Morgan T. Burks

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.

First-generation hybrid compact Compton imager

At Lawrence Livermore National Laboratory, we are pursuing the development of a gamma-ray imaging system using the Compton effect. We have built our first generation hybrid Compton imaging system, and we have conducted initial calibration and image measurements using this system. In this paper, we present the details of the hybrid Compton imaging system and initial calibration and image measurements

First results from the balloon flight of the NCT prototype

We flew a prototype of the Nuclear Compton Telescope (NCT) on a high altitude balloon from Fort Sumner, New Mexico on 2005 June 1. The NCT prototype is a soft gamma-ray (0.2-15 MeV) telescope designed to study, through spectroscopy, imaging, and timing, astrophysical sources of nuclear line emission and gamma-ray polarization. Our program is designed to develop and test the technologies and analysis techniques crucial for the Advanced Compton Telescope satellite, while studying gamma-ray radiation with very high spectral resolution, moderate angular resolution, and high sensitivity. The NCT prototype utilizes two, 3D imaging germanium detectors (GeDs) in a novel, ultra-compact design optimized for nuclear line emission (0.5-2 MeV) and polarization in the 0.2-0.5 MeV range. Our prototype flight was a critical test of the novel instrument technologies, analysis techniques, and background rejection procedures we have developed for high resolution Compton telescopes.

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.

Pre-flight calibration of the prototype Nuclear Compton Telescope

The Nuclear Compton Telescope (NCT) is a balloon-borne soft gamma-ray (0.2MeV-10MeV) telescope designed to study astrophysical sources of nuclear line emission and polarization. A prototype instrument was successfully launched from Ft. Sumner, NM on June 1, 2005. The NCT prototype consists of two 3D position sensitive High-Purity-Germanium (HPGe) strip detectors fabricated with amorphous Ge contacts. The novel ultra-compact design and new technologies allow NCT to achieve high efficiencies with excellent spectral resolution and background reduction. Energy and positioning calibration data was acquired pre-flight in Fort Sumner, NM after the full instrument integration. Here we discuss our calibration techniques and results, and detector efficiencies. Comparisons with simulations are presented as well.

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

A 4-/spl pi/ field of view Compton imager based on a single planar germanium detector

A Compton imager has been developed based on a single germanium strip detector. The system has the ability to image point or continuous gamma ray sources located anywhere in a 4-/spl pi/ field of view. The effective energy range for imaging is approximately 200 keV to greater than 1 MeV. The system was designed as a prototype for a field deployable system. Therefore, custom electronics were designed and other efforts were made such that the entire system could fit on a small lab cart and be moved by a single person. Possible applications include search and monitoring of radionuclide material. Various images are shown and the ability of the imager to find a source in the presence of high background is demonstrated. The absolute imaging efficiency or the detector at 511 keV is shown to be /spl sim/ 4 * 10/sup -3/ for this system. Of events that deposited their full energy in the detector, it is shown that /spl sim/20% are suitable for imaging using current techniques. Methods to improve the efficiency in future systems are discussed.

Performance of gamma-ray imager using a 38 /spl times/ 38 crossed-strip Ge detector

We have developed a gamma-ray imager that uses a unique hybrid germanium detector as its gamma-ray sensing element. The detector includes an eleven-millimeter thick two-millimeter pitch, 38 /spl times/ 38 crossed-strip, planar, Ge detector followed by a 5-cm thick, 8-cm diameter, coaxial, Ge detector. The two-detector design significantly improves the stopping power for high energy gamma-rays over that of the planar detector alone. The images are created using the coded aperture imaging technique. A shadow mask of 6.1-mm thick Ta is used to provide imaging efficiency out to beyond 500 keV. The imager is designed for use in non-destructive analysis problems where the source configuration and overlying material is not known a priori The images are fully deconvolved to provide a spectrum at each pixel in the image. This information allows NDA techniques to be applied to each line of sight from the source, significantly improving the information obtained from a measurement. In this paper we present the details of the imager design and its performance.