Bernard F. Phlips

The Soft Gamma-Ray Spectral Variability of Cygnus X-1

We have used observations of Cyg X-1 from the Compton Gamma Ray Observatory and BeppoSAX to study the variation in the MeV γ-ray emission between the hard and soft spectral states, using spectra that cover the energy range from 20 keV up to 10 MeV. These data provide evidence for significant spectral variability at energies above 1 MeV. In particular, whereas the hard X-ray flux decreases during the soft state, the flux at energies above 1 MeV increases, resulting in a significantly harder γ-ray spectrum at energies above 1 MeV. This behavior is consistent with the general picture of galactic black hole candidates having two distinct spectral forms at soft γ-ray energies. These data extend this picture, for the first time, to energies above 1 MeV. We have used two different hybrid thermal/nonthermal Comptonization models to fit broadband spectral data obtained in both the hard and soft spectral states. These fits provide a quantitative estimate of the electron distribution and allow us to probe the physical changes that take place during transitions between the low and high X-ray states. We find that there is a significant increase (by a factor of ~4) in the bolometric luminosity as the source moves from the hard state to the soft state. Furthermore, the presence of a nonthermal tail in the Comptonizing electron distribution provides significant constraints on the magnetic field in the source region.

Gray-tone lithography using an optical diffuser and a contact aligner

This paper describes a simple method for the three-dimensional (3D) microfabrication of complex high-aspect structures in a one mask lithography process. The method relies on an unconventional way of performing gray-tone lithography. The main idea is to randomize the collimated light by using an optical diffuser to generate intensity distributions in the photoresist. The resist topography is determined by the density of open and opaque squares in the photomask. The resulting 3D resist is then transferred into 3D silicon structures by using reactive ion etching and deep reactive ion etching.

Germanium strip detector Compton telescope using three-dimensional readout

Compton telescopes using two germanium strip detectors with depth resolution have been demonstrated at the Naval Research Lab. Depth resolution allows interactions to be located to less than 1 mm, down from 1 cm with no depth resolution, which improves the imaging resolution of the telescope substantially. Compton images and reconstructed energy spectra of events in which gamma rays interact three times in the two detectors but did not deposit their full energy (i.e. Three-Compton) are examined. Finally, the multiple modes of the system including a standard Compton telescope and a Three-Compton telescope are compared.

Development and Applications of Position-Sensitive Solid-State Gamma Ray Detectors

The development of high-resolution position-sensitive, solid-state detectors will enable gamma ray detectors with improved sensitivity and imaging capabilities. The gamma ray astrophysics group at NRL has been developing germanium strip detectors for several years. We have shown that three-dimensional locations for gamma ray interactions can be determined with sub-millimeter accuracy, and have also demonstrated imaging capability within a single germanium strip detector. We have also initiated work on thick, silicon strip detectors. This was based on the fact that three sequential interactions can enable the energy and direction cone of the incident gamma ray to be determined, even without total energy deposition of the incident gamma ray. We are also working on low-power ASICs that are required to handle the large number of channels associated with arrays of strip detectors. Progress on this work will be presented, along with applications to high-energy astrophysics, medical imaging, nuclear physics, detection of fissile materials, and monitoring of environmental radioactivity.

Gamma Ray Observations of Cygnus X-1 With the Oriented Scintillation Spectrometer Experiment

Abstract : We report on ~120 days of observations of Cygnus X-1 with OSSE onboard the Compton Observatory. Emission is detected in the range 50 keV to 1 MeV and we find evidence for a continuum of hard X-ray flux levels rather than the existence of distinct flux states. Comparisons of the source spectra with various theoretical models show that an exponentially truncated power law best describes the average spectrum in the OSSE energy band. Although we have measured a new minimum in the hard X-ray flux from the source, no evidence was found for either a broad 1 MeV feature or a narrow 511 keV line previously reported in association with a low flux state. Upper limits on such emission features are an order of magnitude lower than earlier reported detections. The 5.6-day periodicity of the source measured at optical wavelengths was not detected with a sensitivity to the rms modulation fraction of 5% in the 60-140 keV energy band.

Beam test of gamma-ray large area space telescope components

A beam test of GLAST (Gamma-ray Large Area Space Telescope) components was performed at the Stanford Linear Accelerator Center in October, 1997. These beam test components were simple versions of the planned flight hardware. Results on the performance of the tracker, calorimeter, and anti-coincidence charged particle veto are presented.A beam test of GLAST (Gamma-ray Large Area Space Telescope) components was performed at the Stanford Linear Accelerator Center in October, 1997. These beam test components were simple versions of the planned flight hardware. Results on the performance of the tracker, calorimeter, and anti-coincidence charged particle veto are presented.

Thick silicon strip detector Compton imager

We present initial results obtained with double-sided, thick (2 mm) silicon strip detectors used as a Compton imager. A reconstructed image of a gamma ray source and a spectrum of the gamma ray energy are produced at room temperature using the multiple Compton technique. Multiple Compton interactions allow the energy and Compton scattering angle to be reconstructed without having to absorb the energy of the incident gamma ray completely. This extends our work on multiple Compton imagers using germanium strip detectors to silicon detectors that operate at higher temperatures. The detectors are 57 /spl times/ 57 mm in active area and are 2 mm thick with 64 strips per side with a pitch of 0.9 mm. The energy resolution of the detectors for 60 keV gamma rays are 3-4 keV at room temperature and 2.1 keV at -20/spl deg/C. Simulations of the imager are performed in GEANT4, including Doppler broadening, and show agreement with the real data.

Results from the Beam Test of the Engineering Model of the GLAST Large Area Telescope

This paper describes the results of a beam test using the Engineering Model of the GLAST Large Area Telescope, which was installed in a beam of positrons, hadrons and tagged photons at SLAC. The performance of the four subsystems, Anti Coincidence Detector, Silicon Tracker, Calorimeter and Data Acquisition will be described.

Mobile imaging and Spectroscopic Threat Identification (MISTI): System overview

The Mobile Imaging and Spectroscopic Threat Identification (MISTI) system developed to locate radiological threats in urban and rural environments is currently undergoing characterization activities. MISTI is a mobile source detection and imaging system designed to identify and localize a radiological source to within +/- 10 m in range. This requirement is based on a 1 mCi Cs-137 source at 100 m in 20s, while maintaining a false alarm rate of less than one per day. MISTI utilizes the cost effective collection power of NaI for imaging and the sensitivity of high resolution HPGe for spectroscopy. MISTIs data acquisition system was developed with the latest commercially availed hardware that met MISTIs requirements. The performance of crucial software and hardware components is presented along with overall system performance. A synopsis and example of the initial characterization results are presented here.

Coincident Compton nuclear medical imager

A novel method is proposed for nuclear medical imaging. A high resolution Compton imager with large solid angle and high efficiency is used to detect multiple gamma rays from selected radionuclide decays. The direction cone of each gamma ray is re-constructed from the multiple Compton interactions in the detector array. The intersection of three Compton direction cones defines a limited number of positions, often a unique location within the region of interest, for each decay. For positron decay radionuclides that are accompanied by a coincident gamma ray (e.g. /sup 14/O) the location is determined from the intersection of the 511 keV interaction sites and the Compton direction cone of the third gamma ray. This technique has the advantages of 3-dimensional imaging with lower doses, and comparable or improved resolution compared to standard SPECT and PET techniques. The technique will be described, along with a discussion of alternative detectors configurations. Results of simulations showing the capability of this technique are presented.