Characteristic of x-ray tomography performance using CdTe timepix detector

X-ray Computed Tomography (CT) is a non-destructive technique for visualizing interior features within solid objects, and for obtaining digital information on their 3-D geometries and properties. The selection of CdTe Timepix detector has a sufficient performance of imaging detector is based on quality of detector performance and energy resolution. The study of Modulation Transfer Function (MTF) shows a 70% contrast at 4 lp/mm was achieved for the 55 µm pixel pitch detector with the 60 kVp X-ray tube and 5 keV noise level. No significant degradation in performance was observed for X-ray tube energies of 20 – 60 keV. The paper discusses the application of the CdTe Timepix detector to produce a good quality image of X-ray tomography imaging

Human frontal eye fields and spatial priming of pop-out

"Priming of pop-out" is a form of implicit memory that facilitates detection of a recently inspected search target. Repeated presentation of a target's features or its spatial position improves detection speed (feature/spatial priming). This study investigated a role for the human frontal eye fields (FEFs) in the priming of color pop-out. To test the hypothesis that the FEFs play a role in short-term memory storage, transcranial magnetic stimulation (TMS) was applied during the intertrial interval. There was no effect of TMS on either spatial or feature priming. To test whether the FEFs are important when a saccade is being programmed to a repeated target color or location, TMS was applied during the search array. TMS over the left but not the right FEFs abolished spatial priming, but had no effect on feature priming. These findings demonstrate functional specialization of the left FEFs for spatial priming, and distinguish this role from target discrimination and saccade-related processes. The results suggest that the left FEFs integrate a spatial memory signal with an evolving saccade program, which facilitates saccades to a recently inspected location

All-optical coherent population trapping with defect spin ensembles in silicon carbide

Divacancy defects in silicon carbide have long-lived electronic spin states and sharp optical transitions, with properties that are similar to the nitrogen-vacancy defect in diamond. We report experiments on 4H-SiC that investigate all-optical addressing of spin states with the zero-phonon-line transitions. Our magneto-spectroscopy results identify the spin $S=1$ structure of the ground and excited state, and a role for decay via intersystem crossing. We use these results for demonstrating coherent population trapping of spin states with divacancy ensembles that have particular orientations in the SiC crystal.Comment: 28 page document: Pages 1-14 main text (with 3 figures); pages 15-28 supplementary information (with 5 figues). v2 has minor correction

Radiation Induced Damage in GaAs Particle Detectors

The motivation for investigating the use of GaAs as a material for detecting particles in experiments for High Energy Physics (HEP) arose from its perceived resistance to radiation damage. This is a vital requirement for detector materials that are to be used in experiments at future accelerators where the radiation environments would exclude all but the most radiation resistant of detector types.Comment: 5 pages. PS file only - original in WORD Also available at http://ppewww.ph.gla.ac.uk/preprints/97/06

Exploring transmission Kikuchi diffraction using a Timepix detector

Electron backscatter diffraction (EBSD) is a well-established scanning electron microscope (SEM)-based technique [1]. It allows the non-destructive mapping of the crystal structure, texture, crystal phase and strain with a spatial resolution of tens of nanometers. Conventionally this is performed by placing an electron sensitive screen, typically consisting of a phosphor screen combined with a charge coupled device (CCD) camera, in front of a specimen, usually tilted 70° to the normal of the exciting electron beam. Recently, a number of authors have shown that a significant increase in spatial resolution is achievable when Kikuchi diffraction patterns are acquired in transmission geometry; that is when diffraction patterns are generated by electrons transmitted through an electron-transparent, usually thinned, specimen. The resolution of this technique, called transmission Kikuchi diffraction (TKD), has been demonstrated to be better than 10 nm [2,3]. We have recently demonstrated the advantages of a direct electron detector, Timepix [4,5], for the acquisition of standard EBSD patterns [5]. In this article we will discuss the advantages of Timepix to perform TKD and for acquiring spot diffraction patterns and more generally for acquiring scanning transmission electron microscopy micrographs in the SEM. Particularly relevant for TKD, is its very compact size, which allows much more flexibility in the positioning of the detector in the SEM chamber. We will furthermore show recent results using Timepix as a virtual forward scatter detector, and will illustrate the information derivable on producing images through processing of data acquired from different areas of the detector. We will show results from samples ranging from gold nanoparticles to nitride semiconductor nanorods

Preliminary Results for LP VPE X-Ray Detectors

Thick epitaxial layers have been grown using Low Pressure Vapour Phase Epitaxy techniques with low free carrier concentrations . This type of material is attractive as a medium for X-ray detection, because of its high conversion efficiency for X-rays in the medically interesting energy range.Comment: 4 pages. PS file only - original in WORD. Also available at http://ppewww.ph.gla.ac.uk/preprints/97/07

Observation of multiple sausage oscillations in cool postflare loop

Using simultaneous high spatial (1.3 arc sec) and temporal (5 and 10 s) resolution H-alpha observations from the 15 cm Solar Tower Telescope at ARIES, we study the oscillations in the relative intensity to explore the possibility of sausage oscillations in the chromospheric cool postflare loop. We use standard wavelet tool, and find the oscillation period of ~ 587 s near the loop apex, and ~ 349 s near the footpoint. We suggest that the oscillations represent the fundamental and the first harmonics of fast sausage waves in the cool postflare loop. Based on the period ratio P1/P2 ~ 1.68, we estimate the density scale height in the loop as ~ 17 Mm. This value is much higher than the equilibrium scale height corresponding to H-alpha temperature, which probably indicates that the cool postflare loop is not in hydrostatic equilibrium. Seismologically estimated Alfv\'en speed outside the loop is ~ 300-330 km/s. The observation of multiple oscillations may play a crucial role in understanding the dynamics of lower solar atmosphere, complementing such oscillations already reported in the upper solar atmosphere (e.g., hot flaring loops).Comment: 13 pages, 4 figures, accepted in MNRA

Simulations of AGN feedback in galaxy clusters and groups: impact on gas fractions and the Lx-T scaling relation

Recently, rapid observational and theoretical progress has established that black holes (BHs) play a decisive role in the formation and evolution of individual galaxies as well as galaxy groups and clusters. In particular, there is compelling evidence that BHs vigorously interact with their surroundings in the central regions of galaxy clusters, indicating that any realistic model of cluster formation needs to account for these processes. This is also suggested by the failure of previous generations of hydrodynamical simulations without BH physics to simultaneously account for the paucity of strong cooling flows in clusters, the slope and amplitude of the observed cluster scaling relations, and the high-luminosity cut-off of central cluster galaxies. Here we use high-resolution cosmological simulations of a large cluster and group sample to study how BHs affect their host systems. We focus on two specific properties, the halo gas fraction and the X-ray luminosity-temperature scaling relation, both of which are notoriously difficult to reproduce in self-consistent hydrodynamical simulations. We show that BH feedback can solve both of these issues, bringing them in excellent agreement with observations, without alluding to the `cooling only' solution that produces unphysically bright central galaxies. By comparing a large sample of simulated AGN-heated clusters with observations, our new simulation technique should make it possible to reliably calibrate observational biases in cluster surveys, thereby enabling various high-precision cosmological studies of the dark matter and dark energy content of the universe.Comment: 4 pages, 2 figures, minor revisions, ApJL in pres

Metal Mixing and Ejection in Dwarf Galaxies is Dependent on Nucleosynthetic Source

Using a high resolution simulation of an isolated dwarf galaxy, accounting for multi-channel stellar feedback and chemical evolution on a star-by-star basis, we investigate how each of 15 metal species are distributed within our multi-phase interstellar medium (ISM) and ejected from our galaxy by galactic winds. For the first time, we demonstrate that the mass fraction probability distribution functions (PDFs) of individual metal species in the ISM are well described by a piecewise log-normal and power-law distribution. The PDF properties vary within each ISM phase. Hot gas is dominated by recent enrichment, with a significant power-law tail to high metal fractions, while cold gas is predominately log-normal. In addition, elements dominated by asymptotic giant branch (AGB) wind enrichment (e.g. N and Ba) mix less efficiently than elements dominated by supernova enrichment (e.g. $\alpha$ elements and Fe). This result is driven by the differences in source energetics and source locations, particularly the higher chance compared to massive stars for AGB stars to eject material into cold gas. Nearly all of the produced metals are ejected from the galaxy (only 4% are retained), but over 20% of metals dominated by AGB enrichment are retained. In dwarf galaxies, therefore, elements synthesized predominately through AGB winds should be both overabundant and have a larger spread compared to elements synthesized in either core collapse or Type Ia supernovae. We discuss the observational implications of these results, their potential use in developing improved models of galactic chemical evolution, and their generalization to more massive galaxies.Comment: 18 pages, 7 figures (plus 2 page, 2 figure appendix). Accepted to Ap