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

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

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

β-Decay study of neutron-rich bromine and krypton isotopes

Short-lived neutron-rich nuclei including 93Br, 93Kr, and 94Kr were produced in proton-induced fission of 238U at the Holifield Radioactive Ion Beam Facility in Oak Ridge. Their β decay was studied by means of a high-resolution online mass separator and β-γ spectroscopy methods. The half-life of T1/2 = 152(8) ms and β-delayed branching ratio of Pn = 53-8+11% measured for 93Br differs from the previously reported values of T1/2 = 102(10) ms and Pn = 68(7)%. At the same time the half-life of 94Kr T1/2 = 227(14) ms and both the half-life of T1/2 = 1.298(54) s and β-delayed branching ratio of Pn = 1.9-0.2+0.6% of 93Kr are in very good agreement with literature values. The decay properties of 93Br include previously unreported γ transitions following β-delayed neutron emission. © 2013 American Physical Society

New transitions and levels for Tb 163 obtained from β -decay studies

Transitions in Tb163 following β decay of Gd163 were obtained as part of investigations of γ rays emitted following Eu163β decay to Gd163. Detailed analysis of the low-energy structure of Tb163 has been carried out with these data to expand previous β-decay studies and reactions studies of levels in Tb163. Data were collected at the LeRIBSS station of the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory with an array of four Clover HPGe detectors for γ rays and two plastic scintillators for β detection. The γ rays were identified as belonging to Gd163 and Tb163 via mass selection and γ-γ-β, γ-γ, or γ-x-ray coincidence. In total, 38 new γ-ray transitions were observed in Tb163 from 15 newly identified levels and 12 previously identified levels. Potential energy surface calculations were performed which support a rigid prolate deformation. Previously identified unplaced transitions in Tb163 have been placed within the level scheme of Tb163 and additional states and transitions have been identified

A novel experimental system for the KDK measurement of the 40^{40}K decay scheme relevant for rare event searches

Potassium-40 ($^{40}$K) is a long-lived, naturally occurring radioactive isotope. The decay products are prominent backgrounds for many rare event searches, including those involving NaI-based scintillators. $^{40}$K also plays a role in geochronological dating techniques. The branching ratio of the electron capture directly to the ground state of argon-40 has never been measured, which can cause difficulty in interpreting certain results or can lead to lack of precision depending on the field and analysis technique. The KDK (Potassium (K) Decay (DK)) collaboration is measuring this decay. A composite method has a silicon drift detector with an enriched, thermally deposited $^{40}$K source inside the Modular Total Absorption Spectrometer. This setup has been characterized in terms of energy calibration, gamma tagging efficiency, live time and false negatives and positives. A complementary, homogeneous, method is also discussed; it employs a KSr$_2$I$_5$:Eu scintillator as source and detector.Comment: 20 pages, 24 figures, Submitted to NIM

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%