Selenium isotopes record extensive marine suboxia during the Great Oxidation Event
Explore the source record for details and available documents.
Michael A. Kipp↗
SEARCH · Engineering Papers
Results for “SELENIUM ISOTOPES”
Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Localizing the Shape Transition in Neutron-Deficient Selenium
Neutron-deficient selenium isotopes are thought to undergo a rapid shape change from a prolate deformation near the line of beta stability towards oblate deformation around the line of $N=Z$. The point at which this shape change occurs is unknown, with inconsistent predictions from available theoretical models. A common feature in the models is the delicate nature of the point of transition, with the introduction of even a modest spin to the system sufficient to change the ordering of the prolate and oblate configurations. In this paper, we present a measurement of the quadrupole moment of the first-excited state in radioactive $^{72}\mathrm{Se}$—a potential point of transition—by safe Coulomb excitation. This is the first low-energy Coulomb excitation to be performed with a rare-isotope beam at the reaccelerated beam facility at the National Superconducting Cyclotron Laboratory. Finally, by demonstrating a negative spectroscopic quadrupole moment for the first-excited ${2}^{+}$ state, it is found that any low-spin shape change in neutron-deficient selenium does not occur until $^{70}\mathrm{Se}$.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
The LLNL Program in Nuclear Structure and Nuclear Astrophysics (NSNA) (FY19 Annual Report)
Our major efforts in FY18-19 were devoted to carrying out the second GRETINA/CHICO2 campaign at ANL and the JANUS campaign at MSU/NSCL. Among a total of 13 fielded experiments, three were for Ph.D. thesis projects; two students at MSU and one at UNC at Chapel Hill. Two experiments in the JANUS campaign were led by LLNL; namely, the study of nuclear structure in 72 Se and 76 Se using the sub-barrier Coulomb excitation method. The data analysis was completed for both selenium isotopes and the results were published. The 72 Se results have been submitted to DOE/SC/NP as a science highlight and the article won a prestigious LLNL S&T Excellence in Publication Award in FY19. For experiments fielded in FY17 at TRIUMF/ISAC2 using TIGRESS/Bambino, the data analysis was completed recently and the 21 Mg results were published in FY19. For the results on the mirror nuclei 23 Mg and 23 Na, the manuscript was written and is in circulation among the coauthors. We intend to submit it for publication later this calendar year. These efforts address the isospin symmetry in the sd shell nuclei in terms of the electromagnetic properties measured with the sub-barrier Coulomb excitation method. The proposal to upgrade CHICO2 to CHICOX by resizing the detector chamber to be compatible to GRETA was approved in FY17 with a budget spread over three fiscal years starting FY18. The preliminary design was completed but cannot be finalized until the design for the GRETA holding frame is completed by the LBNL GRETA team in FY19. The fabrication will proceed once the physical compatibility of CHICOX to GRETA holding frame is confirmed. The narrative below focuses on the research efforts for which LLNL led or played a key role. This report includes two sections: 1) Evolution of shell structure and collectivity in neutron-rich nuclei, and 2) Investigating nucleosynthesis and the Standard Model with β decay.
42 ENGINEERING↗
Oxidation of Dissolved Tetravalent Selenium by Birnessite: Se Isotope Fractionation and the Effects of pH and Birnessite Structure
Redox reactions control the mobility and bioavailability of selenium (Se) in biogeochemical systems, both modern and ancient. Se isotope ratio measurements (e.g., 82 Se/ 76 Se) have been developed to enhance understanding of biogeochemical transformations and transport of Se. Stable isotope ratios of many elements are known to be powerful indicators of redox reactions, and shifts in 82 Se/ 76 Se have been observed for Se reduction reactions. However, Se isotope shifts caused by naturally relevant oxidation reactions have not been published. Here, we report Se isotope fractionation factors for oxidation of Se(IV) by birnessite. Experiments were conducted at pH = 4.0 and 5.5, with two types of birnessite of contrasting composition at two concentrations of suspended birnessite. The results are consistent with a single 82 Se/ 76 Se fractionation factor, for all times during all experiments, of 0.99767 (±0.0035 2 s.d.). Expressed as ε, the fractionation is 2.33‰ (±0.08‰).
58 GEOSCIENCES↗
TYPE Ia SUPERNOVAE AS SITES OF THE p-PROCESS: TWO-DIMENSIONAL MODELS COUPLED TO NUCLEOSYNTHESIS
Beyond Fe, there is a class of 35 proton-rich nuclides, between {sup 74}Se and {sup 196}Hg, called p-nuclei. They are bypassed by the s and r neutron capture processes and are typically 10-1000 times less abundant than the s- and/or r-isotopes in the solar system. The bulk of p-isotopes is created in the 'gamma processes' by sequences of photodisintegrations and beta decays in explosive conditions in both core collapse supernovae (SNe II) and in Type Ia supernovae (SNe Ia). SNe II contribute to the production of p-nuclei through explosive neon and oxygen burning. However, the major problem in SN II ejecta is a general underproduction of the light p-nuclei for A < 120. We explore SNe Ia as p-process sites in the framework of a two-dimensional SN Ia delayed detonation model as well as pure deflagration models. The white dwarf precursor is assumed to have reached the Chandrasekhar mass in a binary system by mass accretion from a giant/main-sequence companion. We use enhanced s-seed distributions, with seeds directly obtained from a sequence of thermal pulse instabilities both in the asymptotic giant branch phase and in the accreted material. We apply the tracer-particle method to reconstruct the nucleosynthesis by the thermal histories of Lagrangian particles, passively advected in the hydrodynamic calculations. For each particle, we follow the explosive nucleosynthesis with a detailed nuclear reaction network for all isotopes up to {sup 209}Bi. We select tracers within the typical temperature range for p-process production, (1.5-3.7) x 10{sup 9} K, and analyze in detail their behavior, exploring the influence of different s-process distributions on the p-process nucleosynthesis. In addition, we discuss the sensitivity of p-process production to parameters of the explosion mechanism, taking into account the consequences on Fe and alpha elements. We find that SNe Ia can produce a large amount of p-nuclei, both the light p-nuclei below A = 120 and the heavy-p nuclei, at quite flat average production factors, tightly related to the s-process seed distribution. For the first time, we find a stellar source able to produce both light and heavy p-nuclei almost at the same level as {sup 56}Fe, including the debated neutron magic {sup 92,} {sup 94}Mo and {sup 96,} {sup 98}Ru. We also find that there is an important contribution from the p-process nucleosynthesis to the s-only nuclei {sup 80}Kr, {sup 86}Sr, to the neutron magic {sup 90}Zr, and to the neutron-rich {sup 96}Zr. Finally, we investigate the metallicity effect on p-process production in our models. Starting with different s-process seed distributions for two metallicities Z = 0.02 and Z = 0.001, running two-dimensional SN Ia models with different initial composition, we estimate that SNe Ia can contribute to at least 50% of the solar p-process composition. A more detailed analysis of the role of SNe Ia in Galactic chemical evolution of p-nuclei is in preparation.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Nuclear Data Sheets for A = 71
The evaluated spectroscopic data are presented for experimentally known nuclides of mass 71 (Mn,Fe,Co,Ni,Cu,Zn,Ga,Ge,As,Se,Br,Kr). Excited-state data are nonexistent for {sup 71}Mn, {sup 71}Fe, {sup 71}Co and {sup 71}Kr. Significant new data have been added since the last evaluation of A=71 nuclides by M. Bhat (1993Bh02). Excited states are now known in {sup 71}Ni and {sup 71}Cu, and enhanced information about high-spin excitations is available for {sup 71}Ga, {sup 71}As and {sup 71}Br. The decay schemes of radioactive isotopes {sup 71}Mn and {sup 71}Fe are not known at all, and those for {sup 71}Co, {sup 71m}Ni and {sup 71}Kr are not known well. For the decay of {sup 71}Cu, {sup 71g}Ni, and {sup 71}Br, extensive data are available, but in the opinion of the evaluators the decay schemes still seem incomplete. This work supersedes earlier full evaluations of A=71 published by 1988Bh01, 1979Ke06 and 1973Al33, and the last one published in 'update' mode by 1993Bh02.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
OPPORTUNITIES TO CONSTRAIN ASTROPHYSICAL REACTION RATES FOR THE s-PROCESS VIA DETERMINATION OF THE GROUND-STATE CROSS-SECTIONS
Modern models of s-process nucleosynthesis in stars require stellar reaction rates of high precision. Most neutron-capture cross-sections in the s-process have been measured, and for an increasing number of reactions the required precision is achieved. This does not necessarily mean, however, that the stellar rates are constrained equally well, because only the capture of the ground state of a target is measured in the laboratory. Captures of excited states can contribute considerably to stellar rates that are already at typical s-process temperatures. We show that the ground-state contribution X to a stellar rate is the relevant measure to identify reactions that are or could be well constrained by experiments and apply it to (n,{gamma}) reactions in the s-process. We further show that the maximum possible reduction in uncertainty of a rate via determination of the ground-state cross-section is given directly by X. An error analysis of X is presented, and it is found that X is a robust measure with mostly small uncertainties. Several specific examples (neutron capture of {sup 79}Se, {sup 95}Zr, {sup 121}Sn, {sup 187}Os, and {sup 193}Pt) are discussed in detail. The ground-state contributions for a set of 412 neutron-capture reactions around the s-process path are presented in a table. This allows identification of reactions that may be better constrained by experiments and that cannot be constrained solely by measuring ground-state cross-sections (and thus require supplementary studies). General trends and implications are discussed.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Real-Time XANES Measurement of Se Reduction by Zerovalent Iron in a Flow-through Cell, and Accompanying Se Isotope Measurements
An anoxic flow-through cell experiment was conducted to examine mechanisms controlling the real-time reduction of selenate (Se(VI)) by zerovalent iron (ZVI), which is commonly used in permeable reactive barriers to treat dissolved contaminants including Se(VI). Changes in selenium (Se) isotope composition were examined by increasing the influent Se concentration over time, thus changing the proportion of Se removed from solution. At the conclusion of the experiment, an anoxic Se-free solution was pumped through the cell to assess the stability of the reaction products. At all stages, X-ray absorption data were obtained from the solid phase and Se isotope data from the aqueous phase. Reduced Se in the form of adsorbed Se(IV), Fe 2 SeO 4 , Se(0), and iron selenides accumulated on the ZVI over time. A linear regression function was fit to the $δ$ 82/76 Se values of the effluent, yielding an isotopic separation of 9.6‰. A Rayleigh curve was fit to the isotope data from the effluent samples collected during the rinse stage with an effective fractionation of 2.4‰. The results from this experiment can be used to elucidate the effect of multiple concurrent mechanisms on Se isotope behavior.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Extrapolation method in the Monte Carlo Shell Model and its applications
We demonstrate how the energy-variance extrapolation method works using the sequence of the approximated wave functions obtained by the Monte Carlo Shell Model (MCSM), taking {sup 56}Ni with pf-shell as an example. The extrapolation method is shown to work well even in the case that the MCSM shows slow convergence, such as {sup 72}Ge with f5pg9-shell. The structure of {sup 72}Se is also studied including the discussion of the shape-coexistence phenomenon.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Energy Density Functional Study of Nuclear Matrix Elements for Neutrinoless {beta}{beta} Decay
We present an extensive study of nuclear matrix elements (NME) for the neutrinoless double-beta decay of the nuclei {sup 48}Ca, {sup 76}Ge, {sup 82}Se, {sup 96}Zr, {sup 100}Mo, {sup 116}Cd, {sup 124}Sn, {sup 128}Te, {sup 130}Te, {sup 136}Xe, and {sup 150}Nd based on state-of-the-art energy density functional methods using the Gogny D1S functional. Beyond-mean-field effects are included within the generating coordinate method with particle number and angular momentum projection for both initial and final ground states. We obtain a rather constant value for the NMEs around 4.7 with the exception of {sup 48}Ca and {sup 150}Nd, where smaller values are found. We analyze the role of deformation and pairing in the evaluation of the NME and present detailed results for the decay of {sup 150}Nd.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Mechanisms of neutrinoless double-beta decay: A comparative analysis of several nuclei
The neutrinoless double beta decay of several nuclei that are of interest from the experimental point of view ({sup 76}Ge, {sup 82}Se, {sup 100}Mo, {sup 130}Te, and {sup 136}Xe) is investigated on the basis of a general Lorentzinvariant effective Lagrangian describing physics effects beyond the Standard Model. The half-lives and angular-correlation coefficients for electrons are calculated for various decay mechanisms associated, in particular, with the exchange of Majorana neutrinos, supersymmetric particles (with R-parity violation), leptoquarks, and right-handed W{sub R} bosons. The effect of theoretical uncertainties in the values of relevant nuclear matrix elements on decay features is considered.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Direct Mass Measurements of Short-Lived A=2Z-1 Nuclides {sup 63}Ge, {sup 65}As, {sup 67}Se, and {sup 71}Kr and Their Impact on Nucleosynthesis in the rp Process
Mass excesses of short-lived A=2Z-1 nuclei {sup 63}Ge, {sup 65}As, {sup 67}Se, and {sup 71}Kr have been directly measured to be -46 921(37), -46 937(85), -46 580(67), and -46 320(141) keV, respectively. The deduced proton separation energy of -90(85) keV for {sup 65}As shows that this nucleus is only slightly proton unbound. X-ray burst model calculations with the new mass excess of {sup 65}As suggest that the majority of the reaction flow passes through {sup 64}Ge via proton capture, indicating that {sup 64}Ge is not a significant rp-process waiting point.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Sulfur-functionalized solid-phase materials for the selective separation of arsenic and selenium
Radioactive arsenic (As) isotopes are of growing interest for applications in nuclear medicine, national security, and environmental research. Recent efforts at the Facility for Rare Isotope Beams (FRIB) have focused on aqueous harvesting of selenium-72,73 ( 72,73 Se) and their daughter isotopes, arsenic-72,73 ( 72,73 As), which are particularly valuable for medical applications and nuclear data studies, respectively. Both conventional isotope production and harvesting methods require chemical separations to purify radioactive As from parent and co-produced Se radioisotopes. While several solid-phase separation methods for As and Se exist, many depend on complex oxidation state control or highly acidic conditions. This study presents results for sulfur-based solid-phase materials selected to enable uptake at lower acidity and eliminate the need for intricate redox chemistry. Specifically, the performance of three covalently bound sulfur-based ligands were evaluated: (1) thiophenol-polystyrene, (2) propanethiol-silica, and (3) thiourea-silica. Uptake characteristics—including distribution coefficients (Dw), kinetics, and column separation behavior—were assessed using 75 Se and 73 As in hydrochloric (HCl) acid and nitric (HNO 3 ) solutions. The resins demonstrated high-yield (>95%) and high-purity As recovery across a range of HCl concentrations. Comparable results in HNO 3 were achieved when combined with anion exchange chromatography. Furthermore, the potential application of these materials for medical isotope generators was also investigated through ligand stability and repeated elution studies. Overall, sulfur leaching from the resins was negligible at the concentrations relevant for these separations but increased with higher acid concentrations.
Arsenic↗
Low Radioactive Techniques in SuperNEMO: Status of the Radon R and D
Radon is a well-known source of background with respect to the search for neutrinoless double beta decay (0{nu}{beta}{beta}), due to the high Q{sub {beta}} value of one of its daughter nucleus {sup 214}Bi. Radon has been observed and reduced down to 6.5 mBq/m{sup 3} in the NEMO-3 experiment which is looking for the 0{nu}{beta}{beta} process in {sup 100}Mo and in six other isotopes. The SuperNEMO project, a next-generation double beta decay experiment which will also use a tracko-calorimeter technique, has been in an R and D phase since 2006. The goal is to reach a sensitivity of T{sub 1/2}(0{nu})>10{sup 26} y corresponding to an effective Majorana neutrino mass of 0.05-0.1 eV with 100 kg of {sup 82}Se. Such a sensitivity requires in particular to improve the radon radiopurity down to 0.1 mBq/m{sup 3} in the tracking chamber.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Microscopic description of large-amplitude shape-mixing dynamics with inertial functions derived in local quasiparticle random-phase approximation
On the basis of the adiabatic self-consistent collective coordinate method, we develop an efficient microscopic method of deriving the five-dimensional quadrupole collective Hamiltonian and illustrate its usefulness by applying it to the oblate-prolate shape coexistence/mixing phenomena in proton-rich {sup 68,70,72}Se. In this method, the vibrational and rotational collective masses (inertial functions) are determined by local normal modes built on constrained Hartree-Fock-Bogoliubov states. Numerical calculations are carried out using the pairing-plus-quadrupole Hamiltonian including the quadrupole-pairing interaction within the two major-shell active model spaces both for neutrons and protons. It is shown that the time-odd components of the moving mean-field significantly increase the vibrational and rotational collective masses in comparison with the Inglis-Belyaev cranking masses. Solving the collective Schroedinger equation, we evaluate excitation spectra, quadrupole transitions, and moments. The results of the numerical calculation are in excellent agreement with recent experimental data and indicate that the low-lying states of these nuclei are characterized as an intermediate situation between the oblate-prolate shape coexistence and the so-called {gamma} unstable situation where large-amplitude triaxial-shape fluctuations play a dominant role.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Isospin symmetry breaking at high spins in the mirror pair {sup 67}Se and {sup 67}As
Recent experimental data have revealed large mirror energy differences (MED) between high-spin states in the mirror nuclei {sup 67}Se and {sup 67}As, the heaviest pair where MED have been determined so far. The MED are generally attributed to the isospin symmetry breaking caused by the Coulomb force and by the isospin-nonconserving part of the nucleon-nucleon residual interaction. The different contributions of the various terms have been extensively studied in the fp shell. By employing large-scale shell-model calculations, we show that the inclusion of the g{sub 9/2} orbit causes interference between the electromagnetic spin-orbit and the Coulomb monopole radial terms at high spin. The large MED are attributed to the aligned proton pair excitations from the p{sub 3/2} and f{sub 5/2} orbits to the g{sub 9/2} orbit. The relation of the MED to deformation is discussed.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Chalcogen isotopes reveal limited volatile contribution from late veneer to Earth
The origin of Earth’s volatile elements is highly debated. Comparing the chalcogen isotope ratios in the bulk silicate Earth (BSE) to those of its possible building blocks, chondritic meteorites, allows constraints on the origin of Earth’s volatiles; however, these comparisons are complicated by potential isotopic fractionation during protoplanetary differentiation, which largely remains poorly understood. Using first-principles calculations, we find that core-mantle differentiation does not notably fractionate selenium and tellurium isotopes, while equilibrium evaporation from early planetesimals would enrich selenium and tellurium in heavy isotopes in the BSE. The sulfur, selenium, and tellurium isotopic signatures of the BSE reveal that protoplanetary differentiation plays a key role in establishing most of Earth’s volatile elements, and a late veneer does not substantially contribute to the BSE’s volatile inventory.
58 GEOSCIENCES↗
β -decay study of Kr 94
β decay of neutron-rich nuclide 94 Kr 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 94 Kr, 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. Furthermore, the discussed level structure affects the evolution of β-decay half-lives for neutron-rich selenium, krypton, and strontium isotopes.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗