2,746 research outputs found
Detection of GRB signals with Fluorescence Detectors
Gamma Ray Bursts are being searched in many ground based experiments
detecting the high energy component (GeV TeV energy range) of the photon
bursts. In this paper, Fluorescence Detectors are considered as possible
candidate devices for these searches. It is shown that the GRB photons induce
fluorescence emission of UV photons on a wide range of their spectrum. The
induced fluorescence flux is dominated by GRB photons from 0.1 to about 100 MeV
and, once the extinction through the atmosphere is taken into account, it is
distributed over a wide angular region. This flux can be detected through a
monitor of the diffuse photon flux, provided that its maximum value exceeds a
threshold value, that is primarily determined by the sky brightness above the
detector. The feasibility of this search and the expected rates are discussed
on the basis of the current GRB observations and the existing fluorescence
detectors.Comment: 16 pages 9 eps figure
Improving the Efficiency of Photon Collection by Compton Rescue
A method to improve the efficiency of photon collection in thin planar HPGe detectors was investigated. The method involved implementing a second HPGe detector to collect Compton scattered photons from the primary detector and incorporating coincident interactions in the two detectors that sum to the full energy event into the energy spectrum. This method is termed Compton rescue because the Compton scattered photons make a partial energy deposition in the primary detector and are added back to the spectrum after being detected by the second detector. This research has implications on improving the efficiency of positron annihilation spectroscopy (PAS) techniques including the use of the method in angular correlation of annihilation radiation (ACAR) and Doppler-broadening of annihilation radiation (DBAR) applications. The effect of using Compton rescue on the energy and spatial resolution on these two PAS techniques was investigated. The research was conducted in two phases: simulation, in which a Monte-Carlo program was used to predict the effectiveness of the Compton rescue method based on photon interaction simulations, and experiment, in which a position-sensitive HPGe detector and a large coaxial HPGe detector were used to implement Compton rescue. A two-detector DBAR experiment on single-crystal Ni was conducted using the Compton rescue setup to illustrate its utility
Gigahertz (GHz) hard x-ray imaging using fast scintillators
Gigahertz (GHz) imaging technology will be needed at high-luminosity X-ray and charged particle sources. It is plausible to combine fast scintillators with the latest picosecond detectors and GHz electronics for multi-frame hard Xray imaging and achieve an inter-frame time of less than 10 ns. The time responses and light yield of LYSO, LaBr_3, BaF_2 and ZnO are measured using an MCP-PMT detector. Zinc Oxide (ZnO) is an attractive material for fast hard X-ray imaging based on GEANT4 simulations and previous studies, but the measured light yield from the samples is much lower than expected
Modelling and testing the x-ray performance of CCD and CMOS APS detectors using numerical finite element simulations
Pixellated monolithic silicon detectors operated in a photon-counting regime are useful in spectroscopic imaging applications. Since a high energy incident photon may produce many excess free carriers upon absorption, both energy and spatial information can be recovered by resolving each interaction event. The performance of these devices in terms of both the energy and spatial resolution is in large part determined by the amount of diffusion which occurs during the collection of the charge cloud by the pixels. Past efforts to predict the X-ray performance of imaging sensors have used either analytical solutions to the diffusion equation or simplified monte carlo electron transport models. These methods are computationally attractive and highly useful but may be complemented using more physically detailed models based on TCAD simulations of the devices. Here we present initial results from a model which employs a full transient numerical solution of the classical semiconductor equations to model charge collection in device pixels under stimulation from initially Gaussian photogenerated charge clouds, using commercial TCAD software. Realistic device geometries and doping are included. By mapping the pixel response to different initial interaction positions and charge cloud sizes, the charge splitting behaviour of the model sensor under various illuminations and operating conditions is investigated. Experimental validation of the model is presented from an e2v CCD30-11 device under varying substrate bias, illuminated using an Fe-55 source
An experimental and theoretical study of the dark current and x-ray sensitivity of amorphous selenium x-ray photoconductors
Recently, the world of diagnostic radiography has seen the integration of digital flat panel x-ray image detectors into x-ray imaging systems, replacing analog film screens. These flat panel x-ray imagers (FPXIs) have been shown to produce high quality x-ray images and provide many advantages that are inherent to a fully digital technology. Direct conversion FPXIs based on a photoconductive layer of stabilized amorphous selenium (a-Se) have been commercialized and have proven particularly effective in the field of mammography. In the operation of these detectors, incident x-ray photons are converted directly to charge carriers in the a-Se layer and drifted to electrodes on either side of the layer by a large applied field (10 V/μm). The applied field causes a dark current to flow which is not due to the incident radiation and this becomes a source of noise which can reduce the dynamic range of the detector. The level of dark current in commercialized detectors has been reduced by the deposition of thin n- and p- type blocking layers between the electrodes and the bulk of the a-Se. Despite recent research into the dark current in metal/a-Se/metal sandwich structures, much is still unknown about the true cause and nature of this phenomenon. The work in this Ph.D. thesis describes an experimental and theoretical study of the dark current in these structures. Experiments have been performed on five separate sets of a-Se samples which approximate the photoconductive layer in an FPXI. The dark current has been measured as a function of time, sample structure, applied field, sample thickness and contact metal used. This work has conclusively shown that the dark current is almost entirely due to the injection of charge carriers from the contacts and the contribution of Poole-Frenkel enhanced bulk thermal generation is negligible. There is also evidence that while the dark current is initially controlled by the injection of holes from the positive contact, several minutes after the application of the bias, the dark current due to hole injection may decay to the point where the electron current becomes significant and even dominant. These conclusions are supported by numerical calculations of the dark current transients which have been calibrated to match experimental results.
Work detailed in this Ph.D. thesis also focuses on Monte Carlo modeling of the x-ray sensitivity of a-Se FPXIs. The higher the x-ray sensitivity of a detector, the lower the radiation dose required to acquire an acceptable image. FPXIs can experience a decrease in the x-ray sensitivity of the photoconductive layer with accumulating exposure, leading to a phenomenon known as “ghosting”. Modeling this decrease in sensitivity can uncover the reasons behind it. The Monte Carlo model described in this thesis is a continuation of a previous model which now considers the effects of the n- and p-like blocking layers and the flow of dark current between x-ray exposures. The simulation results explain how deep trapping of photogenerated charge carriers, and the resulting effect on the electric field distribution, contribute to sensitivity loss. The model has shown excellent agreement with experimental data and has accurately predicted a sensitivity recovery once exposure has ceased which is due to primarily to the relaxation of metastable x-ray-induced carrier trap states
Searching for Gamma-Ray counterparts to Gravitational Waves from merging binary neutron stars with the Cherenkov Telescope Array
The merger of binary neutron star (BNS) systems are predicted to be
progenitors of short gamma-ray bursts (GRBs); the definitive probe of this
association came with the recent detection of gravitational waves (GWs) from a
BNS merger by Advanced LIGO and Advanced Virgo (GW170817), in coincidence with
the short GRB 170817A observed by Fermi-GBM and INTEGRAL. Short GRBs are also
expected to emit very-high energy (VHE, > 100 GeV) photons and VHE
electromagnetic (EM) upper limits have been set with observations performed by
ground-based gamma-ray detectors and during the intense EM follow-up campaign
associated with GW170817/GRB 170817A. In the next years, the searches for VHE
EM counterparts will become more effective thanks to the Cherenkov Telescope
Array (CTA): this instrument will be fundamental for the EM follow-up of
transient GW events at VHE, owing to its unprecedented sensitivity, rapid
response (few tens of seconds) and capability to monitor large sky areas via
survey-mode operation. We present a comprehensive study on the prospects for
joint GW and VHE EM observations of merging BNSs with Advanced LIGO, Advanced
Virgo and CTA, based on detailed simulations of the multi-messenger emission
and detection. We propose a new observational strategy optimized on the prior
assumptions about the EM emission. The method can be further generalized to
include other electromagnetic emission models. According to this study CTA will
cover most of the region of the GW skymap for the intermediate and most
energetic on-axis GRBs associated to the GW event. We estimate the expected
joint GW and VHE EM detection rates and we found this rate goes from 0.08 up to
0.5 events per year for the most energetic EM sources.Comment: 26 pages, 8 figures. Submitted to JCA
Research and development of a gamma-ray imaging spectrometer in the MeV range in Barcelona
Copyright 2010 Society of Photo-Optical Instrumentation Engineers (SPIE). One print or electronic copy may be made for personal use only. Systematic electronic or print reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.Gamma-ray astrophysics in the MeV energy range plays an important role for the understanding of cosmic explosions and acceleration mechanisms in a variety of galactic and extragalactic sources, e.g., Supernovae, Classical Novae, Supernova Remnants (SNRs), Gamma-Ray Bursts (GRBs), Pulsars, Active Galactic Nuclei (AGN). Through the development of focusing telescopes in the MeV energy range, it will be possible to reach unprecedented sensitivities, compared with those of the currently operating gamma ray telescopes. In order to achieve the needed performance, a detector with mm spatial resolution and very high peak efficiency is required. It will be also desirable that the detector could detect polarization of the source. Our research and development activities in Barcelona aim to study a gamma-ray imaging spectrometer in the MeV range suited for the focal plane of a gamma-ray telescope mission, based on CdTe pixel detectors arranged in multiple layers with increasing thicknesses, to enhance gamma-ray absorption in the Compton regime. We have developed an initial prototype based on several CdTe module detectors, with 11x11 pixels, a pixel pitch of 1mm and a thickness of 2mm. Each pixel is stud-bump bonded to a fanout board and routed to a readout ASIC to measure pixel position, pulse height and rise time information for each incident gamma-ray photon. We will report on the results of an optimization study based on simulations, to select the optimal thickness of each CdTe detector within the module to get the best energy resolution of the spectrometer.Peer reviewe
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