Klaus-Peter Ziock

Source-search sensitivity of a large-area, coded-aperture, gamma-ray imager

We have recently completed a large-area, coded-aperture, gamma-ray imager for use in searching for radiation sources. The instrument was constructed to verify that weak point sources can be detected at considerable distances if one uses imaging to overcome fluctuations in the natural background. The instrument uses a rank-19, one-dimensional coded aperture to cast shadow patterns onto a 0.57 m/sup 2/ NaI(Tl) detector composed of 57 individual cubes each 10 cm on a side. These are arranged in a 19 /spl times/ 3 array. The mask is composed of four-centimeter thick, one-meter high, 10-cm wide lead blocks. The instrument is mounted in the back of a small truck from which images are obtained as one drives through a region. Results of first measurements obtained with the system are presented.

A large-area PSPMT-based gamma-ray imager with edge reclamation

We describe a coded aperture, gamma-ray imager which uses a CsI(Na) scintillator coupled to a Hamamatsu R3292 position-sensitive photomultiplier tube (PSPMT) as the position-sensitive detector. We have modified the normal resistor divider readout of the PSPMT to allow use of nearly the full 10 cm diameter active area of the PSPMT with a single scintillator crystal one centimeter thick. This is a significant performance improvement over that obtained with the standard readout technique where the linearity and position resolution start to degrade at radii as small as 3.0 cm with a crystal 0.75 cm thick. This represents a recovery of over 60% of the PSPMT active area. The performance increase allows the construction of an imager with a field of view 20 resolution elements in diameter with useful quantum efficiency from 60-700 keV. In this paper we describe the readout technique, its implementation in a coded aperture imager and the performance of that imager.

A gamma-ray imager for arms control

Ascertaining the number of warheads on a missile-delivery systems by a remote, nonvisual inspection may be desirable in some arms-control verification scenarios. A gamma-ray imaging system (GRIS) has been developed to inspect for nuclear warheads by imaging the highly penetrating gamma-radiation emitted by radioactive materials integral to warheads. Such an image may be used to obtain the missile loadout. The authors describe the GRISs operating principles and capabilities, present data, and discuss the advantages and disadvantages of such a system for arms control. They also present field results where GRIS was used on an emplaced Peacekeeper missile. Improvements to GRIS in light of the field trial results are presented.<<ETX>>

Automatic image analysis for detecting and quantifying gamma-ray sources in coded-aperture images

We report the development of an automatic image analysis system that detects gamma-ray source regions in images obtained from a coded aperture, gamma-ray imager. The number of gamma sources in the image is not known prior to analysis. The system counts the number (K) of gamma sources detected in the image and estimates the lower bound for the probability that the number of sources in the image is K. The system consists of a two-stage pattern classification scheme in which the probabilistic neural network is used in the supervised learning mode. The algorithms were developed and tested using real gamma-ray images from controlled experiments in which the number and location of depleted uranium source disks in the scene are known. The novelty of the work lies in the creative combination of algorithms and the successful application of the algorithms to real images of gamma-ray sources.

Computer vision for detecting and quantifying gamma-ray sources in coded-aperture images

We report the development of an automatic image analysis system that detects gamma-ray source regions in images obtained from a coded aperture, gamma-ray imager. The number of gamma sources in the image is plot known prior to analysis. The system counts the number (K) of gamma sources detected in the image and estimates the lower bound for the probability that the number of sources in the image is K. The system consists of a two-stage pattern classification scheme in which the probabilistic neural network is used in the supervised learning mode. The algorithms were developed and tested using real gamma-ray images from controlled experiments in which the number and location of depleted uranium source disks in the scene are known.<<ETX>>

The gamma-ray burst monitor for the Russian Mars 1996 mission

We describe an innovative detector designed primarily for the study of cosmic gamma-ray bursts aboard interplanetary spacecraft. The experiment is a fully functional, low cost, low mass, low-power instrument, which, when combined with other similar instruments, has the capability of localizing bursts to arcminute precision. The burst monitor is based on a photomultiplier tube and a plastic/CsI(Tl) phoswich detector and records photons in the 20-150-keV energy range with time resolutions up to several milliseconds. In addition, it records photon energy spectra. Events in the plastic scintillator are distinguished by a pulse shape discrimination circuit and are used to count electrons and protons. In a planetary orbiter or fly-by mission, this capability can be used to detect the presence of a magnetosphere. We describe in detail a version of this instrument which was built for the Russian Mars 1996 mission.

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.

A germanium gamma ray imager with 3D position sensitivity

A germanium based gamma ray imager with 3D position sensitivity is being developed. The imager will detect gamma rays in the range of /spl sim/10 keV to 1.5 MeV and locate their interactions in the germanium to within 2 mm in X, Y and Z. The instrument is segmented into 39 orthogonal strips in both the X and Y directions with a 2 mm pitch. Each channel is separately instrumented with a miniature low-noise charge-sensitive preamplifier followed by shaping and timing electronics. Germanium based imagers with 2-D position sensitivity have recently been developed and fully instrumented by our group. The current instrument adds the capability for determining the Z position (depth of interaction). The Z coordinate is found by looking at the relative timing of the signals induced by the drifting electrons and holes. A relatively simple analog approach for determining the depth of interaction was implemented. Some of the intended applications for this type of instrument include balloon based imaging experiments and field portable devices for environmental remediation. Therefore, much of the effort was concentrated on developing a detector and electronics scheme that was simple, compact, and low power. The instrument is described with emphasis on the technique used for measuring the Z coordinate. Results obtained from spectroscopy as well as Z position measurements are shown.

GRABIT: a large-area, broad-bandwidth, gamma-ray imaging spectrometer for astrophysics

GRABIT is the gamma-ray bar imaging telescope, a low-energy gamma-ray (60-600 keV) imaging spectrometer that can be extended to a large area at low cost. GRABIT has a gamma-ray position-resolution of 2 mm with an energy resolution typical of alkali-halide detectors (18% at 60 keV). The complete coded-aperture telescope will have arc-minute angular resolution over a large field of view. In addition, the good energy and angular resolutions are obtained over the full bandwidth of the instrument.

Burstman: a portable GRB detector for really long voyages

The renewal of the Ulysses mission to the year 2001, and the failure of Mars Observer, once more leave the Interplanetary Network with only two widely spaced components. We have therefore developed and begun to build a small GRB detector for the Russian Mars ’96 mission. The first flight unit has now been delivered to Russia for spacecraft tests. Three interesting features of this experiment are first, that it measures both particles and gamma rays, second, that it is not much larger than a Walkman (hence the name), and third, that it is being constructed with support only from discretionary funds at a number of institutes. We discuss the types of measurements that Burstman will make, as well as the quantity and quality of the small error boxes that will be obtained during the two year (nominal) Mars ’96 mission.