216 research outputs found
Nanoscale grains, high irreversibility field, and large critical current density as a function of high energy ball milling time in C-doped magnesium diboride
Magnesium diboride (MgB2) powder was mechanically alloyed by high energy ball
milling with C to a composition of Mg(B0.95C0.05)2 and then sintered at 1000 C
in a hot isostatic press. Milling times varied from 1 minute to 3000 minutes.
Full C incorporation required only 30-60 min of milling. Grain size of sintered
samples decreased with increased milling time to less than 30 nm for 20-50 hrs
of milling. Milling had a weak detrimental effect on connectivity. Strong
irreversibility field (H*) increase (from 13.3 T to 17.2 T at 4.2 K) due to
increased milling time was observed and correlated linearly with inverse grain
size (1/d). As a result, high field Jc benefited greatly from lengthy powder
milling. Jc(8 T, 4.2 K) peaked at > 80,000 A/cm2 with 1200 min of milling
compared with only ~ 26,000 A/cm2 for 60 min of milling. This non-compositional
performance increase is attributed to grain refinement of the unsintered powder
by milling, and to the probable suppression of grain growth by milling-induced
MgO nano-dispersions.Comment: 12 pages, 11 figure
Numerical study of anharmonic vibrational decay in amorphous and paracrystalline silicon
The anharmonic decay rates of atomic vibrations in amorphous silicon (a-Si)
and paracrystalline silicon (p-Si), containing small crystalline grains
embedded in a disordered matrix, are calculated using realistic structural
models. The models are 1000-atom four-coordinated networks relaxed to a local
minimum of the Stillinger-Weber interatomic potential. The vibrational decay
rates are calculated numerically by perturbation theory, taking into account
cubic anharmonicity as the perturbation. The vibrational lifetimes for a-Si are
found to be on picosecond time scales, in agreement with the previous
perturbative and classical molecular dynamics calculations on a 216-atom model.
The calculated decay rates for p-Si are similar to those of a-Si. No modes in
p-Si reside entirely on the crystalline cluster, decoupled from the amorphous
matrix. The localized modes with the largest (up to 59%) weight on the cluster
decay primarily to two diffusons. The numerical results are discussed in
relation to a recent suggestion by van der Voort et al. [Phys. Rev. B {\bf 62},
8072 (2000)] that long vibrational relaxation inferred experimentally may be
due to possible crystalline nanostructures in some types of a-Si.Comment: 9 two-column pages, 13 figure
Response to comment on 'Amphibian fungal panzootic causes catastrophic and ongoing loss of biodiversity'
Lambert et al. question our retrospective and holistic epidemiological assessment of the role of chytridiomycosis in amphibian declines. Their alternative assessment is narrow and provides an incomplete evaluation of evidence. Adopting this approach limits understanding of infectious disease impacts and hampers conservation efforts. We reaffirm that our study provides unambiguous evidence that chytridiomycosis has affected at least 501 amphibian species
Investigating High-Energy Proton-Induced Reactions on Spherical Nuclei: Implications for the Pre-Equilibrium Exciton Model
A number of accelerator-based isotope production facilities utilize 100- to
200-MeV proton beams due to the high production rates enabled by high-intensity
beam capabilities and the greater diversity of isotope production brought on by
the long range of high-energy protons. However, nuclear reaction modeling at
these energies can be challenging because of the interplay between different
reaction modes and a lack of existing guiding cross section data. A Tri-lab
collaboration has been formed among the Lawrence Berkeley, Los Alamos, and
Brookhaven National Laboratories to address these complexities by
characterizing charged-particle nuclear reactions relevant to the production of
established and novel radioisotopes. In the inaugural collaboration
experiments, stacked-targets of niobium foils were irradiated at the Brookhaven
Linac Isotope Producer (E=200 MeV) and the Los Alamos Isotope Production
Facility (E=100 MeV) to measure Nb(p,x) cross sections between 50
and 200 MeV. The measured cross-section results were compared with literature
data as well as the default calculations of the nuclear model codes TALYS, CoH,
EMPIRE, and ALICE. We developed a standardized procedure that determines the
reaction model parameters that best reproduce the most prominent reaction
channels in a physically justifiable manner. The primary focus of the procedure
was to determine the best parametrization for the pre-equilibrium two-component
exciton model. This modeling study revealed a trend toward a relative decrease
for internal transition rates at intermediate proton energies (E=20-60 MeV)
in the current exciton model as compared to the default values. The results of
this work are instrumental for the planning, execution, and analysis essential
to isotope production.Comment: 37 pages, 62 figures. Revised version, published in Physical Review
A Reservoir Species for the Emerging Amphibian Pathogen Batrachochytrium dendrobatidis Thrives in a Landscape Decimated by Disease
Chytridiomycosis, a disease caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd), is driving amphibian declines and extinctions in protected areas globally. The introduction of invasive reservoir species has been implicated in the spread of Bd but does not explain the appearance of the pathogen in remote protected areas. In the high elevation (>1500 m) Sierra Nevada of California, the native Pacific chorus frog, Pseudacris regilla, appears unaffected by chytridiomycosis while sympatric species experience catastrophic declines. We investigated whether P. regilla is a reservoir of Bd by comparing habitat occupancy before and after a major Bd outbreak and measuring infection in P. regilla in the field, monitoring susceptibility of P. regilla to Bd in the laboratory, examining tissues with histology to determine patterns of infection, and using an innovative soak technique to determine individual output of Bd zoospores in water. Pseudacris regilla persists at 100% of sites where a sympatric species has been extirpated from 72% in synchrony with a wave of Bd. In the laboratory, P. regilla carried loads of Bd as much as an order of magnitude higher than loads found lethal to sympatric species. Histology shows heavy Bd infection in patchy areas next to normal skin, a possible mechanism for tolerance. The soak technique was 77.8% effective at detecting Bd in water and showed an average output of 68 zoospores per minute per individual. The results of this study suggest P. regilla should act as a Bd reservoir and provide evidence of a tolerance mechanism in a reservoir species
Context-dependent conservation responses to emerging wildlife diseases
Emerging infectious diseases pose an important threat to wildlife. While established protocols exist for combating outbreaks of human and agricultural pathogens, appropriate management actions before, during, and after the invasion of wildlife pathogens have not been developed. We describe stage-specific goals and management actions that minimize disease impacts on wildlife, and the research required to implement them. Before pathogen arrival, reducing the probability of introduction through quarantine and trade restrictions is key because prevention is more cost effective than subsequent responses. On the invasion front, the main goals are limiting pathogen spread and preventing establishment. In locations experiencing an epidemic, management should focus on reducing transmission and disease, and promoting the development of resistance or tolerance. Finally, if pathogen and host populations reach a stable stage, then recovery of host populations in the face of new threats is paramount. Successful management of wildlife disease requires risk-taking, rapid implementation, and an adaptive approach."Funding was provided by the US National Science Foundation (grants EF-0914866, DGE-0741448, DEB-1115069, DEB-1336290) and the National Institutes of Health (grant 1R010AI090159)."https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/14024
Sodium Chloride Inhibits the Growth and Infective Capacity of the Amphibian Chytrid Fungus and Increases Host Survival Rates
The amphibian chytrid fungus Batrachochytrium dendrobatidis is a recently emerged pathogen that causes the infectious disease chytridiomycosis and has been implicated as a contributing factor in the global amphibian decline. Since its discovery, research has been focused on developing various methods of mitigating the impact of chytridiomycosis on amphibian hosts but little attention has been given to the role of antifungal agents that could be added to the host's environment. Sodium chloride is a known antifungal agent used routinely in the aquaculture industry and this study investigates its potential for use as a disease management tool in amphibian conservation. The effect of 0–5 ppt NaCl on the growth, motility and survival of the chytrid fungus when grown in culture media and its effect on the growth, infection load and survivorship of infected Peron's tree frogs (Litoria peronii) in captivity, was investigated. The results reveal that these concentrations do not negatively affect the survival of the host or the pathogen. However, concentrations greater than 3 ppt significantly reduced the growth and motility of the chytrid fungus compared to 0 ppt. Concentrations of 1–4 ppt NaCl were also associated with significantly lower host infection loads while infected hosts exposed to 3 and 4 ppt NaCl were found to have significantly higher survival rates. These results support the potential for NaCl to be used as an environmentally distributed antifungal agent for the prevention of chytridiomycosis in susceptible amphibian hosts. However, further research is required to identify any negative effects of salt exposure on both target and non-target organisms prior to implementation
Design, construction, and characterization of a compact DD neutron generator designed for 40Ar/39Ar geochronology
A next-generation, high-flux DD neutron generator has been designed,
commissioned, and characterized, and is now operational in a new facility at
the University of California Berkeley. The generator, originally designed for
40Ar/39Ar dating of geological materials, has since served numerous additional
applications, including medical isotope production studies, with others planned
for the near future. In this work, we present an overview of the High Flux
Neutron Generator (HFNG) which includes a variety of simulations, analytical
models, and experimental validation of results. Extensive analysis was
performed in order to characterize the neutron yield, flux, and energy
distribution at specific locations where samples may be loaded for irradiation.
A notable design feature of the HFNG is the possibility for sample irradiation
internal to the cathode, just 8 mm away from the neutron production site, thus
maximizing the neutron flux (n/cm2/s). The generator's maximum neutron flux at
this irradiation position is 2.58e7 n/cm2/s +/- 5% (approximately 3e8 n/s total
yield) as measured via activation of small natural indium foils. However,
future development is aimed at achieving an order of magnitude increase in
flux. Additionally, the deuterium ion beam optics were optimized by simulations
for various extraction configurations in order to achieve a uniform neutron
flux distribution and an acceptable heat load. Finally, experiments were
performed in order to benchmark the modeling and characterization of the HFNG.Comment: 31 pages, 20 figure
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