Scott Douglas Barthelmy

Fabrication of CdZnTe strip detectors for large area arrays

A CdZnTe strip detector array with capabilities for arc second imaging and spectroscopy has been built as a prototype for a space flight gamma ray burst instrument. Two dimensional orthogonal strip detectors with 100 micrometer pitch have been fabricated and tested for a large area array (approximately 60 cm2 with 36 detectors). Details for the fabrication and evaluation of the detectors are presented. Critical issues to be addressed include fabricating metal contacts with low leakage current and with excellent wire bonding yield, achieving high yield for good strips, and surface cleaning and passivation.

CdZnTe background measurements at balloon altitudes

Because of its high atomic number and convenient room temperature operation, CdZnTe has great potential for use in both balloon and space borne hard x-ray (5 - 200 keV) astrophysics experiments. Here we present preliminary results from the first CdZnTe background measurements made by a balloon instrument. Measurements of the CdZnTe internal background are essential to determine which physical processes make the most important background contributions and are critical in the design of future scientific instruments. The PoRTIA CdZnTe balloon instrument was flown three times in three different shielding configurations. PoRTIA was passively shielded during its first flight from Palestine, Texas and actively shielded as a piggyback instrument on the GRIS balloon experiment during flights 2 and 3 from Alice Springs, Australia. PoRTIA flew twice during the Fall 1995 Alice Springs, Australia campaign using the thick GRIS NaI anticoincidence shield. A significant CdZnTe background reduction was achieved during the third flight with PoRTIA placed completely inside the GRIS shield and blocking crystal, and thus completely surrounded by 15 cm of NaI. These background results are presented and contributions from different background processes are discussed.

CdZnTe strip detectors for astrophysical arc second imaging and spectroscopy: detector performance and radiation effects

CdZnTe strip detectors have been fabricated and tested to show the ability for arc second imaging and spectroscopy. Two dimensional CdZnTe strip detectors with 100 micron pitch have been fabricated and wire bonded to readout electronics to demonstrate the ability to localize 22 to 122 keV photons to less than 100 microns. Good spectral resolution has also been achieved. The uniformity and relative efficiency of the strip detector are discussed. Radiation damage effects by intermediate energy neutrons and low energy protons on the surface and bulk performance of CdZnTe devices have been investigated and are presented. Activation and annealing of radiation effects have been seen and are discussed.

Gamma-Ray Limits on Galactic 60Fe Nucleosynthesis and Implications on the Origin of the 26Al Emission

The Gamma Ray Imaging Spectrometer (GRIS) recently observed the gamma-ray emission from the Galactic center region. We have detected the 1809 keV Galactic 26Al emission at a significance level of 6.8-sigma but have found no evidence for emission at 1173 keV and 1332 keV, expected from the decay chain of the nucleosynthetic 60Fe. The isotopic abundances and fluxes are derived for different source distribution models. The resulting abundances are between 2.6+-0.4 and 4.5+-0.7 Solar Masses for 26Al and a 2-sigma upper limit for 60Fe between 1.7 and 3.1 Solar Masses. The measured 26Al emission flux is significantly higher than that derived from the CGRO/COMPTEL 1.8 MeV sky map. This suggests that a fraction of the 26Al emission may come from extended sources with a low surface brightness that are invisible to COMPTEL. We obtain a 60Fe to 26Al flux ratio 2-sigma upper limit of 0.14, which is slightly lower than the 0.16 predicted from current nucleosynthesis models assuming that SNII are the major contributors to the galactic 26Al. Since the uncertainties in the predicted fluxes are large (up to a factor of 2), our measurement is still compatible with the theoretical expectations.

BASIS mission concept for gamma-ray burst imaging and spectroscopy

We are studying a gamma-ray burst mission concept called burst arcsecond imaging and spectroscopy (BASIS) as part of NASAs new mission concepts for astrophysics program. The scientific objectives are to accurately locate bursts, determine their distance scale, and measure the physical characteristics of the emission region. Arcsecond burst positions (angular resolution approximately 30 arcsec, source positions approximately 3 arcsec) will be obtained for approximately 100 bursts per year using the 10 - 100 keV emission. This will allow the first deep, unconfused counterpart searches at other wavelengths. The key technological breakthrough that makes such measurements possible is the development of CdZnTe room-temperature semiconductor detectors with fine (approximately 100 micron) spatial resolution. Fine spectroscopy will be obtained between 0.2 and 150 keV. The 0.2 keV threshold will allow the first measurements of absorption in our galaxy and possible host galaxies, constraining the distance scale and host environment.

Performance of Prototype Segmented CdZnTe Arrays

The Burst and All Sky Imaging Survey (BASIS) is a proposed mission to provide {approximately}3 arc second locations of approximately 90 Gamma-Ray Bursts (GRBs) per year. The BASIS coded aperture imaging system requires a segmented detector plane able to detect the interaction position of (10--150 keV) photons to less than 100 {micro}m. To develop prototype detector arrays with such fine position resolution the authors have fabricated many 15 mm x 15 mm x 2 mm 100 {micro}m pitch CdZnTe strip detectors. They have assembled these fine pitch CdZnTe strip detectors into prototype 2 x 2 and 6 x 6 element arrays read out by ASIC electronics. The assembly and electronics readout of the 6 x 6 flight prototype array will be discussed, and preliminary data illustrating the uniformity and efficiency of the array will be presented.

Position resolution performance of prototype segmented CdZnTe arrays

The burst and all sky imaging survey (BASIS) is a proposed mission to provide plus or minus 3 arc-second locations of an estimated 90 gamma-ray bursts (GRBs) per year. The BASIS coded aperture imaging system requires a segmented detector plane able to detect the position of photon absorption to less than 100 microns. To develop prototype detector arrays with such fine position resolution we have fabricated many 15 mm by 15 mm by 2 mm 100 micron pitch CdZnTe strip detectors. A 2 by 2 prototype 100 micron CdZnTe strip detector array has been fabricated and has been used to test the capabilities of the BASIS imaging system. Preliminary shadowgrams of a 1 mm wide gap between two tungsten straight edges indicate that our position resolution is on the order of 69 micrometers. Both the array and imaging tests are described. A 6 by 6 element CdZnTe detector array is also being fabricated at GSFC. The assembly of this flight prototype array is discussed as well as applications for BASIS.

Arc-second source positions with a prototype BASIS imaging system

The burst and all-sky imaging survey (BASIS) project is a proposed small explorer (SMEX) mission to image the gamma-ray sky in the 10 - 150 keV energy range with high angular and energy resolution. It will be able to determine the locations of gamma-ray bursts (GRBs) to within a few arcseconds, sending accurate positions to ground-based telescopes for simultaneous and follow-up observations within seconds of the beginning of the GRB. It will also produce all-sky maps with 30 arcsecond resolution and 2 milliCrab sensitivity. The instrument uses a two-scale coded aperture mask to modulate gamma-rays falling on a cadmium zinc telluride (CZT) detector plane consisting of both 100 micrometer pitch strip detectors and 4 mm square spectroscopy detectors. The spatial pattern of gamma-rays will be deconvolved with the mask pattern to produce an image. This paper presents results from a prototype of this system, using a mask and strip detectors to produce an image of a radioactive source. The prototype functions as expected, producing images which, when scaled to the dimensions of the proposed instrument, achieve the desired resolution.

BASIS: a new gamma-ray burst imaging and spectroscopy mission concept

We have proposed a gamma-ray burst mission concept called burst arcsecond imaging and spectroscopy (BASIS) in response to NASAs announcement for new mission concept studies. The scientific objectives are to accurately locate bursts, determine their distance scale, and measure the physical characteristics of the emission region. Arcsecond burst positions (angular resolution approximately 30 arcsec, source positions approximately 3 arcsec for greater than 10-6 erg cm-2 bursts) are obtained for about 100 bursts per year using the 10 - 200 keV emission. This allows the first deep, unconfused counterpart searches at other wavelengths. The key technological breakthrough that makes such measurements possible is the development of CdZnTe room-temperature semiconductor detectors with fine (approximately 100 micron) spatial resolution. Fine spectroscopy is obtained between 0.2 keV and 200 keV. The 0.2 keV threshold allows the first measurements of absorption in our galaxy and possible host galaxies, constraining the distance scale and host environment. The mission concept and its scientific objectives are described.

Time-Domain Astronomy with Swift, Fermi and Lobster

The dynamic transient gamma-ray sky is revealing many interesting results, largely due to findings by Fermi and Swift . The list includes new twists on gamma-ray bursts (GRBs), a GeV flare from a symbiotic star, GeV flares from the Crab Nebula, high-energy emission from novae and supernovae, and, within the last year, a new type of object discovered by Swift —a jetted tidal disruption event. In this review we present highlights of these exciting discoveries. A new mission concept called Lobster is also described; it would monitor the X-ray sky at order-of-magnitude higher sensitivity than current missions can.