Publications (104)42.34 Total impact
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Dataset: IEEE 2010 HST GRETA
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Dataset: An Imaging CdZnTe Imaging Detector with coplanar orthogonal strips/Hamel et al.
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Conference Proceeding: Location and identification of radioactive material with the GRETA (Gamma-Ray Experimental Telescope Assembly)
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ABSTRACT: We previously reported Compton imaging measurements of <sup>137</sup>Cs and <sup>60</sup>Co laboratory sources using the full 15-channel readout of the Gamma-Ray Experimental Telescope Assembly (GRETA). We have since modified the detector geometry to accommodate a wider range of Compton scattering angles and implemented a neural networking method for the location of interaction positions with a 3-cm resolution inside the detector volumes. This paper describes the implementation of the neural networking method and culminates in the imaging of <sup>137</sup>Cs, <sup>60</sup>Co, and <sup>252</sup>Cf sources using this technique.Technologies for Homeland Security (HST), 2010 IEEE International Conference on; 12/2010 -
Article: Calibration of the Fast Neutron Imaging Telescope (FNIT) Prototype Detector
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ABSTRACT: The paper describes a novel detector for neutrons in the 1 to 20-MeV energy range with combined imaging and spectroscopic capabilities. The Fast Neutron Imaging Telescope (FNIT) was designed to detect solar neutrons from spacecraft deployed to the inner heliosphere. However, the potential application of this instrument to Special Nuclear Material (SNM) identification was also examined. In either case, neutron detection relies on double elastic neutron-proton (n-p) scattering in liquid scintillator. We optimized the design of FNIT through a combination of Monte Carlo simulations and lab measurements. We then assembled a scaled-down version of the full detector and assessed its performance by exposing it to a neutron beam and an SNM source. The results from these tests, which were used to characterize the response of the complete FNIT detector to fast neutrons, are presented herein.IEEE Transactions on Nuclear Science 11/2009; · 1.45 Impact Factor -
Conference Proceeding: A compton telescope for remote location and identification of radioactive material
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ABSTRACT: The spare detectors from NASA's Compton Gamma-Ray Observatory COMPTEL instrument have been reconfigured to demonstrate the capability at ground level to remotely locate and identify sources of gamma radiation. The gamma-ray experimental telescope assembly (GRETA) employs two 28 cm diameter scintillation detectors separated by 95 cm: one 8.5 cm thick liquid scintillator detector and one 7.5 cm thick NaI(Tl) detector. The assembly electronics and real-time data acquisition system measures the energy deposits and time-of-flight for each coincident detection and compiles histograms of total energy and incident angle as computed using the kinematics of Compton scattering. GRETA's field of view is a cone with full angle approximately 120deg. The sensitive energy range is 0.3 to 2.6 MeV. Energy resolution is ~10% FWHM. The angular resolution is better than 5deg. We have previously reported measurements using a simplified readout configuration that limited GRETA's imaging ability to a rough (~15deg) directional sensitivity. Here we report on measurements using the full (15-channel) readout that can achieve ~1.5-cm position resolution in each scintillator and permits true Compton imaging using COMPTEL software. GRETA has been refurbished to allow for ease of transport and field use with updated real time imaging and identification software. We present most recent laboratory measurements of radioactive sources of concern, <sup>137</sup>Cs and <sup>60</sup>Co, potential candidates used with radiological dispersal devices.Technologies for Homeland Security, 2009. HST '09. IEEE Conference on; 06/2009 -
Article: A Balloon‐Borne Gamma‐Ray Polarimeter for Gamma‐Ray Bursts
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ABSTRACT: We have developed a design for a hard X‐ray polarimeter operating in the energy range from 50 to 500 keV. This modular design, known as GRAPE (Gamma‐Ray Polarimeter Experiment), was successfully demonstrated with partially polarized gamma‐ray sources in the lab and fully polarized photon beams at Argonne National Laboratory. In June of 2007, a GRAPE engineering model flew on a high altitude balloon flight which demonstrated the design and collected background data. A much larger payload is currently under development that will provide a significant level of sensitivity for gamma‐ray bursts on a long‐duration balloon flight. The first balloon flight of the full GRAPE instrument is currently scheduled to take place in the Fall of 2011 from Ft. Sumner, NM.AIP Conference Proceedings. 05/2009; 1133(1):61-63. -
Article: The Soft Gamma-Ray Spectral Variability of Cygnus X-1
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ABSTRACT: We have used observations of Cyg X-1 from the Compton Gamma Ray Observatory and BeppoSAX to study the variation in the MeV γ-ray emission between the hard and soft spectral states, using spectra that cover the energy range from 20 keV up to 10 MeV. These data provide evidence for significant spectral variability at energies above 1 MeV. In particular, whereas the hard X-ray flux decreases during the soft state, the flux at energies above 1 MeV increases, resulting in a significantly harder γ-ray spectrum at energies above 1 MeV. This behavior is consistent with the general picture of galactic black hole candidates having two distinct spectral forms at soft γ-ray energies. These data extend this picture, for the first time, to energies above 1 MeV. We have used two different hybrid thermal/nonthermal Comptonization models to fit broadband spectral data obtained in both the hard and soft spectral states. These fits provide a quantitative estimate of the electron distribution and allow us to probe the physical changes that take place during transitions between the low and high X-ray states. We find that there is a significant increase (by a factor of ~4) in the bolometric luminosity as the source moves from the hard state to the soft state. Furthermore, the presence of a nonthermal tail in the Comptonizing electron distribution provides significant constraints on the magnetic field in the source region.The Astrophysical Journal 12/2008; 572(2):984. · 6.02 Impact Factor -
Article: A High-Sensitivity Measurement of the MeV Gamma-Ray Spectrum of Cygnus X-1
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ABSTRACT: The Compton Gamma Ray Observatory (CGRO) has observed the Cygnus region on several occasions since its launch in 1991. The data collected by the COMPTEL experiment on CGRO represent the most sensitive observations to date of Cygnus X-1 in the 0.75-30 MeV range. A spectrum accumulated by COMPTEL over 10 weeks of observation time shows significant evidence for emission extending out to several MeV. We have combined these data with contemporaneous data from both BATSE and OSSE to produce a broadband γ-ray spectrum, corresponding to the low X-ray state of Cygnus X-1, extending from 50 keV up to ~5 MeV. Although there is no evidence for any broad-line-like emissions in the MeV region, these data further confirm the presence of a hard tail at energies above several hundred keV. In particular, the spectrum at MeV energies can be described as a power law with a photon spectral index of α = -3.2, with no evidence for a cutoff at high energies. For the 200 keV-5 MeV spectrum, we provide a quantitative description of the underlying electron spectrum, in the context of a hybrid thermal/nonthermal model for the emission. The electron spectrum can be described by a thermal Maxwellian with a temperature of kTe = 86 keV and a nonthermal power-law component with a spectral index of pe = 4.5. The spectral data presented here should provide a useful basis for further theoretical modeling.The Astrophysical Journal 12/2008; 543(2):928. · 6.02 Impact Factor -
Article: Calibration of the Gamma-RAy Polarimeter Experiment (GRAPE) at a Polarized Hard X-Ray Beam
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ABSTRACT: The Gamma-RAy Polarimeter Experiment (GRAPE) is a concept for an astronomical hard X-ray Compton polarimeter operating in the 50 - 500 keV energy band. The instrument has been optimized for wide-field polarization measurements of transient outbursts from energetic astrophysical objects such as gamma-ray bursts and solar flares. The GRAPE instrument is composed of identical modules, each of which consists of an array of scintillator elements read out by a multi-anode photomultiplier tube (MAPMT). Incident photons Compton scatter in plastic scintillator elements and are subsequently absorbed in inorganic scintillator elements; a net polarization signal is revealed by a characteristic asymmetry in the azimuthal scattering angles. We have constructed a prototype GRAPE module containing a single CsI(Na) calorimeter element, at the center of the MAPMT, surrounded by 60 plastic elements. The prototype has been combined with custom readout electronics and software to create a complete "engineering model" of the GRAPE instrument. This engineering model has been calibrated using a nearly 100% polarized hard X-ray beam at the Advanced Photon Source at Argonne National Laboratory. We find modulation factors of 0.46 +/- 0.06 and 0.48 +/- 0.03 at 69.5 keV and 129.5 keV, respectively, in good agreement with Monte Carlo simulations. In this paper we present details of the beam test, data analysis, and simulations, and discuss the implications of our results for the further development of the GRAPE concept. Comment: 35 pages, 14 figures, accepted for publication in NIM-A12/2008; -
Article: POET: POlarimeters for Energetic Transients
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ABSTRACT: POET (Polarimeters for Energetic Transients) is a Small Explorer mission concept proposed to NASA in January 2008. The principal scientific goal of POET is to measure GRB polarization between 2 and 500 keV. The payload consists of two wide FoV instruments: a Low Energy Polarimeter (LEP) capable of polarization measurements in the energy range from 2-15 keV and a high energy polarimeter (Gamma-Ray Polarimeter Experiment -- GRAPE) that will measure polarization in the 60-500 keV energy range. Spectra will be measured from 2 keV up to 1 MeV. The POET spacecraft provides a zenith-pointed platform for maximizing the exposure to deep space. Spacecraft rotation will provide a means of effectively dealing with systematics in the polarization response. POET will provide sufficient sensitivity and sky coverage to measure statistically significant polarization for up to 100 GRBs in a two-year mission. Polarization data will also be obtained for solar flares, pulsars and other sources of astronomical interest.10/2008; -
Article: The MEGA Project for Medium Energy Gamma-ray Astronomy
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ABSTRACT: The Medium Energy Gamma-ray Astronomy (MEGA) telescope concept will soon be proposed as a MIDEX mission. This mission would enable a sensitive all-sky survey of the medium-energy gamma-ray sky (0.4–50 MeV) and bridge the huge sensitivity gap between the COMPTEL and OSSE experiments on the Compton Gamma Ray Observatory and the visionary Advanced Compton Telescope (ACT) mission. The scientific goals include compiling a much larger catalog of sources in this energy range, performing far deeper searches for supernovae, better measuring the galactic continuum and line emissions, and identifying the components of the cosmic diffuse gamma-ray emission. MEGA records and images gamma rays by completely tracking Compton and pair creation events in a stack of double-sided Si strip detectors surrounded by a pixellated CsI calorimeter. A prototype instrument has been developed and calibrated in the laboratory and at a gamma-ray beam facility. We present calibration results from the prototype and describe the proposed satellite mission.Chinese Journal of Astronomy and Astrophysics 01/2007; 6(S1):388. · 0.89 Impact Factor -
Article: Design and Testing of a Position-Sensitive Plastic Scintillator Detector for Fast Neutron Imaging
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ABSTRACT: We describe the design and performance of a position sensitive scintillator detector developed for neutron measurements. Several of these detectors are to be used in the assembly of the Fast Neutron Imaging Telescope (FNIT), an instrument with imaging and energy measurement capabilities, sensitive to neutrons in the 2-20 MeV energy range. FNIT was initially conceived to study solar neutrons as a candidate instrument for the Inner Heliospheric Sentinels (IHS) program under formulation at NASA. It is now being adapted to locate Special Nuclear Material (SNM) for homeland security purposes by detecting fission neutrons and reconstructing the image of their source. The detection principle is based on multiple elastic neutron-proton scatterings in organic scintillator. The detector presented here utilizes wavelength-shifting (WLS) fibers, grooved into the plastic scintillator and read out by multianode photomultiplier tubes (MAPMTs) to determine scattering locations. By also measuring the recoil proton and scattered neutron's energies, the direction and energy spectrum of incident neutrons can be determined and discrete sources identifiedIEEE Transactions on Nuclear Science 01/2007; · 1.45 Impact Factor -
Article: Evidence for 56Co Line Emission from the Type Ia Supernova 1991T using COMPTELa
Annals of the New York Academy of Sciences 12/2006; 759(1):397 - 400. · 3.15 Impact Factor -
Conference Proceeding: Medium-Energy Gamma-Ray Astrophysics with the 3-DTI Gamma-Ray Telescope
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ABSTRACT: Gamma-ray observations in the medium energy range (0.50-50.0 MeV) are central to unfolding many outstanding questions in astrophysics. The challenges of medium-energy gamma-ray observations, however, are the low photon statistics and large backgrounds. We review these questions, address the telescope technology requirements, and describe our development of the 3-Dimensional Track Imaging (3-DTI) Compton telescope and its performance for a new medium-energy gamma-ray mission. The 3-DTI is a large-volume time projection chamber (TPC) with a 2-dimensional gas micro-well detector (MWD) readout.Nuclear Science Symposium Conference Record, 2006. IEEE; 12/2006 -
Conference Proceeding: Optimization of Single-Sided Charge-Sharing Strip Detectors
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ABSTRACT: Simulation of the charge sharing properties of single-sided CZT strip detectors with small anode pads are presented. The effect of initial event size, carrier repulsion, diffusion, drift, trapping and detrapping are considered. These simulations indicate that such a detector with a 150 mum pitch will provide good charge sharing between neighboring pads. This is supported by a comparison of simulations and measurements for a similar detector with a coarser pitch of 225 mum that could not provide sufficient sharing. The performance of such a detector used as a gamma-ray imager is discussed.Nuclear Science Symposium Conference Record, 2006. IEEE; 12/2006 -
Conference Proceeding: Position Resolution in LaBr3 and LaCl3 Scintillators Using Position-Sensitive Photomultiplier Tubes
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ABSTRACT: Advanced scintillator materials such as LaBr<sub>3</sub>:Ce and LaCl<sub>3</sub>:Ce hold great promise for future hard X-ray and gamma-ray astrophysics missions due to their high density, high light output, good linearity, and fast decay times. Of particular importance for future space-based imaging instruments, such as coded-aperture telescopes, is the precise spatial location of individual gamma-ray interactions. We have investigated the position and energy resolution achievable within monolithic (5 cm times 5 cm times 1 cm) LaBr<sub>3</sub>:Ce and LaCl<sub>3</sub>:Ce crystals using position-sensitive light readout devices, including a position-sensitive photomultiplier tube and a multi-anode photomultiplier tube. We present the results of these tests and discuss the applicability of such advanced scintillators to future high-energy imaging astrophysics missions.Nuclear Science Symposium Conference Record, 2006. IEEE; 12/2006 -
Conference Proceeding: Simulated Performance of 3-DTI Gamma-Ray Telescope Concepts
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ABSTRACT: We present Monte Carlo simulations of two astronomical gamma-ray telescope concepts based on the Three-Dimensional Track Imager (3-DTI) detector. The 3-DTI consists of a time projection chamber with two-dimensional, crossed-strip micro-well detector readout. The full three-dimensional reconstruction of charged-particle tracks in the gas volume is obtained from transient digitizers, which record the time signature of the charge collected in the wells of each strip. Such detectors hold great promise for advanced Compton telescope (ACT) and advanced pair telescope (APT) concepts due to the very precise measurement of charged particle momenta that is possible (Compton recoil electrons and electron-positron pairs, respectively). We have investigated the performance of baseline ACT and APT designs based on the 3-DTI detector using simulation tools based on GEANT3 and GEANT4, respectively. We present the expected imaging, spectroscopy, polarimetry, and background performance of each design.Nuclear Science Symposium Conference Record, 2006. IEEE; 12/2006 -
Conference Proceeding: Radiation Damage and Activation from Proton Irradiation of Advanced Scintillators
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ABSTRACT: We present results from a proton accelerator beam test to measure radiation damage and activation in advanced scintillator materials. Samples of LaBr<sub>3</sub>:Ce and LaCl<sub>2</sub>:Ce were exposed to protons from 40-250 MeV at the Proton Irradiation Facility of the Paul Scherrer Institute in Switzerland. Twelve energy bands were used to simulate the spectrum of the South Atlantic Anomaly (SAA), with different samples exposed to the equivalent of 4 months, 1 year, and 5 years of SAA passage. No significant decrease in light output was found due to radiation damage, indicating that these new scintillator materials are radiation tolerant. High-resolution spectra of the samples were obtained before and after irradiation with a germanium spectrometer to study activation. We present a detailed analysis of these spectra and a discussion of the suitability of these scintillator materials for detectors in future space missions.Nuclear Science Symposium Conference Record, 2006. IEEE; 12/2006 -
Article: Prospects for GRB Polarimetry with GRAPE
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ABSTRACT: This paper discusses the latest progress in the development of GRAPE (Gamma‐Ray Polarimeter Experiment), a hard X‐ray Compton Polarimeter. The purpose of GRAPE is to measure the polarization of hard X‐rays in the 50–300 keV energy range. We are particularly interested in X‐rays that are emitted from solar flares and gamma‐ray bursts (GRBs). Accurately measuring the polarization of the emitted radiation from these sources will lead to a better understating of both the emission mechanisms and source geometries. The GRAPE design consists of an array of plastic scintillators surrounding a central high‐Z crystal scintillator. We can monitor individual Compton scatters that occur in the plastics and determine whether the photon is photo absorbed by the high‐Z crystal or not. A Compton scattered photon that is immediately photo absorbed by the high‐Z crystal constitutes a valid event. These valid events provide us with the interaction locations of each incident photon and ultimately produces a modulation pattern for the Compton scattering of the polarized radiation. Comparing with Monte Carlo simulations of a 100% polarized beam, the level of polarization of the measured beam can then be determined. The complete array is mounted on a flat‐panel multi‐anode photomultiplier tube (MAPMT) that can measure the deposited energies resulting from the photon interactions. The design of the detector allows for a large field‐of‐view (> π steradian), at the same time offering the ability to be close‐packed with multiple modules in order to reduce deadspace. We present in this paper the latest laboratory results obtained from GRAPE using partially polarized radiation sources along with a brief description of our future plans for the GRAPE design. © 2006 American Institute of PhysicsAIP Conference Proceedings. 05/2006; 836(1):654-659. -
Article: The CASTER Black Hole Finder Probe
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ABSTRACT: The primary scientific mission of the Black Hole Finder Probe (BHFP), part of the NASA Beyond Einstein program, is to survey the local Universe for black holes over a wide range of mass and accretion rate. One approach to such a survey is a hard X-ray coded-aperture imaging mission operating in the 10--600 keV energy band, a spectral range that is considered to be especially useful in the detection of black hole sources. The development of new inorganic scintillator materials provides improved performance (for example, with regards to energy resolution and timing) that is well suited to the BHFP science requirements. Detection planes formed with these materials coupled with a new generation of readout devices represent a major advancement in the performance capabilities of scintillator-based gamma cameras. Here, we discuss the Coded Aperture Survey Telescope for Energetic Radiation (CASTER), a concept that represents a BHFP based on the use of the latest scintillator technology.11/2005; 37:1215.
Top co-authors
Top Journals
Institutions
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1982–2010
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University of New Hampshire
- • Space Science Center
- • Department of Physics
Durham, NH, USA
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1998–2006
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Université de Montréal
- Department of Physics
Montréal, Quebec, Canada
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2005
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University of Alabama in Huntsville
Huntsville, AL, USA
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