An LED-based Flasher System for VERITAS

We describe a flasher system designed for use in monitoring the gains of the photomultiplier tubes used in the VERITAS gamma-ray telescopes. This system uses blue light-emitting diodes (LEDs) so it can be operated at much higher rates than a traditional laser-based system. Calibration information can be obtained with better statistical precision with reduced loss of observing time. The LEDs are also much less expensive than a laser. The design features of the new system are presented, along with measurements made with a prototype mounted on one of the VERITAS telescopes.Comment: Accepted for publication in Nuclear Instruments and Methods in Physics Research

Measurement of the parity violating 6S-7S transition amplitude in cesium achieved within 2 \times 10^{-13} atomic-unit accuracy by stimulated-emission detection

We exploit the process of asymmetry amplification by stimulated emission which provides an original method for parity violation (PV) measurements in a highly forbidden atomic transition. The method involves measurements of a chiral, transient, optical gain of a cesium vapor on the 7S-6P_{3/2} transition, probed after it is excited by an intense, linearly polarized, collinear laser, tuned to resonance for one hyperfine line of the forbidden 6S-7S transition in a longitudinal electric field. We report here a 3.5 fold increase, of the one-second-measurement sensitivity, and subsequent reduction by a factor of 3.5 of the statistical accuracy compared with our previous result [J. Gu\'ena et al., Phys. Rev. Lett. 90, 143001 (2003)]. Decisive improvements to the set-up include an increased repetition rate, better extinction of the probe beam at the end of the probe pulse and, for the first time to our knowledge, the following: a polarization-tilt magnifier, quasi-suppression of beam reflections at the cell windows, and a Cs cell with electrically conductive windows. We also present real-time tests of systematic effects, consistency checks on the data, as well as a 1% accurate measurement of the electric field seen by the atoms, from atomic signals. PV measurements performed in seven different vapor cells agree within the statistical error. Our present result is compatible with the more precise Boulder result within our present relative statistical accuracy of 2.6%, corresponding to a 2 \times 10^{-13} atomic-unit uncertainty in E_1^{pv}. Theoretical motivations for further measurements are emphasized and we give a brief overview of a recent proposal that would allow the uncertainty to be reduced to the 0.1% level by creating conditions where asymmetry amplification is much greater.Comment: Article 21 pages, 6 figures, 3 tables Typos, addition of few comments and little more data (1 week) leading to a slight reduction of the error bar Accepted for publication in Phys.Rev.

The evidence for automated grading in diabetic retinopathy screening

Peer reviewedPostprin

Double label autoradiography : an improvement

A convenient method for double-label autoradiography is described that uses an aqueous mountant, Gelutol (polyvinyl alcohol), which keeps the gelatin spacer in the final autoradiograph permanently swollen to a thickness of around 18 microns in contrast to its 5 microns thickness during exposure of the autoradiograph. This greatly improves optical discrimination between upper and lower layers without the loss of sensitivity or resolution that would result if a 18 microns spacer were used during exposure

Parity violating pion electroproduction off the nucleon

Parity violating (PV) contributions due to interference between $\gamma$ and $Z^0$ exchange are calculated for pion electroproduction off the nucleon. A phenomenological model with effective Lagrangians is used to determine the resulting asymmetry for the energy region between threshold and $\Delta(1232)$ resonance. The $\Delta$ resonance is treated as a Rarita-Schwinger field with phenomenological $N \Delta$ transition currents. The background contributions are given by the usual Born terms using the pseudovector $\pi N$ Lagrangian. Numerical results for the asymmetry are presented.Comment: 17 pages, RevTeX, 6 figures (in separate file figs.uu), uses epsf, accepted for publication in Z. Phys.

Atomic Hydrogen Cleaning of Polarized GaAs Photocathodes

Atomic hydrogen cleaning followed by heat cleaning at 450$^\circ$C was used to prepare negative-electron-affinity GaAs photocathodes. When hydrogen ions were eliminated, quantum efficiencies of 15% were obtained for bulk GaAs cathodes, higher than the results obtained using conventional 600$^\circ$C heat cleaning. The low-temperature cleaning technique was successfully applied to thin, strained GaAs cathodes used for producing highly polarized electrons. No depolarization was observed even when the optimum cleaning time of about 30 seconds was extended by a factor of 100

Three-Dimensional Thermoluminescence Spectra and Their Application in the Study of Some Sedimentary Quartz

The intensity of thermoluminescence emission is a function of both temperature and photon energy (or wavelength) and therefore lends itself to a three-dimensional (3-D) display, either in the form of an isometric plot or a contour diagram. The technique of 3-D thermoluminescence spectrometry is briefly reviewed and the advantages and applications are illustrated by reference to the spectral properties of samples of Australian sedimentary quartz drawn from the context of thermoluminescence dating. In this context, the temperature of emission, the photon energy and the ease with which the emission is bleachable by sunlight are of significance. It is suggested that, in one of the samples, the energy of the photon emitted from one of the luminescence centres depends on the temperature of emission

Assessing Competence of NHS Consultants: Challenges and Possible Solutions

Even a mention of formal assessment of senior clinicians can be a contentious issue, to say least, when revalidation is said to be firmly in place in NHS-UK for almost half a decade. Since revalidation is accepted as a standard modality of assessment of performance, some colleagues in NHS wonder ‘stir up a hornets’ nest,’ when the authors allude to limitations of revalidation because poorly performing senior NHS clinicians may ‘slip through the net.’ NHS consultants have clinical as well as training roles. Fundamentally, this assessment (revalidation) is meant to ensure the safety of the public and mitigates the risk of disciplinary action by the GMC. Unfortunately, a disciplinary action is often the first sign of underperformance. In fact, the Bristol and Shipman inquiries have underscored the importance of the non-clinical and behavioural skills like communication, teamworking, personal organization and leadership are as important as clinical skills. Rather than considered an assessment tool, an annual appraisal is aimed to facilitate and improve the way NHS consultants work and provide services. The authors have to wait for five years, to assess the efficacy of the system that was introduced with much ‘fanfare’ since it was projected as a panacea for poor performance by ’bad doctors.’ The objectives of this article are to contextualize the issue of the underperformance among senior clinicians in the current NHS environment and to conceptualize the idea that their performance as trainers is directly related to their performance as clinicians. It is worth identifying the underlying factors of that are related to, or even better, can predict underperformance and will help evolve a strategy to help those consultants who are underperforming

Cross-country differences in unemployment : fiscal policy, unions, and household preferences in general equilibrium

We develop a five period overlapping generations model with individuals who differ by ability and with an imperfect labour market (union wage setting) for the individuals of lower ability. The model explains human capital formation, hours worked and unemployment within one coherent framework. Its predictions match the differences in the unemployment rate across 12 OECD countries remarkably well. A Shapley decomposition of these differences reveals an almost equal role for fiscal policy variables and union preferences. As to fiscal policy, differences in unemployment benefits play a much more important role than tax differences. Differences in households’ taste for leisure are unimportant