Fiona A. Harrison

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>>

Performance optimization for hard x-ray/soft gamma-ray detectors

This paper discusses the optimization of the performance of imaging scintillation detectors used in the hard X-ray/soft gamma-ray (20-300) keV region of the spectrum. In these devices, absorption of an incident gamma-ray within an alkali halide crystal induces a scintillation light distribution which is centroided by an imaging photomultiplier tube mounted to the crystal. The ultimate imaging resolution is strongly affected by the detailed propagation of the scintillation light within the crystal and at the interface between the crystal and the phototube face plate. A number of refined techniques for preparing the scintillation crystals so as to optimize the imaging resolution have been investigated. The results indicate very good agreement with relatively simple models of the light propagation. It is shown that it is possible to achieve resolution consistent with the most optimistic models.

The Gamma Ray Arcminute Telescope Imaging System (GRATIS) detector performance and imaging

We are constructing a telescope with arcminute resolution in the soft 7-ray band. This telescope will provide the first images of cosmic sources on arcminute scales in the energy range 30 - 200 keV. Arcminute resolution gives us the capability, for the first time, to image truly diffuse emission in this wavelength band, opening up exciting new scientific opportunities. The payload consists of ~36 independent coaligned modules. Each module consists of an alkali halide scintillating crystal collimated to a one dimensional coded aperture mask. Every module is fixed in the telescope with a different rotational orientation, allowing reconstruction of two dimensional images. For our application this novel approach has several advantages over a standard two dimensional coded aperture implementation. We have simulated this system and demonstrated the fidelity and robustness of the reconstructions. In addition, we have refined the GRATIS detector technology and achieved improved spatial resolution over that previously reported.

The Gamma-Ray Arc-Minute Imaging System (GRATIS): Mechanical Design And Expected Performance

We are constructing a balloon experiment, GRATIS, which will perform the first arcminute imaging of cosmic sources in the 30 - 200 keV energy band. Observations conducted with GRATIS can provide data relevant to several key problems in high energy astrophysics including the physical processes responsible for the high energy tail observed in the soft gamma-ray spectra of clusters of galaxies and the origin of both the diffuse and point source components of the gamma-ray emission from the Galactic Center. This paper discusses the scientific motivations in more detail, outlines the experiment, discusses several aspects of the design and construction of hardware components, gives an overview of the stabilized platform, and shows the expected performance and sensitivity.

Effective area calibration of the nuclear spectroscopic telescope array (NuSTAR)

The Nuclear Spectroscopic Telescope ARray (NuSTAR) has been in orbit for 6 years, and with the calibration data accumulated over that period we have taken a new look at the effective area calibration. The NuSTAR 10-m focal length is achieved using an extendible mast, which flexes due to solar illumination. This results in individual observations sampling a range of off-axis angles rather than a particular off-axis angle. In our new approach, we have split over 50 individual Crab observations into segments at particular off-axis angles. We combine segments from different observations at the same off-axis angle to generate a new set of synthetic spectra, which we use to calibrate the vignetting function of the optics against the canonical Crab spectrum.

Characterization of Redlen CZT detectors for x-ray astronomy

We present the results of ongoing characterization of Cadmium Zinc Telluride (CZT) semiconductors produced by Redlen Technologies for use in X-ray astronomy. The fully fabricated hybrid detectors consist of CZT crystals with a collecting area of 2 cm x 2 cm and thickness of 3mm mounted on a custom ASIC originally designed for the Nuclear Spectroscopic Telescope Array (NuSTAR) mission, which launched in 2012. We present the results of electronic noise, inter-pixel conductance, and leakage current tests as well as spectral calibration using an 241Am source. Despite high electronic noise due to errors in fabrication, we are able to compare characteristics of the Redlen CZT detectors to those of the CZT detectors produced by eV Products aboard NuSTAR.

Characterization of Redlen CZT detectors for hard x-ray astronomy

We present the results of ongoing characterization of Cadmium Zinc Telluride (CZT) semiconductors produced by Redlen Technologies. In particular we hope to determine their viability for future X-ray astronomy missions such as the High Energy X-ray Probe (HEX-P). The fully fabricated hybrid detectors consist of CZT crystals with a collecting area of 2 cm × 2 cm and thickness of 3 mm mounted on a custom pixelated ASIC originally designed for the Nuclear Spectroscopic Telescope Array (NuSTAR) mission, which launched in 2012. We present the results of inter-pixel conductance and leakage current tests as well as spectral characterization using an 241Am source. Although further calibration and testing is necessary to determine the capabilities of these detectors, preliminary results indicate that Redlen CZT will be able to achieve spectral resolution and noise levels comparable to those of the CZT detectors currently in use aboard NuSTAR.

Imaging germanium telescope array for gamma‐rays (IGETAGRAY)

The Germanium Drift Chamber (GDC) is a gamma‐ray detector with excellent energy and one‐dimensional spatial resolution. Due to recent developments in coded aperture optics, it is feasible to couple one‐dimensional coded apertures and GDCs in a special array geometry producing a telescope with true two‐dimensional imaging. This Imaging Germanium Telescope Array for Gamma‐rays (IGETAGRAY) has made a comparable field of view and sensitivity to true two‐dimensional systems, but simplified engineering requirements. IGETAGRAY will make possible high sensitivity spectroscopy of the gamma‐ray sky.