526 research outputs found
Origin of resolution enhancement by co-doping of scintillators: Insight from electronic structure calculations
It was recently shown that the energy resolution of Ce-doped LaBr
scintillator radiation detectors can be crucially improved by co-doping with
Sr, Ca, or Ba. Here we outline a mechanism for this enhancement on the basis of
electronic structure calculations. We show that (i) Br vacancies are the
primary electron traps during the initial stage of thermalization of hot
carriers, prior to hole capture by Ce dopants; (ii) isolated Br vacancies are
associated with deep levels; (iii) Sr doping increases the Br vacancy
concentration by several orders of magnitude; (iv) binds
to resulting in a stable neutral complex; and (v) association
with Sr causes the deep vacancy level to move toward the conduction band edge.
The latter is essential for reducing the effective carrier density available
for Auger quenching during thermalization of hot carriers. Subsequent
de-trapping of electrons from complexes then
can activate Ce dopants that have previously captured a hole leading to
luminescence. This mechanism implies an overall reduction of Auger quenching of
free carriers, which is expected to improve the linearity of the photon light
yield with respect to the energy of incident electron or photon
Pressure-induced phase transition in the electronic structure of palladium nitride
We present a combined theoretical and experimental study of the electronic
structure and equation of state (EOS) of crystalline PdN2. The compound forms
above 58 GPa in the pyrite structure and is metastable down to 11 GPa. We show
that the EOS cannot be accurately described within either the local density or
generalized gradient approximations. The Heyd-Scuseria-Ernzerhof
exchange-correlation functional (HSE06), however, provides very good agreement
with experimental data. We explain the strong pressure dependence of the Raman
intensities in terms of a similar dependence of the calculated band gap, which
closes just below 11 GPa. At this pressure, the HSE06 functional predicts a
first-order isostructural transition accompanied by a pronounced elastic
instability of the longitudinal-acoustic branches that provides the mechanism
for the experimentally observed decomposition. Using an extensive Wannier
function analysis, we show that the structural transformation is driven by a
phase transition of the electronic structure, which is manifested by a
discontinuous change in the hybridization between Pd-d and N-p electrons as
well as a conversion from single to triple bonded nitrogen dimers. We argue for
the possible existence of a critical point for the isostructural transition, at
which massive fluctuations in both the electronic as well as the structural
degrees of freedom are expected.Comment: 9 pages, 12 figures. Revised version corrects minor typographical
error
Subjective perceptions as prognostic factors of time to fitness for work during a 4-year period after inpatient rehabilitation for orthopaedic trauma.
INTRODUCTION: Time to fitness for work (TFW) was measured as the number of days that were paid as compensation for work disability during the 4 years after discharge from the rehabilitation clinic in a population of patients hospitalised for rehabilitation after orthopaedic trauma. The aim of this study was to test whether some psychological variables can be used as potential early prognostic factors of TFW.
MATERIAL AND METHODS: A Cox proportional hazards model was used to estimate the associations between predictive variables and TFW. Predictors were global health, pain at hospitalisation and pain decrease during the stay (all continuous and standardised by subtracting the mean and dividing by two standard deviations), perceived severity of the trauma and expectation of a positive evolution (both binary variables).
RESULTS: Full data were available for 807 inpatients (660 men, 147 women). TFW was positively associated with better perceived health (hazard ratio [HR] 1.16, 95% confidence interval [CI] 1.13-1.19), pain decrease (HR 1.46, 95% CI 1.30-1.64) and expectation of a positive evolution (HR 1.50, 95% CI 1.32-1.70) and negatively associated with pain at hospitalisation (HR 0.67, 95% CI 0.59-0.76) and high perceived severity (HR 0.72, 95% CI 0.61-0.85).
DISCUSSION: The present results provide some evidence that work disability during a four-year period after rehabilitation may be predicted by prerehabilitation perceptions of general health, pain, injury severity, as well as positive expectation of evolution
Maine’s Potential to Be a Global Leader in Sustainable Seaweed Harvesting and Management
A multitude of macroalgae (i.e., seaweed) species that are harvested in Maine are economically and ecologically important. Currently, management of these resources in Maine is focused on rockweed (Ascophyllum nodosum). This seaweed grows in abundance along intertidal rocky shores and provides a number of benefits including serving as habitat and nursery for many species including fin-fish, coastal protection from storms and sea level rise, carbon sequestration, and is a harvestable natural resource used primarily in fertilizer and animal feed production. It is critical that these functions and benefits are maintained and harvesting activities managed to ensure a sustainable fishery and healthy ecosystem. In 2014, Maine drafted a rockweed fisheries management plan; however, this plan has yet to be adopted. The delay in adoption is now an opportunity to re-examine the plan from an ecosystem-based management (EBM) perspective. We compare EBM strategies to the existing rockweed fisheries management plan, and identify areas of synergy as well as gaps in the management of Maine’s seaweeds and harvesting activities. Based on this analysis we propose Maine adopt an EBM approach and strategies for wild as well as farmed seaweeds
Investigation of the chemical vicinity of crystal defects in ion-irradiated Mg and AZ31 with coincident Doppler broadening spectroscopy
Crystal defects in magnesium and magnesium based alloys like AZ31 are of
major importance for the understanding of their macroscopic properties. We have
investigated defects and their chemical surrounding in Mg and AZ31 on an atomic
scale with Doppler broadening spectroscopy of the positron annihilation
radiation. In these Doppler spectra the chemical information and the defect
contribution have to be thoroughly separated. For this reason samples of
annealed Mg were irradiated with Mg-ions in order to create exclusively
defects. In addition Al- and Zn-ion irradiation on Mg-samples was performed in
order to create samples with defects and impurity atoms. The ion irradiated
area on the samples was investigated with laterally and depth resolved positron
Doppler broadening spectroscopy (DBS) and compared with preceding
SRIM-simulations of the vacancy distribution, which are in excellent agreement.
The investigation of the chemical vicinity of crystal defects in AZ31 was
performed with coincident Doppler broadening spectroscopy (CDBS) by comparing
Mg-ion irradiated AZ31 with Mg-ion irradiated Mg. No formation of
solute-vacancy complexes was found due to the ion irradiation, despite the high
defect mobility.Comment: Submitted to Physical Review B on March 20 20076. Revised version
submitted on September 28 2007. Accepted on October 19 200
High-entropy alloys as high-temperature thermoelectric materials
Thermoelectric (TE) generators that efficiently recycle a large portion of waste heat will be an important complementary energy technology in the future. While many efficient TE materials exist in the lower temperature region, few are efficient at high temperatures. Here, we present the high temperature properties of high-entropy alloys (HEAs), as a potential new class of high temperature TE materials. We show that their TE properties can be controlled significantly by changing the valence electron concentration (VEC) of the system with appropriate substitutional elements. Both the electrical and thermal transport properties in this system were found to decrease with a lower VEC number. Overall, the large microstructural complexity and lower average VEC in these types of alloys can potentially be used to lower both the total and the lattice thermal conductivity. These findings highlight the possibility to exploit HEAs as a new class of future high temperature TE materials
- …