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Synthesis, Structural Characterization, and Gas-Phase Unimolecular Reactivity of the Silver Hydride Nanocluster [Ag3((PPh2)2CH2)3(μ3-H)](BF4)2.
No full textA bis(diphenylphosphino)methane-ligated trinuclear silver hydride nanocluster, [Ag3((Ph2P)2CH2)3(μ3-H)](BF4)2, featuring three silver(I) ions coordinated to a μ3-hydride, and its deuteride analogue, [Ag3((Ph2P)2CH2)3(μ3-D)](BF4)2, have been isolated and structurally characterized using electrospray ionization mass spectrometry (ESI-MS), X-ray crystallography, NMR and IR spectroscopy. The position of the deuteride in [Ag3((Ph2P)2CH2)3(μ3-D)](BF4)2 was determined by neutron diffraction. ESI-MS of [Ag3L3(μ3-H/D)](BF4)2 [L = ((Ph2P)2CH2)2] produces [Ag3L3(μ3-H/D)]2+ and [Ag3L3(μ3-H/D)(BF4)]+. A rich gas-phase ion chemistry of [Ag3L3(μ3-H/D)]2+ is observed under conditions of collision-induced dissociation (CID) and electron-capture dissociation (ECD). CID gives rise to the following complementary ion pairs: [Ag3L2]+ and [L+(H/D)]+; [Ag2(H/D)L2]+ and [AgL]+; [Ag2(H/D)L]+ and [AgL2]+. ECD gives rise to a number of dissociation channels including loss of the bis(phosphine) ligand, fragmentation of a coordinated bis(phosphine) ligand via C–P bond activation, and loss of a hydrogen (deuterium) atom with concomitant formation of [Ag3L3]+. Under CID conditions, [Ag3L3(μ3-H/D)(BF4)]+ fragments via ligand loss, the combined loss of a ligand and [H,B,F4], and cluster fragmentation to give [Ag2(BF4)L2]+ and [Ag2(L-H)L]+ [where (L-H) = (Ph2P)2CH–]. © 2014, American Chemical Societ
A novel silicon microdosimeter using 3D sensitive volumes: modeling the response in neutron fields typical of aviation
No full textA 4th generation silicon microdosimeter has been designed by the Centre for Medical Radiation Physics (CMRP) at the University of Wollongong using three dimensional (3D) Sensitive Volumes (SVs). This new microdosimeter design has the advantage of well-defined 3D SVs as well as the elimination of lateral charge diffusion by removal of silicon laterally adjacent to the 3D SVs. The gaps between the sensitive volumes are to be backfilled with PolyMethyl MethAcrylate (PMMA) to produce a surrounding tissue equivalent medium. The advantage of this design avoids the generation of secondary particles from inactive silicon lateral to SVs. The response of the microdosimeter to the neutron field from 252Cf, Pu-Be sources and an avionic radiation environment were simulated using the Geant4 Monte Carlo toolkit for design optimisation. The simulated energy deposition in the SVs from the neutron fields and microdosimetric spectra is presented. The simulation study shows a significant reduction in silicon nuclear recoil contribution to the energy deposition for the novel microdosimeter design. The reduction of silicon recoil events from outside of the SV's will consequently reduce the uncertainty in the calculated dose equivalent. The simulations have demonstrated that a 3D silicon microdosimeter surrounded by PMMA can produce microdosimetric spectra similar to those of a tissue equivalent microdosimeter.© 2014, IEEE
Leaching behaviour of and Cs disposition in a UMo powellite glass–ceramic
No full textA UMo powellite glass–ceramic designed by French workers to immobilise Mo-rich intermediate-level waste was found to be quite leach resistant in water at 90 °C with the dissolution of Cs, Mo, Na, B and Ca not exceeding 2 g/L in normalised PCT tests. 133Cs solid state nuclear magnetic resonance and scanning electron microscopy (SEM) showed the Cs to inhabit the glass phase. The microstructures were not greatly affected by cooling rates between 1 and 5 °C/min or by introducing 10 times as much Cs and Sr. Protracted leach tests at 90 °C showed surface alteration as evidenced by SEM and particularly transmission electron microscopy; the main alteration phase was a Zn aluminosilicate but several other alteration phases were evident. Voidage in the alteration layers was indicated from enhanced lifetimes in po sitron annihilation lifetime spectroscopy. © 2014, Elsevier B.V
He and Au ion radiation damage in sodalite, Na4Al3Si3O12Cl
No full textSodalite, a candidate ceramic for the immobilisation of pyroprocessing nuclear waste, showed no observable lattice dilatation in grazing incidence X-ray diffraction when irradiated with up to 1017 5 MeV He ions/cm2. However micro-Raman scattering showed considerable spectral broadening characteristic of radiation damage near the end of the ∼22 μm He range. Partial amorphism plus nepheline formation was observed in grazing incidence X-ray diffraction when sodalite was irradiated by 1016 12 MeV Au ions/cm2. Nepheline appeared less susceptible to 12 MeV Au ion damage than sodalite, with ∼25% less amorphous fraction at 1016 ions/cm2. © 2014, Elsevier B.V
Observations on the Zirconium Hydride Precipitation and Distribution in Zircaloy-4
No full textHydride precipitation and distribution in hot-rolled and annealed Zircaloy-4 plate samples artificially induced by gaseous hydrogen charging were studied primarily by neutron
tomography, scanning electron microscopy (SEM), and SEM-based electron backscattered
diffraction techniques. The precipitated hydride platelet (d-ZrH1.66) at a hydrogen pressure of 20 atm was found following the {111}d-ZrH1.66//(0001)a-Zr with the surrounding a-Zr
matrix. The microstructural characterization indicated that hydrides with a relatively uniform
distribution were precipitated on the rolling-transverse section of the plate, whereas, on the normal-transverse section, a hydride concentration gradient was present with a dense
hydride layer near the surface. Further, the neutron tomography investigations clearly
identified the nonuniform spatial distribution of hydrides. Thin hydride layers preferentially
formed on the sample surface, and the concentrated hydrides precipitating at the edges/corner of the sample were observed. The causes for the localized hydride accumulation werealso discussed. © 2014, The Minerals, Metals & Materials Society and ASM International 2013
Microstructure and texture analysis of [delta]-hydride precipitation in Zircaloy-4 materials by electron microscopy and neutron diffraction
No full textThis work presents a detailed microstructure and texture study of various hydrided Zircaloy-4 materials by neutron diffraction and microscopy. The results show that the precipitated [delta]-ZrH1.66 generally follows the [delta](111)//[alpha](0001) and [delta][1{\overline 1}0]//[alpha][11{\overline 2}0] orientation relationship with the [alpha]-Zr matrix. The [delta]-hydride displays a weak texture that is determined by the texture of the [alpha]-Zr matrix, and this dependence essentially originates from the observed orientation correlation between [alpha]-Zr and [delta]-hydride. Neutron diffraction line profile analysis and high-resolution transmission electron microscopy observations reveal a significant number of dislocations present in the [delta]-hydride, with an estimated average density one order of magnitude higher than that in the [alpha]-Zr matrix, which contributes to the accommodation of the substantial misfit strains associated with hydride precipitation in the [alpha]-Zr matrix. The present observations provide an insight into the behaviour of [delta]-hydride precipitation in zirconium alloys and may help with understanding the induced embrittling effect of hydrides.© 2014, International Union of Crystallography
Recent developments in the modelling of radionuclide uptake, radiation dose and effects in wildlife
No full textOf the ~600 scientific publications on the Fukushima event, more than 80% relate to themes of transport of radionuclides in environmental media, transfer to wildlife and foodstuffs, and dose to environmental receptors. This focus reflects a continued need for development and harmonisation of radiological modelling approaches such as has been underway through recent IAEA and ICRP initiatives (e.g. EMRAS I and II, MODARIA).
Key developments in improving the understanding of uptake of radionuclides in wildlife include establishing the Wildlife Transfer Parameter Database and related IAEA handbook on transfer to wildlife. These sources provide access to a comprehensive collection of transfer parameters, including input from Australian sources (www.wildlifetransferdatabase.org). Key improvements were highlighted in a recent Journal of Environmental Radioactivity special issue (Vol. 121).
Dose modelling for wildlife continues to be challenged by the high diversity of biotic types (plankton to whales) and the breadth of exposure scenarios in diverse ecosystems. Modelling codes (e.g. ERICA Tool, RESRAD-Biota) are undergoing updates of their transfer parameters, improvement of capabilities such as probabilistic analysis (e.g. Monte Carlo), and harmonization of approaches through IAEA model testing exercises (e.g., Little Forest Burial Ground biota dose modelling assessment). A recent development has been the use of voxel dosimetry approaches which build on the standard simplified ellipsoid approach by modelling the absorbed doses in individual organs.
Recent improvements in defining dose effects to environmental receptors have focused on updating the FREDERICA Radiation Effects Database. The more comprehensive data have allowed for the updating/development of new Species Sensitivity Distributions that better support the benchmark values for potential dose effects, and for improving estimation of population effects (rather than individuals) upon which the environmental protection strategies are based
The tricontinuous 3ths(5) phase: a new morphology in copolymer melts
No full textSelf-assembly remains the most efficient route to the formation of ordered nanostructures, including the double gyroid network phase in diblock copolymers based on two intergrown network domains. Here we use self-consistent field theory to show that a tricontinuous structure with monoclinic symmetry, called 3ths(5), based on the intergrowth of three distorted ths nets, is an equilibrium phase of triblock star-copolymer melts when an extended molecular core is introduced. The introduction of the core enhances the role of chain stretching by enforcing larger structural length scales, thus destabilizing the hexagonal columnar phase in favor of morphologies with less packing frustration. This study further demonstrates that the introduction of molecular cores is a general concept for tuning the relative importance of entropic and enthalpic free energy contributions, hence providing a tool to stabilize an extended repertoire of self-assembled nanostructured materials. © 2014, American Chemical Society
Land-use affects the radiocarbon age, storage and depth distribution of soil organic carbon in Eastern Australia
Get PDFLand-use has been shown to affect soil organic carbon (SOC) storage, with natural systems generally storing larger quantities of SOC than anthropogenically managed systems in surface soils. However, these effects are often
difficult to detect deeper in the soil profile. Little is known regarding the effects of land-use on the radiocarbon
age of SOC, both at the surface and deeper in the soil profile. We investigated the storage, radiocarbon content
and depth distribution of soil organic carbon from across the state of NSW, Australia. A total of 100 profiles
were analysed for total SOC concentration at numerous depths (up to 1 m) and a machine learning approach
implementing tree ensemble methods was used to identify the key drivers of SOC depth distribution. Surface SOC
storage was strongly associated with climate (predominately precipitation, to a lesser degree relative humidity and
temperature), whereas SOC depth distribution was predominately influenced by land-use, soil type and to a lesser
extent temperature. A subset of 12 soil profiles from a range of climate zones were analysed for radiocarbon content
with a view to contrasting three land-use systems: natural, cleared/grazed and cropped. Radiocarbon content was
affected strongly by land-use, with effects most pronounced at depth. Native systems appeared to have the youngest
carbon throughout the profile, with cropped and grazed systems having older SOC. Radiocarbon content was also
strongly associated with SOC content. Our results indicate that natural systems act as a carbon pump into the soil,
injecting young, fresh organic carbon which is vertically distributed throughout the profile. In contrast, managed
systems are deprived of this input and are depleted in SOC at all depths, leading to higher radiocarbon ages
throughout the profile
Losses and recovery of organic carbon from a seagrass ecosystem following disturbance
No full textSeagrasses are among the Earth's most efficient and long-term carbon sinks, but coastal development threatens this capacity. We report new evidence that disturbance to seagrass ecosystems causes release of ancient carbon. In a seagrass ecosystem that had been disturbed 50 years ago, we found that soil carbon stocks declined by 72%, which, according to radiocarbon dating, had taken hundreds to thousands of years to accumulate. Disturbed soils harboured different benthic bacterial communities (according to 16S rRNA sequence analysis), with higher proportions of aerobic heterotrophs compared with undisturbed. Fingerprinting of the carbon (via stable isotopes) suggested that the contribution of autochthonous carbon (carbon produced through plant primary production) to the soil carbon pool was less in disturbed areas compared with seagrass and recovered areas. Seagrass areas that had recovered from disturbance had slightly lower (35%) carbon levels than undisturbed, but more than twice as much as the disturbed areas, which is encouraging for restoration efforts. Slow rates of seagrass recovery imply the need to transplant seagrass, rather than waiting for recovery via natural processes. This study empirically demonstrates that disturbance to seagrass ecosystems can cause release of ancient carbon, with potentially major global warming consequences.© 2015, The Royal Society