Philip P. Dunphy

A balloon-borne coded aperture telescope for low-energy gamma-ray astronomy

Abstract A telescope for imaging cosmic γ-ray emission over the energy range 160 keV to 9.3 MeV has been developed and successfully flown on a high altitude balloon over Palestine, Texas on 1 October, 1984. This instrument consists of a coded mask based on a 5 × 7 uniformly redundant array (URA) and a scintillator array consisting of 35 bismuth germanate (BGO) detectors. The telescope can image sources with an intrinsic resolution of 3.8° within a 15.2° × 22.8° field of view. The properties of the instrument are described and its imaging capability is demonstrated with results from an observation of the region of the Crab Nebula. The imaging response to the Crab was found to be well represented by a bivariate Gaussian function of full width at half-maximum (FWHM) 4.8°. The centroid of the response was determined to a precision of ± 12 arc min.

A Coded Aperture Gamma Ray Telescope

A gamma ray telescope is being developed to operate in the energy range 100 keV to 5 MeV utilizing coded aperture imaging. The design incorporates a mask pattern based on a Uniformly Redundant Array (URA), which has been shown to have ideal imaging characteristics. A mask-anti-mask procedure is used to eliminate the effects of any possible systematic variations in detector background rates. The detector array is composed of 35 elements of the high-Z material Bismuth Germanate (BGO). Results of laboratory testing of the imaging properties will be presented. A southern hemisphere balloon flight is planned for 1982 with the goal of observing the 0.511 MeV radiation from the Galactic Center. Computer calculations show that a point source of this radiation can be located to within ±1°.

Gamma-ray observations of the Crab Region using a coded-aperture telescope

The region of the Galactic anticenter, including the Crab Nebula, was observed during a balloon flight of the University of New Hampshire Directional Gamma-Ray Telescope employing the coded-aperture imaging technique to image celestial gamma-radiation between 160 keV and 9.3 MeV. The background systematics are treated with a simple and relatively straightforward correction procedure. The results demonstrate that the coded-aperture procedure is a viable approach for imaging not only point sources of radiation, but also extended sources of emission. The results for the Crabs photon spectrum are consistent with a power-law spectrum. Upper limits on the flux levels of line emission at 405 keV and 1050 keV and on the flux from the X-ray binary source A0535 + 26 and diffuse Galactic emission from the anticenter region are derived. 35 references.

A liquid xenon time projection chamber for γ-ray imaging in astrophysics: present status and future directions

Abstract A liquid xenon time projection chamber (LXeTPC) has been developed to image cosmic γ-rays in the energy band 0.2–20 MeV. Its performance as Gamma Ray Imaging Telescope (LXeGRIT instrument) has been tested during a high altitude balloon flight (Spring ’99, New Mexico). The detector, with 400 cm 2 area and 7 cm drift gap, is filled with 7 l high purity LXe. Both ionization and scintillation light signals are detected to measure the energy deposits and the three spatial coordinates of individual γ-ray interactions within the sensitive volume. During the pre-flight calibration experiments the LXeGRIT instrument was extensively tested with γ-ray sources in the laboratory: a 10% FWHM energy resolution at 1 MeV was determined, scaling with 1/ E . The detector shows a linear response in the energy range 511 keV–4.4 MeV.

Spectroscopy and imaging performance of the Liquid Xenon Gamma-Ray Imaging Telescope (LXeGRIT)

LXeGRIT is a balloon-borne Compton telescope based on a liquid xenon time projection chamber (LXeTPC) for imaging cosmic (gamma) -rays in the energy band of 0.2 - 20 MeV. The detector, with 400 cm2 area and 7 cm drift gap, is filled with high purity LXe. Both ionization and scintillation light signal are detected to measure the energy deposits and the three spatial coordinates of individual (gamma) -ray interactions within the sensitive volume. The TPC has been characterized with repeated measurements of its spectral and Compton imaging response to (gamma) -rays from radioactive sources such as 22Na, 137Cs, 88Y and Am-Be. The detector shows a linear response to g-rays in the energy range 511 keV - 4.4 MeV, with an energy resolution (FWHM) of (Delta) E/E equals 8.8% (root)1MeV/E. Compton imaging of 88Y (gamma) -ray events with two detected interactions is consistent with an angular resolution of approximately 3 degrees (RMS) at 1.8 MeV.

High-energy gamma-ray emission from solar flares: Constraining the accelerated proton spectrum

Using a multi-component model to describe the γ-ray emission, we investigate the flares of December 16, 1988 and March 6, 1989 which exhibited unambiguous evidence of neutral pion decay. The observations are then combined with theoretical calculations of pion production to constrain the accelerated proton spectra.The detection of π0 emisson alone can indicate much about the energy distribution and spectral variation of the protons accelerated to pion producing energies. Here both the intensity and detailed spectral shape of the Doppler-broadened π0 decay feature are used to determine the spectral form of the accelerated proton energy distribution. The Doppler width of this γ-ray emission provides a unique diagnostic of the spectral shape at high energies, independent of any normalisation. To our knowledge, this is the first time that this diagnostic has been used to constrain the proton spectra. The form of the energetic proton distribution is found to be severely limited by the observed intensity and Doppler width of the π0 decay emission, demonstrating effectively the diagnostic capabilities of the π0 decay γ-rays.The spectral index derived from the γ-ray intensity is found to be much harder than that derived from the Doppler width. To reconcile this apparent discrepancy we investigate the effects of introducing a high-energy cut-off in the accelerated proton distribution. With cut-off energies of around 0.5–0.8 GeV and relatively hard spectra, the observed intensities and broadening can be reproduced with a single energetic proton distribution above the pion production threshold.

Gamma-ray observations of Cygnus X-1 and Cygnus X-3 using a coded-aperture telescope

A balloon-borne coded-aperture telescope, measuring gamma-ray photons in the 160 keV to 9.3 MeV range, was used to observe the Cygnus region of the sky on October 1 and 2, 1984. In the 2-9.3-MeV band, evidence is found for a hard spectral component with a mean flux level at the top of the atmosphere of 7.4 + or - 2.5 x 10 to the -7th photons/sq cm per s per keV, inconsistent with the predictions of the inverse Compton models normally used to describe the X-ray emission. Both Cyg X-1 and Cyg X-3 could be observed simultaneously with the telescope. The results are used to establish 1-sigma upper flux limits on the spectral emission from Cyg X-3.

LXeGRIT Compton telescope prototype: current status and future prospects

LXeGRIT is the first prototype of a novel concept of Compton telescope, based on the complete 3D reconstruction of the sequence of interactions of individual γ-rays in one position sensitive detector. This balloon-borne telescope consists of an unshielded time projection chamber with an active volume of 400 cm2 × 7 cm filled with high purity liquid xenon. Four VUV PMTs detect the fast xenon scintillation light signal, providing the event trigger. 124 wires and 4 anodes detect the ionization signals, providing the event spatial coordinates and total energy. In the period 1999-2001, LXeGRIT has been extensively tested both in the laboratory and at balloon altitude, and its response in the MeV region has been thoroughly characterized. Here we summarize some of the results on pre-flight calibration, even reconstruction techniques, and performance during a 27 hour balloon flight on October 4-5. We further present briefly the on-going efforts directed to improve the performance of this prototype towards the requirements for a base module of a next-generation Compton telescope.

Liquid Xenon Gamma-Ray Imaging Telescope (LXeGRIT) for medium energy astrophysics

As part of our ongoing research program to develop a liquid xenon gamma-ray imaging telescope (LXe-GRIT) for medium energy astrophysics, we have built a liquid xenon time projection chamber (LXeTPC) with a total volume of 10 liters and a sensitive are of 20 cm by 20 cm. The detector has been successfully tested with gamma-ray sources in the laboratory and is currently being prepared as balloon-borne payload for imaging MeV gamma-ray emission from the Crab Nebula, Cygnus X-1 and the Orion molecular cloud region. The LXe-TPC, sensitive to gamma-rays from 300 keV to 30 MeV, measures the energy and the 3-D location of each gamma-ray interaction with a resolution of 6% FWHM and 1 mm RMS at 1 MeV, within a 1 sr FOV. Its detection efficiency for Compton events is about 4% in the 1 - 3 MeV, an energy band of great astrophysical interest for both continuum and line emission. Its 3 sigma continuum sensitivity of 1.8 multiplied by 10-7 ph cm-2s-1keV-1 for a nominal 10 hr observation time, will allow us to study a variety of sources with an imaging accuracy as good as 1 degree. We plan to pursue a vigorous program of balloon flights with this telescope to achieve the maximum science return while continuing a strong R&D laboratory program on LXe technology. The ultimate goal is an optimized design of a satellite implementation of a liquid xenon gamma-ray imaging instrument that will lead to drastic improvements in sensitivity and angular resolution in the 0.3 - 30 MeV band and beyond.

A gamma-ray detector for in-situ measurement of 137Cs radioactivity in snowfields and glaciers

Abstract The rate of snow deposition at various cold regions on the earth is an important quantity for glaciological and climatological studies. Radioactive debris from above-ground tests of nuclear weapons (mainly 1954–1970) and from the Chernobyl accident (1986) have been deposited on glaciers and snowfields, where they can be used as time and depth markers to determine the subsequent accumulation of snow. We discuss a technique to locate these markers that has been used just recently — in-situ measurement of γ-rays from 137 Cs. These γ-rays, which are associated with radioactive fallout, have a distinctive depth profile and serve as markers of the historical nuclear events. The γ-ray measurement involves lowering a scintillation detector down a borehole in the snow or ice and recording the response to the 137 Cs γ-rays as a function of depth. The in-situ measurement can be done relatively quickly and can replace sample retrieval, or it can be used to decide which ice or snow samples should be transported for later analysis in the laboratory. The feasibility of in-situ γ-ray measurement has been demonstrated at sites in the French Alps and Greenland. We report on a portable detector system that is being developed for use in Antarctica. It is based, as much as possible, on inexpensive, commercially available detectors and electronics. The advantages and disadvantages of this approach are discussed. The problems involved with making these measurements in a harsh environment and the steps taken to deal with them are also presented.