arXiv subjects

P. F. Bloser

Publications and source records attributed to P. F. Bloser.

At least 19 recordsLinked to original sources

arXiv2021

Performance Evaluation of Pulse Shape Discrimination Capable Organic Scintillators for Space Applications

Scintillators with pulse-shape discrimination (PSD) capability are of great interest to many fields in the scientific community. The ability to discern a gamma ray from a neutron using PSD varies between different types of scintillator materials and dopants. A new generation of organic scintillator materials with PSD capability were studied to determine their radiation hardness to ionizing and non-ionizing radiation. The PSD capability, average pulse shapes, and light output of four types of organic scintillator were characterized before and after neutron and gamma-ray irradiation. The main goal of this investigation is to study the effects of long-term irradiation that may be experienced in space applications on the light output and particle discriminating capabilities of each material. EJ-270, EJ-276, organic glass, and Stilbene were tested. Damage due to non-ionizing (neutron) radiation was not observed in any of the scintillators up to $2.56\times10^{11}$ n/cm$^2$, except for Stilbene which showed a small (12$\%$) decrease in light output. All scintillators presented some light output reduction after ionizing (gamma-ray) irradiation, with reductions of 17$\%$ (EJ-276 and OGS), 32$\%$ (EJ-270), and 42$\%$ (Stilbene) observed immediately after 100 kRad.

physics.ins-det↗

arXiv2019

Ex Luna, Scientia: The Lunar Occultation eXplorer (LOX)

LOX is a lunar-orbiting astrophysics mission that will probe the cosmos at MeV energies. It is guided by open questions regarding thermonuclear, or Type-Ia, supernovae (SNeIa) and will characterize these inherently radioactive objects by enabling a systematic survey of SNeIa at gamma-ray energies for the first time. Astronomical investigations from lunar orbit afford new opportunities to advance our understanding of the cosmos. The foundation of LOX is an observational approach well suited to the all-sky monitoring demands of supernova investigations and time-domain astronomy. Its inherently wide field-of-view and continuous all-sky monitoring provides an innovative way of addressing decadal survey questions at MeV energies (0.1-10 MeV). The LOX approach achieves high sensitivity with a simple, high-heritage instrument design that eliminates the need for complex, position-sensitive detectors, kinematic event reconstruction, masks, or other insensitive detector mass, while also mitigating technology development, implementation complexity, and their associated costs. LOX can be realized within existing programs, like Explorer.

astro-ph.IM↗

arXiv2016

Balloon Flight Test of a Compton Telescope Based on Scintillators with Silicon Photomultiplier Readouts

We present the results of the first high-altitude balloon flight test of a concept for an advanced Compton telescope making use of modern scintillator materials with silicon photomultiplier (SiPM) readouts. There is a need in the fields of high-energy astronomy and solar physics for new medium-energy gamma-ray (~0.4 - 10 MeV) detectors capable of making sensitive observations. A fast scintillator- based Compton telescope with SiPM readouts is a promising solution to this instrumentation challenge, since the fast response of the scintillators permits the rejection of background via time-of-flight (ToF) discrimination. The Solar Compton Telescope (SolCompT) prototype was designed to demonstrate stable performance of this technology under balloon-flight conditions. The SolCompT instrument was a simple two-element Compton telescope, consisting of an approximately one-inch cylindrical stilbene crystal for a scattering detector and a one-inch cubic LaBr3:Ce crystal for a calorimeter detector. Both scintillator detectors were read out by 2 x 2 arrays of Hamamatsu S11828-3344 MPPC devices. Custom front-end electronics provided optimum signal rise time and linearity, and custom power supplies automatically adjusted the SiPM bias voltage to compensate for temperature-induced gain variations. A tagged calibration source, consisting of ~240 nCi of Co-60 embedded in plastic scintillator, was placed in the field of view and provided a known source of gamma rays to measure in flight. The SolCompT balloon payload was launched on 24 August 2014 from Fort Sumner, NM, and spent ~3.75 hours at a float altitude of ~123,000 feet. The instrument performed well throughout the flight. After correcting for small (~10%) residual gain variations, we measured an in-flight ToF resolution of ~760 ps (FWHM). Advanced scintillators with SiPM readouts continue to show great promise for future gamma-ray instruments.

astro-ph.IM↗

arXiv2008

Calibration of the Gamma-RAy Polarimeter Experiment (GRAPE) at a Polarized Hard X-Ray Beam

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.

astro-ph↗

arXiv2005

Status and Future Prospects for Gamma-Ray Polarimetry

The recent detection of linear polarization from GRB120206 has piqued the interest of the community in this relatively unexplored avenue of research. Here, we review the current status and prospects for GRB polarimetry at hard X-ray and soft gamma-ray energies. After reviewing the most recent results, we present a brief survey of current and planned experiments that are capable of making GRB polarization measurements in the energy range between 30 keV and 30 MeV.

astro-ph↗

arXiv2005

GRAPE - A Balloon-Borne Gamma-Ray Polarimeter Experiment

This paper reviews the development status of GRAPE (the 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), although GRAPE could also be employed in the study of other astrophysical sources. Accurately measuring the polarization of the emitted radiation will lead to a better understating of both emission mechanisms and source geometries. The GRAPE design consists of an array of plastic scintillators surrounding a central high-Z crystal scintillator. The azimuthal distribution of photon scatters from the plastic array into the central calorimeter provides a measure of the polarization fraction and polarization angle of the incident radiation. The design of the detector provides sensitivity over a large field-of-view (>pi steradian). The design facilitates the fabrication of large area arrays with minimal deadspace. This paper presents the latest design concept and the most recent results from laboratory tests of a GRAPE science model.

astro-ph↗

arXiv2004

Pixelized Gas Micro-well Detectors for Advanced Gamma-ray Telescopes

We describe possible applications of pixelized micro-well detectors (PMWDs) as three-dimensional charged particle trackers in advanced gamma-ray telescope concepts. A micro-well detector consists of an array of individual micro-patterned gas proportional counters opposite a planar drift electrode. When combined with pixelized thin film transistor (TFT) array readouts, large gas volumes may be imaged with very good spatial and energy resolution at reasonable cost. The third dimension is determined by timing the drift of the ionization electrons. The primary advantage of this technique is the very low scattering that the charged particles experience in a gas tracking volume, and the very accurate determination of the initial particle momenta that is thus achieved. We consider two applications of PMWDs to gamma-ray astronomy: 1) A tracker for an Advanced Compton Telescope (ACT) in which the recoil electron from the initial Compton scatter may be accurately tracked, greatly reducing the telescope's point spread function and increasing its polarization sensitivity; and 2) an Advanced Pair Telescope (APT) whose angular resolution is limited primarily by the nuclear recoil and which achieves useful polarization sensitivity near 100 MeV. We have performed Geant4 simulations of both these concepts to estimate their angular resolution and sensitivity for reasonable mission designs.

astro-ph↗

arXiv2004

EXITE2 Observation of the SIGMA Source GRS 1227+025

We report the EXITE2 hard X-ray imaging of the sky around 3C273. A 2h observation on May 8, 1997, shows a $\sim$260 mCrab source detected at $\sim4σ$ in each of two bands (50-70 and 70-93 keV) and located $\sim$30' from 3C273 and consistent in position with the SIGMA source GRS1227+025. The EXITE2 spectrum is consistent with a power law with photon index 3 and large low energy absorption, as indicated by the GRANAT/SIGMA results. No source was detected in more sensitive followup EXITE2 observations in 2000 and 2001 with 3$σ$ upper limits of 190 and 65 mCrab, respectively. Comparison with the flux detected by SIGMA shows the source to be highly variable, suggesting it may be non-thermal and beamed and thus the first example of a ``type 2'' (absorbed) Blazar. Alternatively it might be (an unprecedented) very highly absorbed binary system undergoing accretion disk instability outbursts, possibly either a magnetic CV, or a black hole X-ray nova.

astro-ph↗

arXiv2003

Applications of Gas Imaging Micro-Well Detectors to an Advanced Compton Telescope

We present a concept for an Advanced Compton Telescope (ACT) based on the use of pixelized gas micro-well detectors to form a three-dimensional electron track imager. A micro-well detector consists of an array of individual micro-patterned proportional counters opposite a planar drift electrode. When combined with thin film transistor array readouts, large gas volumes may be imaged with very good spatial and energy resolution at reasonable cost. The third dimension is determined from the drift time of the ionization electrons. The primary advantage of this approach is the excellent tracking of the Compton recoil electron that is possible in a gas volume. Such good electron tracking allows us to reduce the point spread function of a single incident photon dramatically, greatly improving the imaging capability and sensitivity. The polarization sensitivity, which relies on events with large Compton scattering angles, is particularly enhanced. We describe a possible ACT implementation of this technique, in which the gas tracking volume is surrounded by a CsI calorimeter, and present our plans to build and test a small prototype over the next three years.

astro-ph↗

arXiv2003

A Concept for a High-Energy Gamma-ray Polarimeter

We present a concept for an imaging gamma-ray polarimeter operating from ~50 MeV to ~1 GeV. Such an instrument would be valuable for the study of high-energy pulsars, active galactic nuclei, supernova remnants, and gamma-ray bursts. The concept makes use of pixelized gas micro-well detectors, under development at Goddard Space Flight Center, to record the electron-positron tracks from pair-production events in a large gas volume. Pixelized micro-well detectors have the potential to form large-volume 3-D track imagers with ~100 micron (rms) position resolution at moderate cost. The combination of high spatial resolution and a continuous low-density gas medium permits many thousands of measurements per radiation length, allowing the particle tracks to be imaged accurately before multiple scattering masks their original directions. The polarization of the incoming radiation may then be determined from the azimuthal distribution of the electron-positron pairs. We have performed Geant4 simulations of these processes to estimate the polarization sensitivity of a simple telescope geometry at 100 MeV.

astro-ph↗

arXiv2003

X-ray Dip Monitoring of XB 1916-053

We report on the long term monitoring of X-ray dips from the ultracompact low-mass X-ray binary (LMXB) XB 1916-053. Roughly one-month interval observations were carried out with the Rossi X-ray Timing Explorer (RXTE) during 1996, during which the source varied between dim, hard states and more luminous, soft states. The dip spectra and dip lightcurves were compared against both the broadband luminosity and the derived mass accretion rate Mdot. The dips spectra could be fitted by an absorbed blackbody plus cut-off power law non-dip spectral model, with additional absorption ranging from 0 to >100 x 10^22 cm^-2. The amount of additional blackbody absorption was found to vary with the source luminosity. Our results are consistent with an obscuration of the inner disk region by a partially ionized outer disk. The size of the corona, derived from the dip ingress times, was found to be ~10^9 cm. The corona size did not correlate with the coronal temperature, but seemed to increase when \Mdot also increased. We discuss our findings in the context of an evaporated accretion disk corona model and an ADAF-type model.

astro-ph↗

arXiv2003

Development of Silicon Strip Detectors for a Medium Energy Gamma-ray Telescope

We report on the design, production, and testing of advanced double-sided silicon strip detectors under development at the Max-Planck-Institute as part of the Medium Energy Gamma-ray Astronomy (MEGA) project. The detectors are designed to form a stack, the "tracker," with the goal of recording the paths of energetic electrons produced by Compton-scatter and pair-production interactions. Each layer of the tracker is composed of a 3 x 3 array of 500 micron thick silicon wafers, each 6 cm x 6 cm and fitted with 128 orthogonal p and n strips on opposite sides (470 micron pitch). The strips are biased using the punch-through principle and AC-coupled via metal strips separated from the strip implant by an insulating oxide/nitride layer. The strips from adjacent wafers in the 3 x 3 array are wire-bonded in series and read out by 128-channel TA1.1 ASICs, creating a total 19 cm x 19 cm position-sensitive area. At 20 degrees C a typical energy resolution of 15-20 keV FWHM, a position resolution of 290 microns, and a time resolution of ~1 microsec is observed.

astro-ph↗

arXiv2001

The MEGA Advanced Compton Telescope Project

The goal of the Medium Energy Gamma-ray Astronomy (MEGA) telescope is to improve sensitivity at medium gamma-ray energies (0.4-50 MeV) by at least an order of magnitude over that of COMPTEL. This will be achieved with a new compact design that allows for a very wide field of view, permitting a sensitive all-sky survey and the monitoring of transient and variable sources. The key science objectives for MEGA include the investigation of cosmic high-energy particle accelerators, studies of nucleosynthesis sites using gamma-ray lines, and determination of the large-scale structure of galactic and cosmic diffuse background emission. MEGA records and images gamma-ray events by completely tracking both Compton and pair creation interactions in a tracker of double-sided silicon strip detectors and a calorimeter of CsI crystals able to resolve in three dimensions. We present initial laboratory calibration results from a small prototype MEGA telescope.

astro-ph↗

arXiv2001

Balloon Flight Background Measurement with Actively-Shielded Planar and Imaging CZT Detectors

We present results from the flight of two prototype CZT detectors on a scientific balloon payload in September 2000. The first detector, referred to as ``CZT1,'' consisted of a 10 mm x 10 mm x 2 mm CZT crystal with a single gold planar electrode readout. This detector was shielded by a combination of a passive collimator surrounded by plastic scintillator and a thick BGO crystal in the rear. The second detector, ``CZT2,'' comprised two 10 mm x 10 mm x 5 mm CZT crystals, one made of eV Products high pressure Bridgman material and the other of IMARAD horizontal Bridgman material, each fashioned with a 4 x 4 array of gold pixels on a 2.5 mm pitch. The pixellated detectors were flip-chip-mounted side by side and read out by a 32-channel ASIC. This detector was also shielded by a passive/plastic collimator in the front, but used only additional passive/plastic shielding in the rear. Both experiments were flown from Ft. Sumner, NM on September 19, 2000 on a 24 hour balloon flight. CZT1 recorded a non-vetoed background level at 100 keV of ~1e-3 cts/cm2/s/keV. Raising the BGO threshold from 50 keV to ~1 MeV produced only an 18% increase in this level. CZT2 recorded a background at 100 keV of ~4e-3 cts/cm2/s/keV in the eV Products detector and ~6e-3 cts/cm2/s/keV in the IMARAD detector. Both CZT1 and CZT2 spectra were in basic agreement with Monte Carlo simulations, though both recorded systematically higher count rates at high energy than predicted. No lines were observed, indicating that neutron capture reactions, at least those producing decay lines at a few 100 keV, are not significant components of the CZT background. Comparison of the CZT1 and CZT2 spectra indicates that passive/plastic shielding may provide adequately low background levels for many applications.

astro-ph↗

arXiv2001

Development of an XSPEC-Based Spectral Analysis System for the Coded-Aperture Hard X-ray Balloon Payload EXITE2

We present the spectral analysis system for the second-generation Energetic X-ray Imaging Telescope Experiment (EXITE2) balloon payload. EXITE2 is an imaging hard X-ray telescope using a coded-aperture mask and a NaI/CsI phoswich detector operating in the energy range 20--600 keV. The instrument was flown on a high-altitude scientific balloon from Ft. Sumner, NM on 1997 May 7-8. We describe the details of the EXITE2 spectral analysis system, with emphasis on those aspects peculiar to coded-aperture instruments. In particular, we have made our analysis compatible with the standard X-ray spectral fitting package XSPEC by generating a response matrix in the appropriate format including all the effects of a coded-aperture system. The use of XSPEC, which may be a first for coded-aperture data, permits great flexibility in the fitting of spectral models. The additional effects of our phoswich system, or any other detector-specific considerations, may be easily included as well. We test our spectral analysis using observations of the Crab Nebula, and find that the EXITE2 Crab spectrum is consistent with those recorded by previous instruments operating in this energy range.

astro-ph↗

arXiv2000

Timing Analysis of the Light Curve of the Dipping-Bursting X-ray Binary X1916-053

We present the timing analysis results for our observations of the x-ray dip source X1916-053 conducted with RXTE between February and October of 1996. Our goal was to finally measure the binary period - as either the x-ray dip period or ~1% longer optical modulation period, thereby establishing if the binary has a precessing disk (SU UMa model) or a third star (triple model). Combined with historical data (1979-96), the x-ray dip period is measured to be 3000.6508 $\pm$ 0.0009 sec with a 2$σ$ upper limit $|\dot P| \leq 2.06 \times 10^{-11}$. From our quasi-simultaneous optical observations (May 14-23, 1996) and historical data (1987-96), we measure the optical modulation period to be 3027.5510 $\pm$ 0.0052 sec with a 2$σ$ upper limit $|\dot P| \leq 2.28 \times 10^{-10}$. The two periods are therefore each stable (over all recorded data) and require a $3.9087 \pm 0.0008$d beat period. This beat period, and several of its harmonics is also observed as variations in the dip shape. Phase modulation of x-ray dips, observed in a 10 consecutive day observation, is highly correlated with the $\sim$3.9d dip shape modulation. The 1987-1996 optical observations show that the optical phase fluctuations are a factor of 3 larger than those in the x-ray. We discuss SU UMa vs. triple models to describe the X1916-053 light curve behavior and conclude that the x-ray dip period, with smaller phase jitter, is probably the binary period so that the required precession is most likely similar to that observed in SU UMa and x-ray nova systems. However the ``precession'' period stability and especially the fact that the times of x-ray bursts may partially cluster to occur just after x-ray dips, continue to suggest that this system may be a hierarchical triple.

astro-ph↗

arXiv2000

Construction and Testing of a Pixellated CZT Detector and Shield for a Hard X-ray Astronomy Balloon Flight

We report on the construction and laboratory testing of pixellated CZT detectors mounted in a flip-chip, tiled fashion and read out by an ASIC, as required for proposed hard X-ray astronomy missions. Two 10 mm x 10 mm x 5 mm detectors were fabricated, one out of standard eV Products high-pressure Bridgman CZT and one out of IMARAD horizontal Bridgman CZT. Each was fashioned with a 4 x 4 array of gold pixels on a 2.5 mm pitch with a surrounding guard ring. The detectors were mounted side by side on a carrier card, such that the pixel pitch was preserved, and read out by a 32-channel VA-TA ASIC from IDE AS Corp. controlled by a PC/104 single-board computer. A passive shield/collimator surrounded by plastic scintillator encloses the detectors on five sides and provides a ~40deg field of view. Thus this experiment tests key techniques required for future hard X-ray survey instruments. The experiment was taken to Ft Sumner, NM in May 2000 in preparation for a scientific balloon flight aboard the joint Harvard-MSFC EXITE2/HERO payload. Although we did not receive a flight opportunity, and are currently scheduled to fly in September 2000, we present our calibration data in the flight configuration together with data analysis techniques and simulations of the expected flight background spectrum.

astro-ph↗

arXiv2000

Development of Gold Contacted Flip-chip Detectors with IMARAD CZT

We present initial results from our evaluation of a gold-contacted pixellated detector using cadmium zinc telluride substrate produced by IMARAD Imaging Systems. The Horizontal Bridgman (HB) grown crystals from IMARAD have been shown to produce high resolution photopeaks, but they are also seen to have large leakage current. Our previous tests with IMARAD CZT showed that the use of indium anodes and gold cathode improved the resistivity compared to the standard indium-contacted detectors. We seek to test whether simple evaporated gold contacts alone could also reduce the leakage current and thus improve the spectral resolution, especially in the 10-100 keV energy range. We have fabricated several metal-semiconductor-metal (MSM) detectors with a 4x4 array of pixels on 10x10 mm substrates. Measurements of the detectors' leakage current, spectral response, and temperature sensitivity are presented and compared to IMARAD's ohmic contact detector and gold contact MSM detectors made of High Pressure Bridgman (HPB) material. Finally, we show preliminary results from a tiled flip-chip pixellated detector made using the IMARAD detectors.

astro-ph↗