1,222 research outputs found
Characterization of Thin p-on-p Radiation Detectors with Active Edges
Active edge p-on-p silicon pixel detectors with thickness of 100 m were
fabricated on 150 mm Float zone silicon wafers at VTT. By combining measured
results and TCAD simulations, a detailed study of electric field distributions
and charge collection performances as a function of applied voltage in a p-on-p
detector was carried out. A comparison with the results of a more conventional
active edge p-on-n pixel sensor is presented. The results from 3D spatial
mapping show that at pixel-to-edge distances less than 100 m the sensitive
volume is extended to the physical edge of the detector when the applied
voltage is above full depletion. The results from a spectroscopic measurement
demonstrate a good functionality of the edge pixels. The interpixel isolation
above full depletion and the breakdown voltage were found to be equal to the
p-on-n sensor while lower charge collection was observed in the p-on-p pixel
sensor below 80 V. Simulations indicated this to be partly a result of a more
favourable weighting field in the p-on-n sensor and partly of lower hole
lifetimes in the p-bulk.Comment: 23 pages, 16 figures, 1 tabl
Ground state and excitation dynamics in Ag doped helium clusters
We present a quantum Monte Carlo study of the structure and energetics of
silver doped helium clusters AgHe for up to 100. Our simulations show
the first solvation shell of the Ag atom to be composed by roughly 20 He atoms,
and to possess a structured angular distribution. Moreover, the electronic
PS and PS
electronic transitions of the embedded silver impurity have been studied as a
function of the number ofhelium atoms. The computed spectra show a redshift for
and an increasing blueshift for larger clusters, a feature
attributed to the effect of the second solvation shell of He atoms. For the
largest cluster, the computed excitation spectrum is found in excellent
agreement with the ones recorded in superfluid He clusters and bulk. No
signature of the direct formation of proposed AgHe exciplex is present in
the computed spectra of AgHe.Comment: 4 Pages, 3 Figures, submitted to Phys. Rev. Let
Shock Deformation in Zircon, a Comparison of Results from Shock-Reverberation and Single-Shock Experiments
The utility of the mineral zircon, ZrSiO4, as a shock-metamorphic geobarometer and geochronometer, has been steadily growing within the planetary science community. Zircon is an accessory phase found in many terrestrial rock types, lunar samples, lunar meteorites, martian meteorites and various other achondrites. Because zircon is refractory and has a high closure temperature for Pb diffusion, it has been used to determine the ages of some of the oldest material on Earth and elsewhere in the Solar System. Furthermore, major (O) and trace-element (REE, Ti, Hf) abundances and isotope compositions of zircon help characterize the petrogenetic environments and sources from which they crystallized. The response of zircon to impact-induced shock deformation is predominantly crystallographic, including dislocation creep and the formation of planar and sub-planar, low-angle grain boundaries; the formation of mechanical {112} twins; transformation to the high pressure polymorph reidite; the development of polycrystalline microtextures; and dissociation to the oxide constituents SiO2 and ZrO2. Shock microstructures can also variably affect the U- Pb isotope systematics of zircon and, in some instances, be used to constrain the impact age. While numerous studies have characterized shock deformation in zircon recovered from a variety of terrestrial impact craters and ejecta deposits and Apollo samples, experimental studies of shock deformation in zircon are limited to a handful of examples in the literature. In addition, the formation conditions (e.g., P, T) of various shock microstructures, such as planar-deformation bands, twins, and reidite lamellae, remain poorly con-strained. Furthermore, previous shocked-zircon experimental charges have not been analyzed using modern analytical equipment. This study will therefore under-take an new set of zircon shock experiments, which will then be microstructurally characterized using state-of-the-art instrumentation within the Astromaterials Research and Exploration Science Division (ARES), NASA Johnson Space Center
Real-time in-vivo Ό-imaging with Medipix2
Abstract An X-ray micro-radiographic system based on the Medipix2 semiconductor pixel detector for dynamic high spatial resolution and for high contrast imaging has been developed. Our system is based on a micro-focus and nano-focus X-ray tube and the hybrid single-photon counting silicon pixel detector Medipix2 (matrix 256Ă256 sq. pixels of 55 ÎŒm pitch). This compact table-top system stands promising as a new tool in the field of small animal imaging as well as in the in-vivo observation of dynamic processes inside living organisms. The main advantages of these Medipix2 pixel detectors include: high sensitivity to low-energy X-ray photons; position sensitive and noiseless single-photon detection with preselected photon energies; single-quantum counting in each pixel performed by digital counter (therefore there is no dark current); digital integration (providing unlimited dynamic range and absolute linearity in device response to number of photons, high sensitivity and high contrast); real-time digital information, high-speed digital communication and data transfer. We improve the picture quality with the help of statistical data analysis and extended the calibration of individual pixels response. 2D and 3D radiographic images of samples demonstrate the potential and applicability of our system for precise in-vivo X-ray high-resolution dynamic diagnostic and biological studies. Obtained results are shown on small animal and organic samples
Rydberg spectroscopy of barium monofluoride
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 1995.by Zygmunt J. Jakubek.Ph.D
BrachyView, A novel inbody imaging system for HDR prostate brachytherapy: Design and Monte Carlo feasibility study
Purpose: High dose rate (HDR) brachytherapy is a form of radiation therapy for treating prostate cancer whereby a high activity radiation source is moved between predefined positions inside applicators inserted within the treatment volume. Accurate positioning of the source is essential in delivering the desired dose to the target area while avoiding radiation injury to the surrounding tissue. In this paper, HDR BrachyView, a novel inbody dosimetric imaging system for real time monitoring and verification of the radioactive seed position in HDR prostate brachytherapy treatment is introduced. The current prototype consists of a 15 à 60 mm2 silicon pixel detector with a multipinhole tungsten collimator placed 6.5 mm above the detector. Seven identical pinholes allow full imaging coverage of the entire treatment volume. The combined pinhole and pixel sensor arrangement is geometrically designed to be able to resolve the three-dimensional location of the source. The probe may be rotated to keep the whole prostate within the transverse plane. The purpose of this paper is to demonstrate the efficacy of the design through computer simulation, and to estimate the accuracy in resolving the source position (in detector plane and in 3D space) as part of the feasibility study for the BrachyView project. Methods: Monte Carlo simulations were performed using the GEANT4 radiation transport model, with a 192Ir source placed in different locations within a prostate phantom. A geometrically accurate model of the detector and collimator were constructed. Simulations were conducted with a single pinhole to evaluate the pinhole design and the signal to background ratio obtained. Second, a pair of adjacent pinholes were simulated to evaluate the error in calculated source location. Results: Simulation results show that accurate determination of the true source position is easily obtainable within the typical one second source dwell time. The maximum error in the estimated projection position was found to be 0.95 mm in the imaging (detector) plane, resulting in a maximum source positioning estimation error of 1.48 mm. Conclusions: HDR BrachyView is a feasible design for real-time source tracking in HDR prostate brachytherapy. It is capable of resolving the source position within a subsecond dwell time. In combination with anatomical information obtained from transrectal ultrasound imaging, HDR BrachyView adds a significant quality assurance capability to HDR brachytherapy treatment systems. © 2013 American Association of Physicists in Medicine
Experimental Tests of Neutron Shielding for the ATLAS Forward Region
Experimental tests devoted to the optimization of the neutron shielding for the ATLAS forward region were performed at the CERN-PS with a 4 GeV/c proton beam. Spectra of fast neutrons, slow neutrons and gamma rays escaping a block of iron (404080 cm) shielded with different types of neutron and gamma shields (pure polyethylene - PE, borated polyethylene - BPE, lithium filled polyethylene - LiPE, lead, iron) were measured by means of plastic scintillators, a Bonner spectrometer, a HPGe detector and a slow neutron detector. Effectiveness of different types of shielding agaisnt neutrons and -rays were compared. The idea of a segmented outer layer shielding (iron, BPE, iron, LiPE) for the ATLAS Forward Region was also tested
Single hadron response measurement and calorimeter jet energy scale uncertainty with the ATLAS detector at the LHC
The uncertainty on the calorimeter energy response to jets of particles is
derived for the ATLAS experiment at the Large Hadron Collider (LHC). First, the
calorimeter response to single isolated charged hadrons is measured and
compared to the Monte Carlo simulation using proton-proton collisions at
centre-of-mass energies of sqrt(s) = 900 GeV and 7 TeV collected during 2009
and 2010. Then, using the decay of K_s and Lambda particles, the calorimeter
response to specific types of particles (positively and negatively charged
pions, protons, and anti-protons) is measured and compared to the Monte Carlo
predictions. Finally, the jet energy scale uncertainty is determined by
propagating the response uncertainty for single charged and neutral particles
to jets. The response uncertainty is 2-5% for central isolated hadrons and 1-3%
for the final calorimeter jet energy scale.Comment: 24 pages plus author list (36 pages total), 23 figures, 1 table,
submitted to European Physical Journal
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