Comparing fission-product yields from photon-induced fission of 240Pu and neutron-induced fission of 239Pu as a test of the Bohr hypothesis in nuclear fission

Jack Silano,Anton Tonchev,Nicolas Schunck,Roger Henderson,Sean Finch,Fnu Krishichayan,Matthew Gooden,Calvin Howell,Werner Tornow,T Kawano

doi:10.1051/epjconf/202024201008

Abstract

Fission product yields (FPYs) are a uniquely sensitive probe of the fission process, with well established dependence on the species of nucleus undergoing fission, its excitation energy and spin. Thus FPYs are well suited for testing Bohr’s hypothesis in the context of nuclear fission, which states that the decay of a compound nucleus with a given excitation energy, spin and parity is independent of its formation. Using FPYs, we have performed a new highprecision test of the combined effects of the entrance channel, spin and parity on the fission process from two of the most commonly used particles to induce fission neutrons and photons. The 239 Pu(n,f) reaction at En = 4.6 MeV and the 240 Pu(γ,f) reaction at Eγ = 11.2 MeV were used to produce a 240 Pu∗ compound nucleus with the same excitation energy. The FPYs from these two reactions were measured using quasimonoenergetic neutron beams from the TUNL’s FN tandem Van de Graaff accelerator and quasimonenergetic photon beams from the High Intensity γ-ray Source (HIγS) facility. The FPYs from these two reactions are compared quantitatively for the first time.

Highlights

Since the discovery of nuclear fission in 1938, there has been extensive efforts to develop a theoretical description of the fission mechanism
All fission theory relies on the "Bohr hypothesis", which states that the decay of a compound nucleus for a given excitation energy, spin, and parity is independent of its formation [2]
In this work we propose to test Bohr’s hypothesis in the context of nuclear fission by comparing the fission product yields (FPYs) from a compound 240Pu∗ nucleus produced via two different nuclear reactions: 239Pu(n,f) and 240Pu(γ,f)

Summary

Introduction

Since the discovery of nuclear fission in 1938, there has been extensive efforts to develop a theoretical description of the fission mechanism. An essential assumption of this theory is that the nucleus proceeds through the fission process in a well defined quantum state. The state of the nucleus influences the fission process, while the formation mechanism of the compound nucleus does not. All fission theory relies on the "Bohr hypothesis", which states that the decay of a compound nucleus for a given excitation energy, spin, and parity is independent of its formation [2]. The Bohr hypothesis is a reasonable assumption for nuclear fission, where the timescale for the formation of the compound nucleus (∼ 10−16 seconds) is long compared to the time needed to reach an equilibrium. In this work we propose to test Bohr’s hypothesis in the context of nuclear fission by comparing the fission product yields (FPYs) from a compound 240Pu∗ nucleus produced via two different nuclear reactions: 239Pu(n,f) and 240Pu(γ,f)

Experiment

Dual-fission chambers

Monoenergetic photon beams from the HIγS facility

Detection of γ-rays from the β-decay of fission products

Results and discussion

Conclusion & Future Work