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 ~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.

We report on the development of high-sensitivity and compact Compton imaging systems built of large and position-sensitive Si(Li) and HPGe detectors. The primary goal of this effort is to provide improved capabilities in the passive detection of nuclear materials for homeland security. Our detectors are implemented in double-sided strip configuration, which—along with digital signal processing—provides energies and three-dimensional position information of individual γ-ray interactions. γ-Ray tracking algorithms then determine the scattering sequence of the γ-ray, which in turn allows us—employing the Compton scattering formula—to reconstruct a cone of possible incident angles and ultimately an image. This Compton imaging concept enables large-field-of-view γ-ray imaging without the use of a heavy collimator or aperture. The intrinsically high-energy resolution of the detectors used, the excellent position resolution we have demonstrated, both combined with the high efficiency of large-volume detectors is the basis for high Compton imaging sensitivity. These capabilities are being developed to identify and localize potential threat sources and to potentially increase the sensitivity in detecting weak sources out of the midst of natural, medical, or commercial sources. γ-ray imaging provides a new degree of freedom to distinguish between spatial and temporal background fluctuations and compact threat sources.

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.

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-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 of the detector at 511 keV is shown to be similar to 4 * 10(-3) for this system. Of events that deposited their full energy in the detector, it is shown that similar to20% are suitable for imaging using current techniques. Methods to improve the efficiency in future systems are discussed.

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.

We have evaluated a collimator-less gamma-ray imaging system, which is based on thin layers of double-sided strip HPGe detectors. The positions of individual gamma-ray interactions will be deduced by the strip addresses and the Ge layers which fired: Therefore, high bandwidth pulse processing is not required as in thick Ge detectors. While the drawback of such a device is the increased number of electronics channels to be read out and processed, there are several advantages, which are particularly important for remote applications: the operational voltage can be greatly reduced to fully deplete the detector and no high bandwidth signal processing electronics is required to determine positions. Only a charge sensitive preamplifier, a slow pulse shaping amplifier, and a fast discriminator are required on a per channel basis in order to determine photon energy and interaction position in three dimensions. Therefore, the power consumption and circuit board real estate can be minimized. More importantly, since the high bandwidth signal shapes are not used to determine the depth position, lower energy signals can be processed. The processing of these lower energy signals increases the efficiency for the recovery of small angle scattering. Currently, we are studying systems consisting of up to ten 2mm thick Ge layers with 2mm pitch size. The required electronics of the few hundred channels can be integrated to reduce space and power. We envision applications in nuclear nonproliferation and gamma-ray astronomy where ease of operation and low power consumption, and reliability, are crucial.

We describe a coded-aperture based, gamma-ray imager that uses a unique hybrid germanium detector system. A planar, germanium strip detector, 11 mm thick is followed by a coaxial detector. The 19/spl times/19 strip detector (2 mm pitch) is used to determine the location and energy of low energy events. The location of high energy events are determined from the location of the Compton scatter in the planar detector and the energy is determined from the sum of the coaxial and planar energies. With this geometry, we obtain useful quantum efficiency in a position-sensitive mode out to 500 keV. The detector is used with a 19/spl times/17 URA coded aperture to obtain spectrally resolved images in the gamma-ray band. We discuss the performance of the planar detector, the hybrid system and present images taken of laboratory sources.

HESSI will perform spectroscopy from roughly 3 keV to 17 MeV, well over three orders of magnitude in energy with a single set of 9 cryogenically cooled germanium detectors. Here we describe the HESSI spectrometer, discuss the procedures for spectroscopic data analysis, and show the estimated sensitivity of HESSI to important features of flare spectra.