Gilbert Vedrenne

MAX: a gamma-ray lens for nuclear astrophysics

The mission concept MAX is a space borne crystal diffraction telescope, featuring a broad-band Laue lens optimized for the observation of compact sources in two wide energy bands of high astrophysical relevance. For the first time in this domain, gamma-rays will be focused from the large collecting area of a crystal diffraction lens onto a very small detector volume. As a consequence, the background noise is extremely low, making possible unprecedented sensitivities. The primary scientific objective of MAX is the study of type Ia supernovae by measuring intensities, shifts and shapes of their nuclear gamma-ray lines. When finally understood and calibrated, these profoundly radioactive events will be crucial in measuring the size, shape, and age of the Universe. Observing the radioactivities from a substantial sample of supernovae and novae will significantly improve our understanding of explosive nucleosynthesis. Moreover, the sensitive gamma-ray line spectroscopy performed with MAX is expected to clarify the nature of galactic microquasars (e+e- annihilation radiation from the jets), neutrons stars and pulsars, X-ray Binaries, AGN, solar flares and, last but not least, gamma-ray afterglow from gamma-burst counterparts.

Spectrometer SPI of the INTEGRAL mission

INTEGRAL is ESAs high-energy astrophysics mission to be launched into a high eccentric orbit early in the next decade. One of the two missions main telescopes is the gamma-ray spectrometer SPI. This instrument features a compact array of 19 high-purity germanium detectors shielded by a massive anticoincidence system. A coded aperture of the HURA type modulates the astrophysical signal. We present the spectrometer system and its characteristics and discuss the choices that led to the present design. The instrument properties like imaging capability, energy resolution and sensitivity have been evaluated by extensive Monte-Carlo simulations. The expected performance for narrow-line spectroscopy is characterized by an energy resolution of approximately 1.6 keV at 1 MeV, an angular resolution of approximately 2 degrees within a totally coded field of view of approximately 15 degrees, and a sensitivity of (2 - 5) multiplied by 10-6 gamma/(cm2 s) for 4 multiplied by 106 s observation time in the nominal energy range from approximately 20 keV and approximately 8 MeV. With these characteristic features it will be possible for the first time to explore the gamma-ray sky in greater depth and detail than it was possible with previous gamma- ray telescopes like SIGMA, OSSE and COMPTEL. In particular the field of nuclear astrophysics will be addressed with an unprecedented combination of sensitivity and energy. Especially the high-energy resolution allows for the first time measuring gamma-ray line profiles. Such lines are emitted by the debris of nucleosynthesis processes, by the annihilation process near compact objects and by the nuclear interaction between cosmic rays and interstellar matter. Lines of all these processes have been measured so far, but, owing to the relatively poor energy resolution, details of the emission processes in the source regions could not be studied. With the high-resolution spectroscopy of SPI such detailed investigations will be possible opening a wealth of astrophysical investigations.

Neutron-induced nuclear reactions and degradation in germanium detectors

We have measured cross sections of neutron-induced nuclear reactions leading to the delayed production of gamma-ray lines similar to the ones of astrophysical interest. Conclusions were drawn concerning the expected background in the Al-26 1809 keV line and the Be-7 478 keV line in SPI. The neutron-induced degradation of Ge detectors was studied vs. the neutron energy, the neutron fluence and the detector temperature. Performance recovery of the detectors was studied for different annealing temperatures. Optimum temperature and times for annealing were determined.

Degradation and recovery of Ge detectors: tests prior to a space mission

Ge detectors onboard the future INTEGRAL mission of ESA will be deteriorated by secondary neutrons. In the present work, flight model Ge detectors were degraded by accelerator-produced neutrons, and then annealed. The detector recovery was found very sensitive to the annealing temperature. Transients following a high-voltage cut-off were studied as well.

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.

High-resolution gamma-ray and hard x-ray spectrometer for long- duration balloon flights

The elements of a high resolution gamma-ray spectrometer, developed for observations of solar flares, are described. Emphasis is given to those aspects of the system that relate to its operation on a long duration balloon platform. The performance of the system observed in its first flight, launched from McMurdo Station, Antarctica on 10 January, 1992, is discussed. Background characteristics of the antarctic balloon environment are compared with those observed in conventional mid-latitude balloon flights and the general advantages of long duration ballooning are discussed.

GRASP — A gamma-ray astronomy mission dedicated to the fine spectroscopy and positioning of celestial gamma-ray sources

The GRASP Mission - Gamma Ray Astronomy with Spectroscopy and Positioning - will be the first high resolution spectral imager to operate in the gamma-ray region of the spectrum. The instrument covers the photon energy range from approximately 15 keV to more than 100 MeV. A combination of discrete germanium solid state devices and scintillation counters form a position sensitive gamma-ray detection matrix which is operated in conjunction with a coded aperture mask to create arc minute images of the gamma-ray sky with a spectral resolution of typically λ/Δλ ∼1000. The use of a coded mask with a ‘zoom’ facility will permit the combination of field of view and angular resolution to be adjusted to suit the scientific aims of each observation. The respective continuum and line sensitivities will be typically 10−8ph cm−2 s−1 keV−1 and 3 10−6 cm−2 s−1 for point sources of gamma-rays with photon energies close to 1 MeV.

Calibration of the spectrometer aboard the INTEGRAL satellite

SPI, the Spectrometer on board the ESA INTEGRAL satellite, to be launched in October 2002, will study the gamma-ray sky in the 20 keV to 8 MeV energy band with a spectral resolution of 2 keV for photons of 1 MeV, thanks to its 19 germanium detectors spanning an active area of 500 cm2. A coded mask imaging technique provides a 2° angular resolution. The 16° field of view is defined by an active BGO veto shield, furthermore used for background rejection. In April 2001 the flight model of SPI underwent a one-month calibration campaign at CEA in Bruyères le Châtel using low intensity radioactive sources and the CEA accelerator for homogeneity measurements and high intensity radioactive sources for imaging performance measurements. After integration of all scientific payloads (the spectrometer SPI, the imager IBIS and the monitors JEM-X and OMC) on the INTEGRAL satellite, a cross-calibration campaign has been performed at the ESA center in Noordwijk. A set of sources has been placed in the field of view of the different instruments in order to compare their performances and determine their mutual influence. We report on the scientific goals of this calibration activity, and present the measurements performed as well as some preliminary results.

Imaging with the coded aperture gamma-ray spectrometer SPI aboard INTEGRAL

ESAs INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) will be launched in October 2002. Its two main instruments are the imager IBIS and the spectrometer SPI. Both emply coded apertures to obtain directional information on the incoming radiation. SPIs detection plane consists of 19 hexagonal Ge detectors, its coded aperture has 63 tungsten-alloy elements of 30 mm thickness.

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.