Juan E. Naya

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.

On the Nature of the High Velocity(26) Al Near the Galactic Center

Recent observations of the Galactic center region by the GRIS balloon-borne germanium spectrometer have determined that the diffuse 1809 keV emission resulting from the decay of 26Al has an intrinsic width of 5.4 keV FWHM. This line width indicates that the 26Al either is at a temperature of ~4.5 × 108 K or has a nonthermal velocity of ~500 km s-1. Previous authors have suggested that the 26Al must be trapped within dust grains in the interstellar medium (ISM) in order for these conditions to persist over the 106 yr lifetime of the aluminum. We discuss the results of our model, in which 26Al dust grains are produced in Type II supernovae and are subsequently reaccelerated in the ISM by ambient supernova remnant (SNR) shocks. Our results show that dust grains can be maintained at a velocity sufficient to explain the GRIS observation for ISM densities of ~0.2 cm-3, dust-grain sizes near 10-5 cm, and distances between SNR shocks in the ISM of 100-200 pc.

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.

Crystal diffraction lens telescope for focusing nuclear gamma rays

A crystal diffraction lens was constructed at Argonne National Laboratory for use as a telescope to focus nuclear gamma rays. It consists of 600 single crystals of germanium arranged in 8 concentric rings. The mounted angle of each crystal was adjusted to intercept and diffract the incoming gamma rays with an accuracy of a few arcsec. The performance of the lens was tested in two ways. In one case, the gamma rays were focused on a single medium size germanium detector. In the second case, the gamma rays were focused on the central germanium detector of a 3 multiplied by 3 matrix of small germanium detectors. The efficiency, image concentration and image quality, and shape were measured. The tests performed with the 3 by 3 matrix detector system were particularly interesting. The wanted radiation was concentrated in the central detector. The 8 other detectors were used to detect the Compton scattered radiation, and their energy was summed with coincident events in the central detector. This resulted in a detector with the efficiency of a large detector (all 9 elements) and the background of a small detector (only the central element). The use of the 3 multiplied by 3 detector matrix makes it possible to tell if the source is off axis and, if so, to tell in which direction. The crystal lens acts very much like a simple convex lens for visible light. Thus if the source is off to the left then the image will focus off to the right illuminating the detector on the right side: telling one in which direction to point the telescope. Possible applications of this type of crystal lens to balloon and satellite experiments are discussed.

Performance of advanced Ge-spectrometer for nuclear astrophysics

Model calculations for a next generation telescope for high resolution gamma-ray spectroscopy are presented: the sensitivity for narrow lines is based on estimates of the background level and the detection efficiency. The instrumental background rates (continuum and lines) are explained as a sum of various components that depend on cosmic-ray intensity and spectrometer characteristics (e.g. mass distribution around the Ge detectors, passive material, characteristics of the detection system and background reduction techniques). Extended background calculations have been performed both with Monte-Carlo simulations and using empirical, semi-empirical and calculated neutron and proton cross-sections. In order to improve the spectrometer sensitivity several design and background reduction techniques (shield thickness, passive material, active or passive coded mask, enriched or natural Ge detectors) have been compared for an instrument with a fixed detector volume.

Gamma-ray background lines in balloon- and satellite-borne Ge spectrometers

The background spectrum in balloon and satellite Ge spectrometers consists of a continuum with discrete nuclear gamma-ray lines superimposed. Although many background lines can be distinguished in a Ge background spectrum, only a few are at energies of astrophysical interest. In this work, tools for estimating these background lines in different spectrometer configurations are provided. The 511 keV background line is studied in detail since it is one of the most intense background features. This line can be described as the sum of three components: (1) atmospheric 511 keV photons entering through the aperture of the instrument; (2) 511 keV photons produced in the (beta) + decays of unstable nuclei in the passive material inside the shield; and (3) 511 keV photons coming from outside that pass through the shield without scattering and are completely absorbed in the detector. The variation of the different components as a function of the passive material and shield characteristics is studied to provide techniques to determine the instrument configuration that minimizes the background line intensity. The mechanisms producing the 1809 keV Al background line are not as numerous. The background line can be described as the sum of a delayed component originating inside the shield and a prompt component from outside the shield. The use of a thick shield seems to be an appropriate way to reduce this background line. However, the lack of information about relevant cross sections has not allowed us to perform accurate modeling. Finally, the mechanisms that cause other narrow background lines at energies of astrophysical interest (i.e., the 476 keV Be line, 844 keV Al line, 847 Fe line, 1157 keV Ti line, 4.439 MeV C line, and 6.129 MeV O line) are briefly described.

Crystal diffraction telescopes for nuclear astrophysics

Until recently, focusing of gamma-radiation was regarded as an impracticable task. Today, gamma-ray lenses have become feasible and present promising perspectives for future instrumentation. For the first time in high energy astronomy the signal/noise ratio will be dramatically improved as gamma-rays are collected on the large area of a lens from where they are focused onto a small detector. Besides an unprecedented sensitivity, such instruments feature very high angular and energy resolution.

A space bourne crystal diffraction telescope for the energy range of nuclear transitions

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Experimental results obtained with the positron-annihilation- radiation telescope of the Toulouse-Argonne collaboration

We present laboratory measurements obtained with a ground-based prototype of a focusing positron-annihilation-radiation telescope developed by the Toulouse-Argonne collaboration. This balloon-borne telescope has been designed to collect 511-keV photons with an extremely low instrumental background. The telescope features a Laue diffraction lens and a detector module containing a small array of germanium detectors. It will provide a combination of high spatial and energy resolution (15 arc sec and 2 keV, respectively) with a sensitivity of {approximately}3{times}10{sup {minus}5} photons cm{sup {minus}2}s{sup {minus}1}. These features will allow us to resolve a possible narrow 511-keV line both energetically and spatially within a Galactic center ``microquasar`` or in other broad-class annihilators. The ground-based prototype consists of a crystal lens holding small cubes of diffracting germanium crystals and a 3{times}3 germanium array that detects the concentrated beam in the focal plane. Measured performances of the instrument at different line energies (511 keV and 662 keV) are presented and compared with Monte-Carlo simulations. The advantages of a 3{times}3 Ge-detector array with respect to a standard-monoblock detector have been confirmed. The results obtained in the laboratory have strengthened interest in a crystal-diffraction telescope, offering new perspectives for die future of experimental gamma-ray astronomy.

Optimization of the veto shield for the INTEGRAL spectrometer SPI with Monte Carlo simulations

Using Monte-Carlo simulations, an optimization of the mass distribution of the scintillator crystals, which constitute the veto shield of the spectrometer SPI on board of INTEGRAL, has been performed. Special emphasis was put on a realistic model for the radiation environment in the satellite orbit. All the components of the radiation (gamma- rays, protons and electrons) in space were taken into account regarding their relative fluxes. Furthermore the radiation produced by nuclear reactions within the spacecraft structure was estimated using a separate computer code. A simple realistic mass model of the spectrometer with special consideration of the holding structure of the crystals and other material within the opening angle of the spectrometer, was implemented. Different geometries for background reduction were analyzed and the results are presented. Experiments concerning the behavior of the radiation damage in the scintillator crystals are presented. They give important hints for methods to avoid an increase in the background due to the radiation induced degradation of the crystals.