Raj Prasad Manandhar

Development of EXITE2: a large-area imaging phoswich detector/telescope for hard x-ray astronomy

We review design considerations and present preliminary details of the performance of a new imaging system for hard X-ray astronomy in the 20 - 600 keV energy range. The detector is a 40 cm X 40 cm NaI(Tl)/CsI(Na) phoswich module, read out by a 7 X 7 array of square PMTs. The detector comprises the main part of the next generation Energetic X-ray Imaging Telescope Experiment (EXITE2), which had its first flight on 13 June 1993 from Palestine, Texas. Imaging is accomplished via the coded-aperture mask technique. The mask consists of 16 mm square lead/tin/copper pixels arranged in a cyclically repeated 13 X 11 uniformly redundant array pattern at a focal length of 2.5 m, giving 22 arcmin resolution. The field of view, determined by the lead/brass collimator (16 mm pitch) is 4.65 degrees FWHM. We anticipate a 3 sigma sensitivity of 1 X 10-5 photons cm-2 s-1 keV-1 at 100 keV in a 104 sec balloon observation. The electronics incorporate two on-board computers, providing a future capability to record the full data stream and telemeter compressed data. The design of the current detector and electronics allows an upgrade to EXITE3, which adds a proportional counter front-end to achieve lower background and better spatial and spectral resolution below approximately 100 keV.

Techniques For Removing Non-uniform Background In Coded-aperture Imaging On The Energetic X-ray Imaging Telescope Experiment

It is found that the subtractive flat field technique for nonuniform background illumination is generally effective at removing background systematics for stationary mask experiments such as the energetic X-ray imaging experiment. The time dependence of intensity and the two-dimensional shape of the background detector image during the flight are explored. A flat field image is constructed from observations where X-ray sources were absent from the field of view. It is shown that this technique can successfully reduce RMS fluctuations to within a few percent of ideal Poisson statistics. The quality of the flat field does not appear to be a strong function of radius and can be used effectively out to the edge of the detector to remove the strong background ring. >

Hybrid Gas Scintillation Proportional Counteriphoswich Detector For Hard X-Ray Astronomy

We present a concept for a balloon-borne imaging hybrid proportional counter/phoswich detector of medium to hard X-rays. The phoswich would be optically coupled to the exit window of the proportional counter, and both detectors would use a common position-sensitive readout. We anticipate that such a detector could combine the good energy and position resolution and excellent background rejection ability of the proportional counter for incident photon energies <100 keV with the extended response of the phoswich for higher energies. The phoswich could also be used to reject Compton scattering events in the proportional counter. We are studying this detector concept using numerical simulations of a 400 cm2 square prototype detector. We use a Monte Carlo approach, properly accounting for range, penetration depth, and diffusion effects in the proportional counter. We explicitly model the propagation of the scintillation light from the proportional counter through a waveshifter and the phoswich, and its detection with an Anger camera arrangement of Hamamatsu crossed-wire anode photomultiplier tubes. Results from this simulation indicate that current levels of proportional counter and phoswich performance are attainable at small cost in quantum efficiency, compared to a bare phoswich detector. We also report some rough calculations of the background rejection efficiency in the xenon and the phoswich.

Simulations and measurements of the background encountered by a high-altitude balloon-borne experiment for hard X-ray astronomy

Abstract We have modelled the hard X-ray background expected for a high-altitude balloon flight of the Energetic X-ray Imaging Telescope Experiment (EXITE2), an imaging phoswich detector/telescope for the 20–600 keV energy range. Photon- and neutron-induced contributions to the background are considered. We describe the code and the results of a series of simulations with different shielding configurations. The simulated hard X-ray background for the actual flight configuration agrees reasonably well (within a factor of ∼2) with the results measured on the first flight of EXITE2 from Palestine, Texas. The measured background flux at 100 keV is ∼4 × 10 −4 counts cm −2 s −1 keV −1 .

Hybrid gas detector/phoswich for hard x-ray astronomy

A hybrid detector, which combines an optical avalanche chamber with a phoswich, is currently under development. The optical avalanche chamber--a proportional counter designed to give large quantities of light photons during charge multiplication, mounts on the front of the scintillator and gives response at low energies, while the solid scintillator takes over at energies where the gas becomes transparent (> 90 keV). Both sections of the hybrid will be read out by a common set of photomultipliers under the phoswich. The addition of the gas section to the phoswich improves the energy resolution of the instrument by a factor of 2.5 at 25 keV and the spatial resolution by a factor of 10 at the same energy. The net result is an instrument with broad band coverage and high sensitivity which will be used for coded aperture imaging on long duration balloon flights.

Development of a hybrid gas detector/phoswich for hard x-ray astronomy

A hybrid detector is under development for use as a balloon-borne instrument in hard x-ray astronomy. The detector provides broad band coverage by coupling an optical avalanche chamber to a phoswich. The optical avalanche chamber yields superior instrument response at low energies while the scintillator takes over at the higher energies where the gas becomes transparent: at 25 keV, the addition of the gas chamber improves the energy resolution by a factor of 2.5 and the spatial resolution by a factor of 10 as compared to the stand-alone response of the phoswich. A half-scale prototype instrument is being constructed for test purposes and to help resolve a number of design questions involving the coupling of the two components.