Ethan L. Hull

Real-time generation of images with pixel-by-pixel spectra for a coded aperture imager with high spectral resolution

The capabilities ofa coded aperture imager are significantly enhanced when a detector with excellent energy resolution is used. We are constructing such an imager with a 1.1 cm thick, crossed-strip, planar detector which has 38 strips of 2 mm pitch in each dimension followed by a large coaxial detector. Full value from this system is obtained only when the images are ‘‘fully deconvolved’’ meaning that the energy spectrum is available from each pixel in the image. The large number ofenergy bins associated with the spectral resolution ofthe detector, and the fixed pixel size, present significant computational challenges in generating an image in a timely manner at the conclusion ofa data acquisition. The long computation times currently preclude the generation ofintermediate images during the acquisition itself. We have solved this problem by building the images on-line as each event comes in using pre-imaged arrays ofthe system response. The generation ofthese arrays and the use off ractional mask-to-detector pixel sampling is discussed. r 2003 Published by Elsevier Science B.V.

Long-term radiation damage to a spaceborne germanium spectrometer

Abstract The Transient Gamma-Ray Spectrometer aboard the Wind spacecraft in deep space has observed gamma-ray bursts and solar events for four years. The germanium detector in the instrument has gradually deteriorated from exposure to the ≈10 8 p / cm 2 / yr (>100 MeV ) cosmic-ray flux. Low-energy tailing and loss of efficiency, attributed to hole trapping and conversion of the germanium from n- to p-type as a result of crystal damage, were observed. Raising the detector bias voltage ameliorated both difficulties and restored the spectrometer to working operation. Together, these observations extend our understanding of the effects of radiation damage to include the previously unsuccessfully studied regime of long-term operation in space.

Signal interpolation in germanium detectors for improved 3-D position resolution

A technique has been implemented for improving the 3-D position resolution in a germanium strip detector. By using the signals induced on multiple electrodes, the position of the drifting charges can be interpolated to a resolution smaller than the width of the strips. Interpolation allows for a desired position resolution to be achieved with the fewest number of electronic channels. Applications include portable and space-based instruments where power, cooling, and mass are at a premium. Measurements were made on a fully instrumented 19/spl times/19 planar germanium strip detector. This system is used as a Compton imager and detects gamma-ray sources anywhere in a 4-pi field of view. It is shown that, using interpolation, the point spread function for a 662 keV point source improves from 25/spl deg/ to 10/spl deg/ FWHM under specified conditions. However, it is shown that many interactions occur too close together to be effectively interpolated. This and other practical limitations are discussed. In addition, an electrostatic model has been developed and shown to be in good agreement with measurements. This model was used to optimize detector design, predicting signal to noise ratios, and check the calibration of the signals.

The nuclear compton telescope: A balloon-borne soft γ-ray spectrometer, polarimeter, and imager

Our collaboration has begun the design and development of a prototype high resolution Compton telescope utilizing 3-D imaging germanium detectors. The Nuclear Compton Telescope (NCT) is a balloon-borne soft gamma-ray (0.2–15 MeV) telescope designed to study astrophysical sources of nuclear line emission and polarization. NCT is a prototype design for the Advanced Compton Telescope, to study gamma-ray radiation with very high spectral resolution, moderate angular resolution, and high sensitivity. The instrument has a novel, ultra-compact design optimized for studying nuclear line emission in the critical 0.5–2 MeV range, and polarization in the 0.2–0.5 MeV range. We have proposed to develop and fly NCT on a conventional US balloon flight in Summer of 2004. This first flight will perform gamma-ray polarization measurements the Crab nebula, Crab pulsar, and Cyg X-1, and 26Al emission from the Cygnus Region. This flight will critically test the novel instrument technologies and analysis techniques we have dev...

A Germanium Orthogonal Strip Detector System for Gamma‐Ray Imaging

A coded aperture, germanium‐detector based gamma‐ray imaging system has been designed, fabricated, and tested. The detector, cryostat, and signal processing electronics are discussed in this paper. The latest version orthogonal strip planar detector is 11‐millimeters thick, having 38×38 strips of 2‐millimeter pitch. The planar detector was fabricated using amorphous germanium contacts. The strips on each face of the detector lie in a chorded‐circular pattern to more efficiently utilize the area of the 10‐cm diameter germanium crystal. The detector is held in a mount that allows convenient installation and removal of the detector, lending itself to eventual tiling of such detectors into large arrays. The cryostat includes provisions to install a large volume coaxial germanium detector immediately behind the planar detector in the same cryostat. Many gamma rays Compton scatter from the planar detector into the coaxial detector. The energies of these coincident interactions are summed to increase the gamma‐r...

Upcoming balloon flight of the nuclear Compton telescope

Our collaboration is developing a 2-detector prototype high resolution Compton telescope utilizing 3D imaging germanium detectors (GeDs) for a test balloon flight in Spring 2003. This instrument is a prototype for a full 12-GeD instrument, the Nuclear Compton Telescope. NCT is a balloon-borne soft gamma-ray (0.2-15 MeV) telescope designed to study astrophysical sources of nuclear line emission and polarization. The NCT program is designed to develop and test the technologies and analysis techniques crucial for the Advanced Compton Telescope, while studying gamma-ray radiation with very high spectral resolution, moderate angular resolution, and high sensitivity. NCT has a novel, ultra-compact design optimized for studying nuclear line emission in the critical 0.5-2 MeV range, and polarization in the 0.2-0.5 MeV range. This prototype flight will critically test the novel instrument technologies, analysis techniques, and background rejection procedures we have developed for high resolution Compton telescopes. We present the design and expected performance of this prototype NCT instrument.

Thermal design and performance of the gamma-ray spectrometer for the MESSENGER spacecraft

A gamma-ray spectrometer (GRS) has been built and delivered to the MESSENGER spacecraft which launched on August 3, 2004, from Cape Canaveral, Florida. The GRS, a part of seven scientific instruments on board MESSENGER, is based on a coaxial high-purity germanium detector. Gamma-ray detectors based on germanium have the advantage of providing excellent energy resolution, which is critical to achieving the science goals of the mission. However, germanium has the disadvantage that it must operated at cryogenic temperatures (typically /spl sim/80 K). This requirement is easy to satisfy in the laboratory but difficult near Mercury, which has an extremely hot thermal radiation environment. To cool the detector, a Stirling cycle mechanical cooler is employed. In addition, radiation and conduction techniques are used to reduce the GRS heat load. Before delivering the flight sensor, a complete thermal prototype was built and tested. The results of these tests, including thermal design, radiative and conductive heat loads, and cooler performance, are described.

3D positioning germanium detectors for gamma-ray astronomy

We have developed germanium detector technologies for use in the Nuclear Compton Telescope (NCT) - a balloon-borne soft γ-ray (0.2-10 MeV) telescope to study astrophysical sources of nuclear line emission and polarization. The heart of NCT is an array of twelve large volume cross strip germanium detectors, designed to provide 3-D positions for each photon interaction with ~1mm resolution while maintaining the high spectral resolution of germanium. Here we discuss the detailed performance of our prototype 19x19 strip detector, including laboratory tests, calibrations, and numerical simulations. In addition to the x and y positions provided by the orthogonal strips, the interaction depth (z-position) in the detector is measured using the relative timing of the anode and cathode charge collection signals. We describe laboratory calibrations of the depth discrimination using collimated sources with different characteristic energies, and compare the measurements to detailed Monte Carlo simulations and charge collection routines tracing electron-hole pairs from the interaction site to the electrodes. We have also investigated the effects of charge sharing and loss between electrodes, and present these in comparison to charge collection simulations. Detailed analysis of strip-to-strip uniformity in both efficiency and spectral resolution are also presented.

Imaging performance of the Si/Ge hybrid Compton imager

The point spread function (PSF) of a fully-instrumented silicon/germanium Compton telescope has been measured as a function of energy and angle. Overall, the resolution was 3deg to 4deg FWHM over most of the energy range and field of view. The various contributions to the resolution have been quantified. These contributions include the energy and position uncertainty of the detector; source energy; Doppler broadening; and the 1/r broadening characteristic of Compton back-projection. Furthermore, a distortion of the PSF is observed for sources imaged off-axis from the detector. These contributions are discussed and compared to theory and simulations

Germanium orthogonal strip detector system for gamma-ray imaging

A germanium-detector based, gamma-ray imaging system has been designed, fabricated, and tested. The detector, cryostat, electronics, readout, and imaging software are discussed. An 11 millimeter thick, 2 millimeter pitch 19x19 orthogonal strip planar germanium detector is used in front of a coaxial detector to provide broad energy coverage. The planar detector was fabricated using amorphous germanium contacts. Each channel is read out with a compact, low noise external FET preamplifier specially designed for this detector. A bank of shaping amplifiers, fast amplifiers, and fast leading edge discriminators were designed and fabricated to process the signals from preamplifiers. The readout system coordinates time coincident x-y strip addresses with an x-strip spectroscopy signal and a spectroscopy signal from the coaxial detector. This information is sent to a computer where an image is formed. Preliminary shadow and pinhole images demonstrate the viability of a germanium based imaging system. The excellent energy resolution of the germanium detector system provides isotopic imaging.