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

Evolution of the bilayer nu = 1 quantum Hall state under charge imbalance

We use high-mobility bilayer hole systems with negligible tunneling to examine how the bilayer nu = 1 quantum Hall state evolves as charge is transferred from one layer to the other at constant total density. We map bilayer nu = 1 state stability versus imbalance for five total densities spanning the range from strongly interlayer coherent to incoherent. We observe competition between single-layer correlations and interlayer coherence. Most significantly, we find that bilayer systems that are incoherent at balance can develop spontaneous interlayer coherence with imbalance, in agreement with recent theoretical predictions.Comment: 4 pages, 4 figure

Effects of Force Level and Hand Dominance on Bilateral Transfer of a Fine Motor Skill

Our research is about bilateral transfer, a concept in motor learning where skills learned by one limb are "transferred", allowing the opposite limb to benefit from what was learned by the first limb. Previous research into bilateral transfer has raised questions about whether specific aspects of motor coordination are or are not transferred. We wanted to see whether learning to control pinch force by the thumb and index finger is transferable, and if it is, whether the learning transfers equally from either hand. We also want to look into the effects of different force levels on the degree of transfer. We designed a task using a program that takes force levels as inputs and has the participant trace shapes on a screen. By having participants perform with one hand, then practice with the other, and finally perform again with the initial hand, we can measure transfer as the difference in performance before and after practice with the other hand.Kinesiology and Health Educatio

Radio-frequency dressing of multiple Feshbach resonances

We demonstrate and theoretically analyze the dressing of several proximate Feshbach resonances in Rb-87 using radio-frequency (rf) radiation. We present accurate measurements and characterizations of the resonances, and the dramatic changes in scattering properties that can arise through the rf dressing. Our scattering theory analysis yields quantitative agreement with the experimental data. We also present a simple interpretation of our results in terms of rf-coupled bound states interacting with the collision threshold.Comment: 4+ pages, 3 figures, 1 table; revised introduction & references to reflect published versio

Simulations of a Scintillator Compton Gamma Imager for Safety and Security

We are designing an all-scintillator Compton gamma imager for use in security investigations and remediation actions involving radioactive threat material. To satisfy requirements for a rugged and portable instrument, we have chosen solid scintillator for the active volumes of both the scatter and absorber detectors. Using the BEAMnrc/EGSnrc Monte Carlo simulation package, we have constructed models using four different materials for the scatter detector: LaBr_3, NaI, CaF_2 and PVT. We have compared the detector performances using angular resolution, efficiency, and image resolution. We find that while PVT provides worse performance than that of the detectors based entirely on inorganic scintillators, all of the materials investigated for the scatter detector have the potential to provide performance adequate for our purposes.Comment: Revised text and figures, Presented at SORMA West 2008, Published in IEEE Transactions on Nuclear Scienc

ELECTRON MICROSCOPIC AUTORADIOGRAPHY OF GERMINAL CENTER CELLS IN MOUSE SPLEEN

The fine structure of tritiated thymidine-labeled cells in antigen-stimulated mouse spleen germinal centers is described. In studies on the ultrastructural level, two labeled cell types found in germinal centers are observed. Large lymphocytes are characterized by their very numerous free ribosomes, a paucity of endoplasmic reticulum, relatively few mitochondria, and a poorly developed Golgi region. The nuclei are large and vesicular, and large nucleoli are present. A second labeled cell type appears to contain more mitochondria and has a higher development of the Golgi area. The nucleus contains large, numerous blocks of chromatin, indicative of a more differentiated cell type. Reticular cells, both phagocytic and non-phagocytic, were not observed to be labeled in the germinal centers