1,367 research outputs found
INITIAL PRODUCTION OF DEFECTS IN ALKALI-HALIDES - F AND H CENTER PRODUCTION BY NON-RADIATIVE DECAY OF SELF-TRAPPED EXCITON
Radiation damage in KCl can be produced by the decay of a self-trapped exciton into an F centre and an H centre. The authors present calculations of the energies of the states involved for various stages in the evolution of the damage. These lead to important conclusions about the very rapid damage process, and support strongly Itoh and Saidoh's suggestion (1973) that damage proceeds through an excited hole state. The results also help in understanding the prompt decay of F and H pairs at low temperatures, the thermal annihilation of F and H centres, the effects of optical excitation of the self-trapped exciton, and some of the trends within the alkali halides. The calculations use a self-consistent semi-empirical molecular-orbital method. A large cluster of ions is used (either 42 or 57 ions) plus long-range Madelung terms. The ion positions were obtained from separate lattice-relaxation calculations with the HADES code. The choice of CNDO parameters and the adequacy of the method were checked by a number of separate predictions
Seeding rate and seeding depth
Non-Peer Reviewe
Further evidence for linearly-dispersive Cooper pairs
A recent Bose-Einstein condensation (BEC) model of several cuprate
superconductors is based on bosonic Cooper pairs (CPs) moving in 3D with a
quadratic energy-momentum (dispersion) relation. The 3D BEC condensate-fraction
vs. temperature (T/Tc, where Tc is the BEC transition temperature) formula
poorly fits penetration-depth data for two cuprates in the range (1/2, 1]. We
show how these fits are dramatically improved assuming cuprates to be quasi-2D,
and how equally good fits obtain for conventional 3D and quasi-1D nanotube
superconducting data, provided the correct CP dispersion is assumed in BEC at
their assumed corresponding dimensionalities. This is offered as additional
concrete empirical evidence for linearly-dispersive pairs in another recent BEC
scenario of superconductors within which a BCS condensate turns out to be a
very special case.Comment: 9 pages, 1 figur
Codeine misuse and dependence in South Africa – learning from substance abuse treatment admissions
Background. Misuse of prescription and over-the-counter codeine-containing products is a global public health issue.Objectives. To investigate the extent of treatment demand related to the misuse of codeine or codeine dependence in South Africa (SA) and the profile of patients seeking treatment, so as to understand the nature and extent of the problem.Method. Data were collected from centres participating in the South African Community Epidemiology Network on Drug Use in 2014. A total of 17 260 admissions were recorded.Results. There were 435 recorded treatment admissions for codeine misuse or dependence as a primary or secondary substance of abuse (2.5% of all admissions). Of treatment admissions, 137 (0.8%) involved codeine as the primary substance of abuse; 74.9% of patients were males, with an even spread across population groups. Ages ranged from 11 to 70 years, with the highest proportion aged 20 - 29 years; >40% were referred by self, family and/or friends, and 26.7% by health professionals; and 36.8% had received treatment previously. The majority reported misuse of tablets/capsules, with 17.6% reporting misuse of syrups. Oral use comprised 96.6% and daily use 63.1%.Conclusions. Data from treatment admissions related to codeine misuse and dependence are informative, but provide an incomplete picture of the nature and extent of codeine-related problems in SA. Other data sources must be considered before further regulatory/policy changes regarding codeine are implemented
Transitions between lifetime alcohol use, regular use and remission: Results from the 2004 South African Stress and Health Survey
Background. Hazardous alcohol consumption presents a serious threat to the health and wellbeing of all people and is linked to chronic and acute health problems.Objectives. To: (i) estimate the prevalence of alcohol use disorders and remission from alcohol abuse and dependence in the South African (SA) population; and (ii) determine whether age of onset, education, sex and level of cohort alcohol use are associated with commencement of use, regularity of use, and transitions to and remission from more harmful levels of use.Methods. The study was a nationally representative sample of 4 315 individuals aged ≥18 years. In a multistage, area probability sample of adults, data were collected from 4 311 alcohol users using the World Mental Health Survey Initiative version of the Composite International Diagnostic Interview version 3.0. All analyses were carried out using SAS version 9.4.Results. Of the respondents, 40.6% indicated lifetime use of alcohol, 35.3% reported regular use, and 8.8% met diagnostic criteria for alcohol abuse and 2.7% for alcohol dependence. The prevalence of remission from lifetime abuse without dependence was 55.9%. The median age of onset of alcohol use was 20 years, with transition from use to regular use occurring within ~1 - 3 years. The results suggest that males, students (compared with those who had completed a high level of education) and greater alcohol use in the respondent’s birth cohort were all associated with increased odds of commencing alcohol use. For transitions from use to regular use, increased odds were associated with males, greater birth cohort alcohol use, low education and later (>21 years) onset of first alcohol use.Conclusions. Our findings suggest that cohort alcohol use is associated with transition to commencement of use and from use to regular use in the general SA population. The study further highlighted the need for interventions among males and university students, given that hazardous alcohol consumption seems to be the most prevalent public health issue encountered by university students and males
The DEEP2 Galaxy Redshift Survey: Mean Ages and Metallicities of Red Field Galaxies at z ~ 0.9 from Stacked Keck/DEIMOS Spectra
As part of the DEEP2 galaxy redshift survey, we analyze absorption line
strengths in stacked Keck/DEIMOS spectra of red field galaxies with weak to no
emission lines, at redshifts 0.7 <= z <= 1. Comparison with models of stellar
population synthesis shows that red galaxies at z ~ 0.9 have mean
luminosity-weighted ages of the order of only 1 Gyr and at least solar
metallicities. This result cannot be reconciled with a scenario where all stars
evolved passively after forming at very high z. Rather, a significant fraction
of stars can be no more than 1 Gyr old, which means that star formation
continued to at least z ~ 1.2. Furthermore, a comparison of these distant
galaxies with a local SDSS sample, using stellar populations synthesis models,
shows that the drop in the equivalent width of Hdelta from z ~ 0.9 to 0.1 is
less than predicted by passively evolving models. This admits of two
interpretations: either each individual galaxy experiences continuing low-level
star formation, or the red-sequence galaxy population from z ~ 0.9 to 0.1 is
continually being added to by new galaxies with younger stars.Comment: A few typos were corrected and numbers in Table 1 were revise
Foregrounds for observations of the cosmological 21 cm line: II. Westerbork observations of the fields around 3C196 and the North Celestial Pole
In the coming years a new insight into galaxy formation and the thermal
history of the Universe is expected to come from the detection of the highly
redshifted cosmological 21 cm line. The cosmological 21 cm line signal is
buried under Galactic and extragalactic foregrounds which are likely to be a
few orders of magnitude brighter. Strategies and techniques for effective
subtraction of these foreground sources require a detailed knowledge of their
structure in both intensity and polarization on the relevant angular scales of
1-30 arcmin. We present results from observations conducted with the Westerbork
telescope in the 140-160 MHz range with 2 arcmin resolution in two fields
located at intermediate Galactic latitude, centred around the bright quasar
3C196 and the North Celestial Pole. They were observed with the purpose of
characterizing the foreground properties in sky areas where actual observations
of the cosmological 21 cm line could be carried out. The polarization data were
analysed through the rotation measure synthesis technique. We have computed
total intensity and polarization angular power spectra. Total intensity maps
were carefully calibrated, reaching a high dynamic range, 150000:1 in the case
of the 3C196 field. [abridged]Comment: 20 pages, 22 figures, accepted for publication in A&A. A version with
full resolution figures is available at
http://www.astro.rug.nl/~bernardi/NCP_3C196/bernardi.pd
Radio Recombination Lines at Decametre Wavelengths: Prospects for the Future
This paper considers the suitability of a number of emerging and future
instruments for the study of radio recombination lines (RRLs) at frequencies
below 200 MHz. These lines arise only in low-density regions of the ionized
interstellar medium, and they may represent a frequency-dependent foreground
for next-generation experiments trying to detect H I signals from the Epoch of
Reionization and Dark Ages ("21-cm cosmology"). We summarize existing
decametre-wavelength observations of RRLs, which have detected only carbon
RRLs. We then show that, for an interferometric array, the primary instrumental
factor limiting detection and study of the RRLs is the areal filling factor of
the array. We consider the Long Wavelength Array (LWA-1), the LOw Frequency
ARray (LOFAR), the low-frequency component of the Square Kilometre Array
(SKA-lo), and a future Lunar Radio Array (LRA), all of which will operate at
decametre wavelengths. These arrays offer digital signal processing, which
should produce more stable and better defined spectral bandpasses; larger
frequency tuning ranges; and better angular resolution than that of the
previous generation of instruments that have been used in the past for RRL
observations. Detecting Galactic carbon RRLs, with optical depths at the level
of 10^-3, appears feasible for all of these arrays, with integration times of
no more than 100 hr. The SKA-lo and LRA, and the LWA-1 and LOFAR at the lowest
frequencies, should have a high enough filling factor to detect lines with much
lower optical depths, of order 10^-4 in a few hundred hours. The amount of
RRL-hosting gas present in the Galaxy at the high Galactic latitudes likely to
be targeted in 21-cm cosmology studies is currently unknown. If present,
however, the spectral fluctuations from RRLs could be comparable to or exceed
the anticipated H I signals.Comment: 9 pages; Astron. & Astrophys., in pres
THE LATTICE-RELAXATION ENERGY ASSOCIATED WITH SELF-TRAPPING OF A POSITIVE HOLE AND AN EXCITON IN ALKALI-HALIDES
The authors have evaluated the upper and lower bounds to the lattice relaxation energy gained on the self-trapping of excitons, using experimental values of transition energies for free and self-trapped excitons and theoretical values for lattice relaxation energies and optical transition energies. The lattice relaxation energy upon self-trapping of an exciton in alkali halides proves to be appreciably larger than that of a self-trapped hole. They discuss the implications for a number of solid state processes including the production mechanism of F and H centres and the desorption of halogen atoms following valence electron excitation
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