Interpretation of mercury (Hg) geochemistry in environmental systems remains a challenge. This is largely associated with the inability to identify specific Hg transformation processes and species ...using established analytical methods in Hg geochemistry (total Hg and Hg speciation). In this study, we demonstrate the improved Hg geochemical interpretation, particularly related to process tracing, that can be achieved when Hg stable isotope analyses are complemented by a suite of more established methods and applied to both solid- (soil) and liquid-phases (groundwater) across two Hg
2+
-chloride (HgCl
2
) contaminated sites with distinct geological and physicochemical properties. This novel approach allowed us to identify processes such as Hg
2+
(
i.e.
, HgCl
2
) sorption to the solid-phase, Hg
2+
speciation changes associated with changes in groundwater level and redox conditions (particularly in the upper aquifer and capillary fringe), Hg
2+
reduction to Hg
0
, and dark abiotic redox equilibration between Hg
0
and Hg(
ii
). Hg stable isotope analyses play a critical role in our ability to distinguish, or trace, these
in situ
processes. While we caution against the non-critical use of Hg isotope data for source tracing in environmental systems, due to potentially variable source signatures and overprinting by transformation processes, our study demonstrates the benefits of combining multiple analytical approaches, including Hg isotope ratios as a process tracer, to obtain an improved picture of the enigmatic geochemical behavior and fate of Hg at contaminated legacy sites.
A holistic multi-analyses (led by Hg stable isotope analysis), multi-media, multi-site approach to improving contaminated site Hg geochemistry, particularly process tracing.
Reduced transition probabilities have been extracted between excited, yrast states in the N=Z+2 nucleus 94Pd. The transitions of interest were observed following decays of the Iπ=14+, Ex=2129-keV ...isomeric state, which was populated following the projectile fragmentation of a 124Xe primary beam at the GSI Helmholtzzentrum für Schwerionenforschung accelerator facility as part of FAIR Phase-0. Experimental information regarding the reduced E2 transition strengths for the decays of the yrast 8+ and 6+ states was determined following isomer-delayed Eγ1−Eγ2−△T2,1 coincidence method, using the LaBr3(Ce)-based FATIMA fast-timing coincidence gamma-ray array, which allowed direct determination of lifetimes of states in 94Pd using the Generalized Centroid Difference (GCD) method. The experimental value for the half-life of the yrast 8+ state of 755(106) ps results in a reduced transition probability of B(E2:8→+6+) = 205−25+34 e2fm4, which enables a precise verification of shell-model calculations for this unique system, lying directly between the N=Z line and the N=50 neutron shell closure. The determined B(E2) value provides an insight into the purity of (g9/2)n configurations in competition with admixtures from excitations between the (lower) N=3pf and (higher) N=4gds orbitals for the first time. The results indicate weak collectivity expected for near-zero quadrupole deformation and an increasing importance of the T=0 proton-neutron interaction at N=48.
The population of isomeric states in the prompt decay of fission fragments-so-called isomeric yield ratios (IYRs)-is known to be sensitive to the angular momentum J that the fragment emerged with, ...and may therefore contain valuable information on the mechanism behind the fission process. In this work, we investigate how changes in the fissioning system impact the measured IYRs of fission fragments to learn more about what parameters affect angular momentum generation. To enable this, a new technique for measuring IYRs is first demonstrated. It is based on the time of arrival of discrete gamma rays, and has the advantage that it enables the study of the IYR as a function of properties of the partner nucleus. This technique is used to extract the IYR of 134Te, strongly populated in actinide fission, from the three different fissioning systems: 232Th(n, f), 238U(n, f), at two different neutron energies, as well as 252Cf(sf). The impacts of changing the fissioning system, the compound nuclear excitation energy, the minimum J of the binary partner, and the number of neutrons emitted on the IYR of 134Te are determined. The decay code TALYS is used in combination with the fission simulation code FREYA to calculate the primary fragment angular momentum from the IYR. We find that the IYR of 134Te has a slope of 0.004 +/- 0.002 with increase in compound nucleus (CN) mass. When investigating the impact on the IYR of increased CN excitation energy, we find no change with an energy increase similar to the difference between thermal and fast fission. By varying the mass of the partner fragment emerging with 134Te, it is revealed that the IYR of 134Te is independent of the total amount of prompt neutrons emitted from the fragment pair. This indicates that neutrons carry minimal angular momentum away from the fission fragments. Comparisons with the FREYA+TALYS simulations reveal that the average angular momentum in 134Te following 238U(n, f) is 6.0 h over bar . This is not consistent with the value deduced from recent CGMF calculations. Finally, the IYR sensitivity to the angular momentum of the primary fragment is discussed. These results are not only important to help understanding the underlying mechanism in nuclear fission, but can also be used to constrain and benchmark fission models, and are relevant to the gamma -ray heating problem of reactors.
In order to develop Ca isotopes as a tracer for biogeochemical Ca cycling in terrestrial environments and for Ca utilisation in plants, stable calcium isotope ratios were measured in various species ...of alpine plants, including woody species, grasses and herbs. Analysis of plant parts (root, stem, leaf and flower samples) provided information on Ca isotope fractionation within plants and seasonal sampling of leaves revealed temporal variation in leaf Ca isotopic composition. There was significant Ca isotope fractionation between soil and root tissue △44/42 Caroot–soil ≈ –0.40‰ in all investigated species, whereas Ca isotope fractionation between roots and leaves was species dependent. Samples of leaf tissue collected throughout the growing season also highlighted species differences: Ca isotope ratios increased with leaf age in woody species but remained constant in herbs and grasses. The Ca isotope fractionation between roots and soils can be explained by a preferential binding of light Ca isotopes to root adsorption sites. The observed differences in whole plant Ca isotopic compositions both within and between species may be attributed to several potential factors including root cation exchange capacity, the presence of a woody stem, the presence of Ca oxalate, and the levels of mycorrhizal infection. Thus, the impact of plants on the Ca biogeochemical cycle in soils, and ultimately the Ca isotope signature of the weathering flux from terrestrial environments, will depend on the species present and the stage of vegetation succession.
We report on spectroscopic information and lifetime measurements in the neutron-rich 135,137,139I isotopes. This is the first lifetime data on iodine isotopes beyond N=82. Excited states were ...populated in fast neutron-induced fission of 238U at the ALTO facility of IJCLab with the LICORNE neutron source and detected using the hybrid ν-ball spectrometer. The level schemes of the 135,137,139I isotopes are revised in terms of excited states with up to maximum spin-parity of (33/2+), populated for the first time in fast neutron-induced fission. We provide first results on the lifetimes of the (9/2+1) and (13/2+1) states in 137I and 139I, and the (17/2+1) state in 137I. In addition, we give upper lifetime limits for the (11/2+1) states in 135−139I, the (15/2+1) state in 137I, the (17/2+1) state in 139I, and reexamine the (29/2+1) state in 137I. The isomeric data in 135I are reinvestigated, such as the previously known (15/2+1) and (23/2−1) isomers with T1/2 of 1.64(14) and 4.6(7) ns, respectively, as obtained in this work. The new spectroscopic information is compared to that from spontaneous or thermal-neutron induced fission and discussed in the context of large scale shell-model (LSSM) calculations for the region beyond 132Sn, indicating the behavior of collectivity for the three valence-proton iodine chain with N=82,84,86.
The Zirconium isotopes across the N=56,58 neutron subshell closures have been of special interest since years, sparked by the near doubly-magic features of
96
Zr and the subsequent rapid onset of ...collectivity with a deformed ground-state structure already in
100
Zr. Recent state-of-the-art shell model approaches did not only correctly describe this shape-phase transition in the Zr isotopic chain, but alsothe coexistence of non-collective structures and pronounced collectivity especially
in
96,98
Zr. Theisotope
98
Zr is located on the transition from spherical to deformed ground state structures. We summarize recent experimental work to obtain the B(E2) excitation strengths of the first 2
+
state of
98
Zr, including a new experiment employing the recoil-distance Doppler-shift method following a two-neutron transfer reaction.
Nuclei are complex quantum objects due to complex nucleon-nucleon interactions. They can undergo rather rapid changes in structure as a function of nucleon number. A well known region of such a shape ...transition is the rare-earth region around N = 90, where accessible nuclei range from spherical nuclei at the closed neutron shell at N = 82 to deformed nuclei. For a better understanding of this phenomenon, it is of interest to study empirical signatures like the E2 transition strength \(B(E2;{2}_{1}^{+}\to {0}_{1}^{+})\) or the E0 excitation strength \({\rho }^{2}(E0;{0}_{1}^{+}\to {0}_{2}^{+})\). The nuclide 152Gd with 88 neutrons is located close to the quantum phase transition at N = 90. The lifetime \(\tau ({0}_{2}^{+})\) of 152Gd has been measured using fast electronic scintillation timing (FEST) with an array of HPGe- and LaBr3- detectors. Excited states of 152Gd were populated via an (α,n)-reaction on a gold-backed 149Sm target. The measured lifetime of \(\tau ({0}_{2}^{+})=96(6)\text{ps}\) corresponds to a reduced transition strength of \(B(E2;{0}_{2}^{+}\to {2}_{1}^{+})=111(7)\) W.u. and an E0 transition strength of ρ 2(E0) = 39(3) · 10−3 to the ground state. This result provides experimental support for the validity of a correlation between E0 and E2 strengths that is a novel indicator for a quantum phase transition. This work was published as J. Wiederhold et al., Phys. Rev. C 94, 044302 (2016).
Iron isotope fractionation during dissolution of goethite (α-FeOOH) was studied in laboratory batch experiments. Proton-promoted (HCl), ligand-controlled (oxalate dark), and reductive (oxalate light) ...dissolution mechanisms were compared in order to understand the behavior of iron isotopes during natural weathering reactions. Multicollector ICP-MS was used to measure iron isotope ratios of dissolved iron in solution. The influence of kinetic and equilibrium isotope fractionation during different time scales of dissolution was investigated. Proton-promoted dissolution did not cause iron isotope fractionation, concurrently demonstrating the isotopic homogeneity of the goethite substrate. In contrast, both ligand-controlled and reductive dissolution of goethite resulted in significant iron isotope fractionation. The kinetic isotope effect, which caused an enrichment of light isotopes in the early dissolved fractions, was modeled with an enrichment factor for the 57Fe/54Fe ratio of −2.6‰ between reactive surface sites and solution. Later dissolved fractions of the ligand-controlled experiments exhibit a reverse trend with a depletion of light isotopes of ∼0.5‰ in solution. We interpret this as an equilibrium isotope effect between Fe(III)−oxalate complexes in solution and the goethite surface. In conclusion, different dissolution mechanisms cause diverse iron isotope fractionation effects and likely influence the iron isotope signature of natural soil and weathering environments.
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