We have observed fallout from the recent Fukushima Dai-ichi reactor accident in samples of rainwater collected in the San Francisco Bay area. Gamma ray spectra measured from these samples show clear evidence of fission products – 131,132I, 132Te, and 134,137Cs. The activity levels we have measured for these isotopes are very low and pose no health risk to the public.
Background: Neutron-induced fission cross-section data are needed in various fields of applied and basic nuclear science. However, cross sections of short-lived nuclei are difficult to measure directly due to experimental constraints. Purpose: The first experimental determination of the neutron-induced fission cross section of 239 Np at nonthermal energies was performed. This minor actinide is the waiting point to 240 Pu production in a nuclear reactor. Method: The surrogate ratio method was employed to indirectly deduce the 239 Np(n, f ) cross section. The surrogate reactions used were 236 U( 3 He, p) and 238 U( 3 He, p) with the reference cross section given by the well-known 237 Np(n, f ) cross section. The ratio of observed fission reactions resulting from the two formed compound nuclei, 238 Np and 240 Np, was multiplied by the directly measured 237 Np(n, f ) cross section to determine the 239 Np(n, f ) cross section. Results: The 239 Np(n, f ) cross section was determined with an uncertainty ranging between 4% and 30% over the energy range of 0.5-20 MeV. The resulting cross section agrees closest with the JENDL-4.0 evaluation.
Conclusions:The measured cross section falls in between the existing evaluations, but it does not match any evaluation exactly (with JENDL-4.0 being the closest match); hence reactor codes relying on existing evaluations may under-or overestimate the amount of 240 Pu produced during fuel burnup. The measurement helps constrain nuclear structure parameters used in the evaluations.
Background: Nuclear excitation by electronic transition (NEET) is a rare nuclear excitation that can occur in isotopes containing a low-lying nuclear excited state. Over the past 40 years, several experiments have attempted to measure NEET of 235 U and those experiments have yielded conflicting results.Purpose: An experiment was performed in order to determine whether NEET of 235 U occurs, and to determine its excitation rate.Method: A pulsed Nd:YAG laser operating at 1064 nm with a pulse energy of 790 mJ and a pulse width of 9 ns was used to generate a uranium plasma. The plasma was collected on a catcher plate and electrons from the catcher plate were accelerated and focused onto a microchannel plate detector. An observation of a decay with a 26 minute half-life would suggest the creation of 235m U and the possibility that NEET of 235 U occurred.Results: A 26 minute decay consistent with the decay of 235m U was not observed and there was no evidence that NEET occurred. An upper limit for the NEET rate of 235 U was determined to be λNEET < 1.8 × 10 −4 s −1 with a confidence level of 68.3%.
Conclusions:The upper limit determined from this experiment is consistent with most of the past measurements. Discrepancies between this experiment and past measurements can be explained by assuming past experiments misinterpreted the data.
Measurements of beta-delayed gamma-ray spectra following 14-MeV neutron-induced fissions of 232 Th, 238 U, and 237 Np were conducted at Lawrence Berkeley National Laboratory's 88-Inch Cyclotron. Spectra were collected for times ranging from 1 minute to 14 hours after irradiation. Intensity ratios of gamma-ray lines were extracted from the data that allow identification of the fissioning isotope.
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