The Carnian Humid Episode of the late Triassic: a review

From 1989 to 1994 a series of papers outlined evidence for a brief episode of climate change from arid to humid, and then back to arid, during the Carnian Stage of the late Triassic Epoch. This time of climate change was compared to marine and terrestrial biotic changes, mainly extinction and then radiation of flora and fauna. Subsequently termed, albeit incorrectly, the Carnian Pluvial Event (CPE) by successive authors, interest in this episode of climatic change has increased steadily, with new evidence being published as well as several challenges to the theory. The exact nature of this humid episode, whether reflecting widespread precipitation or more local effects, as well as its ultimate cause, remains equivocal. Bed-by-bed sampling of the Carnian in the Southern Alps (Dolomites) shows the episode began with a negative carbon isotope excursion that lasted for only part of one ammonoid zone (A. austriacum). However, that the Carnian Humid Episode represents a significantly longer period, both environmentally and biotically, is irrefutable. The evidence is strongest in the European, Middle Eastern, Himalayan, North American and Japanese successions, but not always so clear in South America, Antarctica and Australia. The eruption of the Wrangellia Large Igneous Province and global warming (causing increased evaporation in the Tethyan and Panthalassic oceans) are suggested as causes for the humid episode

βdecays of \u3csup\u3e92\u3c/sup\u3eRb, \u3csup\u3e96gs\u3c/sup\u3eY, and \u3csup\u3e142\u3c/sup\u3eCs measured with the modular total absorption spectrometer and the influence of multiplicity on total absorption spectrometry measurements

Total absorption spectroscopy is a technique that helps obtain reliable β-feeding patterns of complex decays important for nuclear structure and astrophysics modeling as well as decay heat analysis in nuclear reactors. The need for improved measurements of β-feeding patterns from fission decay products has come to the forefront of experiments that use nuclear reactors as a source of antineutrinos. Here we present more detailed results, in particular the β-decay measurements of 96gsY, and demonstrate the impact of the β-delayed γ multiplicity on the overall efficiency of Modular Total Absorption Spectrometer used at Oak Ridge National Laboratory to study the decays of fission products abundant during a nuclear fuel cycle

β -decay study of Kr 94

β decay of neutron-rich nuclide Kr94 was reinvestigated by means of a high resolution on-line mass separator and β-γ spectroscopy. In total 22 γ-ray transitions were assigned to the decay of Kr94, and a new isomeric state was identified. The new information allows us to build detailed levels systematics in a chain of odd-odd rubidium isotopes and draw conclusions on nuclear structure for some of the observed states. The discussed level structure affects the evolution of β-decay half-lives for neutron-rich selenium, krypton, and strontium isotopes

Impact of Modular Total Absorption Spectrometer measurements of β decay of fission products on the decay heat and reactor ν e flux calculation

We report the results of a β-decay study of fission products Br86, Kr89, Rb89, Rb90gs, Rb90m, Kr90, Rb92, Xe139, and Cs142 performed with the Modular Total Absorption Spectrometer (MTAS) and on-line mass-separated ion beams. These radioactivities were assessed by the Nuclear Energy Agency as having high priority for decay heat analysis during a nuclear fuel cycle. We observe a substantial increase in β feeding to high excited states in all daughter isotopes in comparison to earlier data. This increases the average γ-ray energy emitted by the decay of fission fragments during the first 10 000 s after fission of U235 and Pu239 by approximately 2% and 1%, respectively, improving agreement between results of calculations and direct observations. New MTAS results reduce the reference reactor νe flux used to analyze reactor νe interaction with detector matter. The reduction determined by the ab initio method for the four nuclear fuel components, U235, U238, Pu239, and Pu241, amounts to 0.976, 0.986, 0.983, and 0.984, respectively

Decays of the Three Top Contributors to the Reactor ν - e High-Energy Spectrum, Rb 92, y 96gs, and Cs 142, Studied with Total Absorption Spectroscopy

We report total absorption spectroscopy measurements of Rb92, Y96gs, and Cs142 β decays, which are the most important contributors to the high energy ν-e spectral shape in nuclear reactors. These three β decays contribute 43% of the ν-e flux near 5.5 MeV emitted by nuclear reactors. This ν-e energy is particularly interesting due to spectral features recently observed in several experiments including the Daya Bay, Double Chooz, and RENO Collaborations. Measurements were conducted at Oak Ridge National Laboratory by means of proton-induced fission of U238 with on-line mass separation of fission fragments and the Modular Total Absorption Spectrometer. We observe a β-decay pattern that is similar to recent measurements of Rb92, with a ground-state to ground-state β feeding of 91(3)%. We verify the Y96gs ground-state to ground-state β feeding of 95.5(20)%. Our measurements substantially modify the β-decay feedings of Cs142, reducing the β feeding to Ba142 states below 2 MeV by 32% when compared with the latest evaluations. Our results increase the discrepancy between the observed and the expected reactor ν-e flux between 5 and 7 MeV, the maximum excess increases from ∼10% to ∼12%

Complete β -decay pattern for the high-priority decay-heat isotopes i 137 and Xe 137 determined using total absorption spectroscopy

Background: An assessment done under the auspices of the Nuclear Energy Agency in 2007 suggested that the β decays of abundant fission products in nuclear reactors may be incomplete. Many of the nuclei are potentially affected by the so called pandemonium effect and their β-γ decay heat should be restudied using the total absorption technique. The fission products I137 and Xe137 were assigned highest priority for restudy due to their large cumulative fission branching fractions. In addition, measuring β-delayed neutron emission probabilities is challenging and any new technique for measuring the β-neutron spectrum and the β-delayed neutron emission probabilities is an important addition to nuclear physics experimental techniques. Purpose: To obtain the complete β-decay pattern of I137 and Xe137 and determine their consequences for reactor decay heat and νe emission. Complete β-decay feeding includes ground state to ground state β feeding with no associated γ rays, ground state to excited states β transitions followed by γ transitions to the daughter nucleus ground state, and β-delayed neutron emission from the daughter nucleus in the case of I137. Method: We measured the complete β-decay intensities of I137 and Xe137 with the Modular Total Absorption Spectrometer at Oak Ridge National Laboratory. We describe a technique for measuring the β-delayed neutron energy spectrum, which also provides a measurement of the β-neutron branching ratio, Pn. Results: We validate the current Evaluated Nuclear Structure Data File (ENSDF) evaluation of Xe137β decay. We find that major changes to the current ENSDF assessment of I137β-decay intensity are required. The average γ energy per β decay for I137β decay (γ decay heat) increases by 19%, from 1050-1250 keV, which increases the average γ energy per U235 fission by 0.11%. We measure a β-delayed neutron branching fraction for I137β decay of 7.9±0.2(fit)±0.4(sys)% and we provide a β-neutron energy spectrum. Conclusions: The Modular Total Absorption Spectrometer measurements of I137 and Xe137 demonstrate the importance of revisiting and remeasuring complex β-decaying fission products with total absorption spectroscopy. We demonstrate the ability of the Modular Total Absorption Spectrometer to measure β-delayed neutron energy spectra

Long-lived isomeric states and quasiparticle band structures in neutron-rich Gd 162,164 nuclei from β decay

Neutron-rich nuclei Eu162,164 were produced by bombarding a proton beam on a U238 target at the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory and mass separating the Eu162,164 products. New level schemes and new γ-ray transitions of the daughters Gd162,164 were identified from β-decay spectroscopy studies. Half-lives of the Eu162,164 were remeasured to clarify the previous ambiguous results. Two quasiparticle band structures were built and compared with neighboring nuclei. The β and γ bands were extended in Gd162 and a γ band was extended in Gd164. Half-lives of the isomeric states at (6-) 1449 keV in Gd162 and (4-) 1096 keV in Gd164 were measured to be 99(3) μs and 0.56(3) μs, respectively. Projected shell model calculations were performed and found to be in good agreement with all of the experimental data