Abstract
The Bohr hypothesis, one of the most fundamental assumptions in nuclear fission theory, states that the decay of a compound nucleus with a given excitation energy, spin and parity is independent of its formation. Using fission product yields (FPYs) as a sensitive probe, we have performed new high precision test of the combined effects of the entrance channel, spin and parity on the fission process. Two different reactions were used in a self-consistent manner to produce a compound 240Pu nucleus with the same excitation energy: neutron induced fission of 239Pu at En = 4.6 MeV and photon-induced fission of 240Pu at Eγ = 11.2 MeV. 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 (HlγS) facility. The first results comparing the FPYs from these two reactions will be presented. Implications for validating the Bohr hypothesis will be discussed.
Highlights
In 1939, Niels Bohr and John Wheeler formulated a theory of neutron-induced nuclear fission based on the hypothesis of the compound nucleus [1]
The "Bohr hypothesis" is at the heart of every current theoretical fission model; it states that the decay of a compound nucleus for a given excitation energy, spin, and parity is independent of its formation [2]
We compared the fission product yields (FPYs) of the same 240Pu compound nucleus produced by two different reactions: (i) n+239Pu and (ii) γ+240Pu
Summary
In 1939, Niels Bohr and John Wheeler formulated a theory of neutron-induced nuclear fission based on the hypothesis of the compound nucleus [1]. The "Bohr hypothesis" is at the heart of every current theoretical fission model; it states that the decay of a compound nucleus for a given excitation energy, spin, and parity is independent of its formation [2]. There is sufficient time for the compound nucleus to "forget" the details of its origin. This hypothesis is widely accepted by theorists and experimentalists, its implications (especially concerning fission product yields) have never been experimentally verified precisely via direct comparison of fission observables. We compared the fission product yields (FPYs) of the same 240Pu compound nucleus produced by two different reactions: (i) n+239Pu and (ii) γ+240Pu. The two Pu isotopes provided a valuable set of nuclides for use in investigating the influence of the spin and parity of the compound nucleus on the detailed fission product yields distribution, including shell and pairing effects and fission dynamics. The only difference between these two reactions was a small mismatch in the spin and parity distributions of the 240Pu
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