $^{239}\mathrm{Pu}(n,f)$ prompt fission neutron spectrum over a range of incident neutron energies, tagging fissions with a parallel-plate avalanche counter and measuring prompt fission neutrons and gamma rays with Chi-Nu, a hemispherical array of EJ-309 liquid scintillator detectors [1, 2].
In this work we further analyze the data set from Ref. [2] and present, for the first time, the change in total gamma-ray multiplicity as a function of inducing neutron energy (2 - 20 MeV) in the $^{239}\mathrm{Pu}(n,f)$ reaction. We present corrections to systematic biases that arise from alpha contamination and neutron beam wraparound and then conclude that the multiplicity increases with inducing neutron energy, albeit more gradually than in the ENDF-B/VIII.0 evaluation [3]. This finding suggests that the excitation of the compound $^{240}\mathrm{Pu}^*$ nucleus contributes to excitation of higher rotational modes in the fragments.
[1] C.Y. Wu, et al. "A multiple parallel-plate avalanche counter for fission-fragment detection". Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 794. (2015): 76-79.
[2] Kelly, K. J. et al. "Measurement of the $^{239}\mathrm{Pu}(n,f)$ prompt fission neutron spectrum from 10 keV to 10 MeV induced by neutrons of energy 1–20 MeV". Phys. Rev. C 102. (2020): 034615.
[3] I. Stetcu, et al. "Evaluation of the Prompt Fission Gamma Properties for Neutron Induced Fission of 235,238U and 239Pu". Nuclear Data Sheets 163. (2020): 261-279.