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Charge collection efficiency degradation induced by MeV ions in semiconductor devices: model and experiment
No full textThis paper investigates both theoretically and experimentally the charge collection efficiency (CCE) degradation in silicon diodes induced by energetic ions. Ion Beam Induced Charge (IBIC) measurements carried out on n- and p-type silicon diodes which were previously irradiated with MeV He ions show evidence that the CCE degradation does not only depend on the mass, energy and fluence of the damaging ion, but also depends on the ion probe species and on the polarization state of the device. A general one-dimensional model is derived, which accounts for the ion-induced defect distribution, the ionization profile of the probing ion and the charge induction mechanism. Using the ionizing and non-ionizing energy loss profiles resulting from simulations based on the binary collision approximation and on the electrostatic/transport parameters of the diode under study as input, the model is able to accurately reproduce the experimental CCE degradation curves without introducing any phenomenological additional term or formula. Although limited to low level of damage, the model is quite general, including the displacement damage approach as a special case and can be applied to any semiconductor device. It provides a method to measure the capture coefficients of the radiation induced recombination centres. They can be considered indexes, which can contribute to assessing the relative radiation hardness of semiconductor materials. © 2016 Elsevier B.V
Intrinsic reduction of the ordered 4f magnetic moments in semiconducting rare-earth nitride thin films: DyN, ErN, and HoN
No full textPolarized neutron reflectometry and x-ray reflectometry were used to determine the nanoscale magnetic and chemical depth profiles of the heavy rare-earth nitrides HoN, ErN, and DyN in the form of 15- to 40-nm-thick films. The net ferromagnetic components are much lower than the predictions of density-functional theory and Hund's rules for a simple ferromagnetic ground state in these 4f ionic materials, which points to the intrinsic contribution of crystal-field effects and noncollinear spin structures. The magnetic moment per rare-earth ion was determined as a function of temperature in the range 5–100 K at fields of 1–4 T. It is demonstrated that the films are stoichiometric within 1–3% and magnetically homogeneous on the nanometer scale.© 2014, American Physical Society
The role of microstructure in the stress relaxation and tempering of laser clad Ti–6Al–4V
No full textIn this work a heat treatment procedure was performed on laser clad Ti–6Al–4V, which resulted in a reduction of tensile residual stresses in the clad samples as well and a decrease in the hardness of the clad layer with an increasing heat treatment time. The reduction in the residual stress was attributed to inter-phase stress relaxation brought about by the growth of the β phase. The reduction in hardness was attributed to an increase in the volume fraction of the soft β phase. Both the reduction in residual stress and hardness due to this heat treatment makes it an attractive method to heat treat both laser clad and additively manufactured Ti–6Al–4V components for improved mechanical properties.© 2014, Elsevier B.V
Evidence for extreme floods in arid subtropical northwest Australia during the Little Ice Age chronozone (CE 1400-1850).
No full textHere we report a ∼2000-year sediment sequence from the Fortescue Marsh (Martuyitha) in the eastern Pilbara region, which we have used to investigate changing hydroclimatic conditions in the arid subtropics of northwest Australia. The Pilbara is located at the intersection of the tropical Indian and Pacific Oceans and its modern rainfall regime is strongly influenced by tropical cyclones, the Intertropical Convergence Zone (ITCZ) and the Indo-Pacific Warm Pool. We identified four distinct periods within the record. The most recent period (P1: CE ∼1990–present) reveals hydroclimatic conditions over recent decades that are the most persistently wet of potentially the last ∼2000 years. During the previous centuries (P2: ∼CE 1600–1990), the Fortescue Marsh was overall drier but likely punctuated by a number of extreme floods, which are defined here as extraordinary, strongly episodic floods in drylands generated by rainfall events of high volume and intensity. The occurrence of extreme floods during this period, which encompasses the Little Ice Age (LIA; CE 1400–1850), is coherent with other southern tropical datasets along the ITCZ over the last 2000 years, suggesting synchronous hydroclimatic changes across the region. This extreme flood period was preceded by several hundred years (P3: ∼CE 700–1600) of less vigorous but more regular flows. The earliest period of the sediment record (P4: ∼CE 100–700) was the most arid, with sedimentary and preservation processes driven by prolonged drought. Our results highlight the importance of developing paleoclimate records from the tropical and sub-tropical arid zone, providing a long-term baseline of hydrological conditions in areas with limited historical observations. © 2016 Elsevier Ltd
Vertical distribution of diatoms in the sediment of Al-Huwaiza Marsh, Southern Iraq and their use as indicators of environmental changes Algological Studies
No full textThe Mesopotamian marshlands are one of the most internationally important wetlands in the Middle East as it constitutes a nursing ground for many species of birds migrating from North Europe as well as for its rich biodiversity. These marshlands have undergone significant environmental changes during the last three decades owing to decreasing freshwater discharge which has led to water quality deterioration and a changing hydrological regime. This has had a considerable effect on the fauna and flora of the marshes, diminishing their ecological value and significance. The work presented here is the first attempt to investigate the marshes environmental condition during the past two centuries using paleolimnology, specifically diatoms as bioindicators in a sediment core. 80 species of diatom belonging to 35 genera were identified. The taxa encountered in the core are a mixture of fresh and brackish water forms. These are epiphytic and benthic taxa, no planktonic species were found. The fossil diatom community shows that the marshes have been exposed to different periods of salinization as well as eutrophication. The dominance of pollution tolerant species in the core indicates poor water quality for the past 160 years. Paleolimnology is important to understand past environmental conditions and forms a milestone for successful future restoration process. Further work is required to extend the time scale to identify natural ecological states and thresholds to guide international aid restoration projects
Alternative least squares methods for determining the meteoric water line, demonstrated using GNIP data
No full textThe relationship between δ2H and δ18O in precipitation at a site, known as the local meteoric water line (LMWL), is normally defined using an ordinary least squares regression (OLSR). However, it has been argued that this form of minimisation is more appropriate when a predictive model is being developed (and there are no measurement errors associated with the independent variable) and that orthogonal regression, also known as major axis regression (MA), or reduced major axis regression (RMA) may be better suited when a relationship is being sought between two variables which are related by underlying physical processes.
The slope of the LMWLs for the GNIP data is examined using the three linear regressions, and the corresponding precipitation weighted regressions. The MA and RMA regressions generally produced larger slopes, with the largest differences for oceanic islands and coastal sites. The difference between the various methods was the least for continental sites. In all considered cases, both for the standard and precipitation weighted regressions, the slope produced by RMA was in between those determined by OLSR and MA, with OLSR producing the smaller slope. Further, the results of both RMA and precipitation weighted RMA were less sensitive to the removal of outliers and values with high leverage statistic.
The results indicate that when a good linear relationship exists between δ2H and δ18O, all considered regressions result in a close fit. When the values are distributed within circles or ellipses on the δ18O–δ2H bivariate plot, as would appear in coastal and oceanic sites from first stage rainout, care needs to be taken as to which regression is utilised. However, in some of these cases, it appears the precipitation weighted MA (and in some cases MA) produces large slopes. In these cases the average Root Mean Sum of Squared Error (rmSSEav) value of the fit can be used as a guide of the suitability of the MA and PWMA for each site. Where the slope of the PWRMA was significantly different to the slope of the OLSR regression (22% of sites; dominated by coastal, island and Mediterranean locations), we believe PWRMA is more suitable.© 2014 Elsevier B.V
Bioaccumulation and retention kinetics of cadmium in the freshwater decapod Macrobrachium australiense
No full textThe potential sources and mechanisms of cadmium bioaccumulation by the native freshwater decapods Macrobrachium species in the waters of the highly turbid Strickland River in Papua New Guinea were examined using 109Cd-labelled water and food sources and the Australian species Macrobrachium australiense as a surrogate. Synthetic river water was spiked with environmentally relevant concentrations of cadmium and animals were exposed for 7 days with daily renewal of test solutions. Dietary assimilation of cadmium was assessed through pulse-chase experiments where prawns were fed separately 109Cd-labelled fine sediment, filamentous algae and carrion (represented by cephalothorax tissue of water-exposed prawns).
M. australiense readily accumulated cadmium from the dissolved phase and the uptake rate increased linearly with increasing exposure concentration. A cadmium uptake rate constant of 0.10 ± 0.05 L/g/d was determined in synthetic river water. During depuration following exposure to dissolved cadmium, efflux rates were low (0.9 ± 5%/d) and were not dependent on exposure concentration. Assimilation efficiencies of dietary sources were comparable for sediment and algae (48–51%), but lower for carrion (28 ± 5%) and efflux rates were low (0.2–2.6%/d) demonstrating that cadmium was well retained by M. australiense. A biokinetic model of cadmium accumulation by M. australiense predicted that for exposures to environmentally relevant cadmium concentrations in the Strickland River, uptake from ingestion of fine sediment and carrion would be the predominant sources of cadmium to the organism. The model predicted the total dietary route would represent 70–80% of bioaccumulated cadmium © 2014, Elsevier B.V
A neutron diffraction study of hydrogen bonding in isostructural potassium and ammonium lanthanoidates
No full textTwo isostructural series of compounds NH4[Ln(cdm)4(H2O)4]·18c6·3H2O (1Ln) and [K(18c6)(H2O)2][Ln(cdm)4(H2O)4]·H2O (2Ln) have been synthesised and structurally characterised (Ln = Gd, Dy, Er, cdm = C(CN)2(CONH2)−). These two classes of compounds are shown to be essentially isostructural to each other despite the change in counter-cation (ammonium vs. potassium) and the ensuing changes in coordination and hydrogen-bonding of water molecules in the structure. Structural data for 1Dy and 2Dy have been obtained using single crystal Laue neutron diffraction, allowing the precise location of all hydrogen atoms to be determined using fully anisotropic models. Both 1 and 2 contain the anionic complex [Ln(cdm)4(H2O)4]− in which the 8-coordinate lanthanoid is coordinated by a ring of four O-bound cdm ligands supported by inter-ligand N–H⋯O hydrogen bonds. The structures pack with complicated hydrogen-bonding networks in which both coordinated and non-coordinated water molecules are hydrogen-bond donors and all of the nitrile groups of the cdm ligands are hydrogen-bond acceptors. The change in cation between ammonium and potassium affects two water molecule sites which are both coordinated in 2Ln but not in 1Ln; in 1Ln one of these water molecules forms a hydrogen bond with NH4+ whilst the other has neither a coordination interaction nor a hydrogen bond to its oxygen atom. © Reproduced by permission of The Royal Society of Chemistry
Late Quaternary glacial history constrains glacio-isostatic rebound in Enderby Land, East Antarctica
No full textMeasurements of the loss or gain of ice mass from large ice sheets are presently achieved through satellite-based techniques such as GRACE (Gravity Recovery and Climate Experiment). The accuracy of these satellite-based measurements to changes in modern ice sheet mass depends on our knowledge of present-day glacio-isostatic crustal uplift rates caused by past ice sheet changes. To improve models of glacio-isostatic rebound in East Antarctica, we investigated ice histories along Rayner Glacier, Enderby Land, and a little explored sector of the ice sheet where GRACE data had suggested significant mass gain during the last decade. Observations from a recent glacial geomorphic reconnaissance coupled with cosmogenic nuclide dating indicate that in the lower part of the Rayner Glacier, Enderby Land, ice heights lowered by at least 300 m and the calving margin retreated by at least 10 km in the early Holocene (~6 to 9 ka B.P.). The magnitude and timing of deglaciation are consistent with ice histories used to model the postglacial rebound corrections for present-day GRACE mass trends. These observations strengthen the body of evidence that suggests ice mass gain in Enderby Land is presently partly offsetting mass loss in other parts of Antarctica.© 2014, American Geophysical Union
MD study of primary damage in L1(0) TiAl structural intermetallics
No full textComputer modelling by molecular dynamics has been applied to study the radiation damage created in collision cascades in L10 TiAl intermetallic compound. Either Al or Ti primary knock-on atoms (PKA) with energy 5 keV ⩽ EPKA ⩽ 20 keV were introduced in the intermetallic crystals at temperatures ranging from 100 K to 900 K. At least 24 different cascade for each (EPKA, T, PKA type) set were modelled in order to simulate a random spatial and temporal distribution of PKAs and provide statistical reliability of the results. The total yield of more than 760 simulated cascades is the largest yet reported for this binary intermetallic material. A comprehensive treatment of the modelling results has been carried out. The number of Frenkel pairs, fraction of Al and Ti vacancies, self-interstitial atoms and anti-sites as a function of (EPKA, T, PKA type) has been established. Preferred formation of Al self-interstitial atoms has been detected in L10 TiAl structural intermetallics exposed to irradiation. © 2012, Elsevier B.V