James M. Ryan

Continued studies of single-sided charge-sharing CZT strip detectors

In this paper, we report progress in the study of thick single-sided charge-sharing cadmium zinc telluride (CZT) strip detector modules designed to perform gamma-ray spectroscopy and 3-D imaging. We report on continuing laboratory and simulation measurements of prototype detectors with 11/spl times/11 unit cells (15/spl times/l5/spl times/7.5 mm/sup 3/). We report preliminary measurements of the 3-D spatial resolution. Our studies are aimed at developing compact, efficient, detector modules for 0.05 to 1 MeV gamma measurements while minimizing the number and complexity of the electronic readout channels. This is particularly important in space-based coded aperture and Compton telescope instruments that require large area, large volume detector arrays. Such arrays will be required for the NASAs Black Hole Finder Probe (BHFP) and Advanced Compton Telescope (ACT). This design requires an anode pattern with contacts whose dimensions and spacing are roughly the size of the ionization charge cloud. The first prototype devices have 125 /spl mu/m anode contacts on 225 /spl mu/m pitch. Our studies conclude that finer pitch contacts will be required to improve imaging efficiency.

Coded-aperture x-ray/gamma-ray telescope for arc- minute localization of gamma-ray bursts

MARGIE will be a large-area, wide field-of-view, hard x- ray/gamma-ray imaging telescope capable of providing accurate positions for faint gamma-ray bursts in near-real- time and of performing a sensitive survey of both steady and transient cosmic sources. The instrument is designed to image faint bursts at the low-intensity end of the log N - log S distribution. MARGIE was recently selected by NASA for a mission-concept study for an Ultra Long Duration Balloon flight. We describe a program to develop an instrument based on the new detector technology of either cadmium zinc telluride room-temperature semiconductors or pixelated cesium iodide scintillators viewed by fast timing charge- coupled devices.

CGRO-Comptel observations of the Centaurus A region

Abstract The sky region containing the active radio-galaxy Centaurus A has repeatedly been observed with the COMPTEL instrument onboard the Compton Gamma Ray Observatory (CGRO). The nine observation periods during the CGRO phases I and II in which Cen A was in the field of view of COMPTEL are spread over 18 months in the years 1991 to 1993. The energy range 0.75 to 30 MeV is covered. Clear evidence for a source with emission up to several MeV is seen from a region coinciding with the position of Cen A. The spectra change significantly over ∼ 6 months between the two observation phases. A possible source confusion with the nearby gamma-ray source MS1312.1-4221 is discussed.

Position Resolution in LaBr/sub 3/ and LaCl/sub 3/ Scintillators Using Position-Sensitive Photomultiplier Tubes

Advanced scintillator materials such as LaBr3:Ce and LaCl3: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) LaBr3:Ce and LaCl3: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.

COMPTEL time-averaged all-sky point source analysis

We use all COMPTEL data from the beginning of the CGRO mission (April ’91) up to the end of CGRO Cycle 6 (November ’97) to carry out all-sky point source analyses in the four standard COMPTEL energy bands for different time periods. We apply our standard maximum-likelihood method to generate all-sky significance and flux maps for point sources by subtracting off the diffuse emission components via model fitting. In addition, fluxes of known sources have been determined for individual CGRO Phases/Cycles to generate lightcurves with a time resolution of the order of one year. The goal of the analysis is to derive quantitative results—significances, fluxes, light curves—of our brightest and most significant sources such as 3C 273, and to search for additional new COMPTEL sources, showing up in time-averaged maps only.

Improved COMPTEL maps of the milky way

In the course of the mission we have gradually developed an analysis method that separates in an iterative manner the celestial emission and the (a priori unknown) instrumental background. It has become our standard analysis tool for point sources. We illustrate here that this method is widely applicable now. It provides mutually consistent sets of model-fitting parameters (spectra) and sky maps, both for continuum and line studies. Because of the wide applicability, it has been possible to make various cross-checks while building up confidence in this procedure.

Simulated Performance of 3-DTI Gamma-Ray Telescope Concepts

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.

A prototype for SONTRAC, a scintillating plastic fiber tracking detector for neutron imaging and spectroscopy

We report on tests of a prototype detector system designed to perform imaging and spectroscopy on 20 to 250 MeV neutrons. Although developed for the study of high-energy solar flare processes, the detection techniques employed for SONTRAC, the SOlar Neutron TRACking experiment, can be applied to measurements in a variety of disciplines including atmospheric physics, radiation therapy and nuclear materials monitoring. The SONTRAC instrument measures the energy and direction of neutrons by detecting double neutron-proton scatters and recording images of the ionization tracks of the recoil protons in a densely packed bundle of scintillating plastic fibers stacked in orthogonal layers. By tracking the recoil protons from individual neutrons, the kinematics of the scatter are determined. This directional information results in a high signal to noise measurement. SONTRAC is also capable of detecting and measuring high-energy gamma rays >20 MeV as a solid-state spark chamber. The self-triggering and track imaging features of a prototype for tracking in two dimensions are demonstrated in calibrations with cosmic-ray muons, 14 to /spl sim/65 MeV neutrons and /spl sim/20 MeV protons.

The prompt cosmic-ray-induced background in the orbiting Compton telescope COMPTEL

The authors report the spectrum of background events that the COMPTEL instrument on Compton Gamma Ray Observatory experiences due to the instantaneous effects of cosmic rays. Other backgrounds are present in the data of this Compton telescope, but the components that closely follow the instantaneous flux of cosmic rays are the most identifiable. The background varies approximately linearly with the cosmic-ray intensity and exhibits a broad feature from about 1 to 10 MeV suggestive of the nuclear nature of the events. Above 10 MeV there is a marked change in the nature of the background that is not understood. It appears that the background changes from nuclear to electromagnetic in nature.

The continued development of a low energy Compton imager for GRB polarization studies

The Gamma Ray Polarimeter Experiment (GRAPE) is designed to investigate gamma-ray bursts (GRB) in the important energy range of 50-500 keV. Our eventual goal is to fly GRAPE on a long duration balloon (LDB) platform to collect data on a significant sample of GRBs. Our experience with two balloon flights (in 2011 and 2014), coupled with further design efforts focused on orbital payloads, has led to an improved polarimeter concept that represents a natural evolution of the current design. The new concept employs a large number of small (2 cm3 ), optically-isolated scintillator cubes, each of which is read out by its own silicon photomultiplier (SiPM). These cubes are stacked in an arrangement that allows the determination of event interaction locations in three dimensions. The resulting three-dimensional location data provides a moderate level of Compton imaging capability (1σ angular resolution of 10-15). This level of imaging can be used to significantly reduce the instrumental background by limiting the impact of the cosmic diffuse flux, dramatically improving the polarization sensitivity. Here we shall describe this concept, some results from initial laboratory studies, and the expected performance parameters. We are currently working to optimize this design in preparation for a prototype balloon flight in the summer of 2020. Our long-term goal (pending acquisition of continued funding) is to fly a prototype balloon payload in the summer of 2020 and to be prepared for a first long duration balloon (LDB) flight at the end of 2021.