Thermal-neutron fission cross section of 26.1-min235Um

Integral cross section measurement of theU235(n,n′)UBélier1, Bond2, Vieira3 et al. 2015Phys. Rev. C3010View full textAdd to dashboardCite

“…But in some situations, excited states need to be considered and the population of these excited states leads to other reaction channels that can change system behavior. In the case of 235 U, the fission cross section for the first excited state, the 26 minutes isomer, exceeds that of the ground state fission cross section by a factor ~2 at thermal neutron energies [1]. At higher energies (up to ~300 keV) calculations predict the fission cross section for the isomer to be 20-50% smaller than that of the ground state [2].…”

First Measurements with a Lead Slowing-Down Spectrometer at LANSCE

“…An interesting example of a short-lived isomer is the first isomer of uranium 235 with a half-life of 26 min. Until now, only the ratio of the isomer fission cross section to the ground-state fission cross section for thermal and cold neutron energies has been measured [2,3]. In the laboratory, this isomer can be produced from the alpha-decay of 239 Pu with an equilibrium ratio being 2x10 -9 .…”

Measuring (n,f) Cross Sections of Short-Lived States

“…One possible approach would be to chemically purify the plutonium, removing the uranium that has built up, and then repurify, extracting the newly created isomer perhaps 10 minutes later. This is a formidable problem in radiochemistry but has been solved on a small scale [6]. The first purification must completely extract all the uranium, which will have been in the ground state (the tolerable fraction of the uranium remaining is less than the ratio of the isomeric half life to the time since fabrication of the sample, or < 0.0006 for a month-old sample), and the second purification, taking no longer than about 10 minutes, must extract 2 ppb of newly produced (isomeric) uranium from plutonium.…”

“…The first purification must completely extract all the uranium, which will have been in the ground state (the tolerable fraction of the uranium remaining is less than the ratio of the isomeric half life to the time since fabrication of the sample, or < 0.0006 for a month-old sample), and the second purification, taking no longer than about 10 minutes, must extract 2 ppb of newly produced (isomeric) uranium from plutonium. We estimate in Section 5 that to measure the cross-section at MeV energies will require isolation of about 100 times as many isomeric nuclei as achieved by [6] for their measurements at thermal energies.…”

“…Previous attempts to collect 235m U from gas flowing through a tube coated with Pu have not succeeded [6,11], perhaps because diffusion in the carrier gas permitted the 235m U to deposit on unintended surfaces, or because flow of the carrier gas swept it around the intended collector. Free atoms will stick to almost any surface they encounter.…”