F. Albernhe

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.

The spectrometer SPI of the INTEGRAL mission

The spectrometer on INTEGRAL (SPI) is one of the two main telescopes of the future INTEGRAL observatory. SPI is made of a compact hexagonal matrix of 19 high-purity germanium detectors shielded by a massive anticoincidence system. A HURA type coded aperture modulates the astrophysical signal. The spectrometer system, its physical characteristics and performances are presented. The instrument properties such as imaging capability, energy resolution and sensitivity have been evaluated by means of extensive Monte-Carlo simulations. With the expected performances of SPI, it will be possible to explore the γ-ray sky in greater depth and detail than it was possible with previous γ-ray telescopes like SIGMA, OSSE and COMPTEL. In particular, the high-energy resolution will allow for the first time the measurement of γ-ray line profiles. Such lines are emitted by the debris of nucleosynthesis and annihilation processes in our Galaxy. Lines from these processes have already been measured, but due to the relatively ...

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.

SPI: A high resolution imaging spectrometer for INTEGRAL

SPI (Spectrometer for INTEGRAL) is a high spectral resolution gamma-ray telescope using cooled germanium detectors that will be flown on board the INTEGRAL mission in 2001. It consists of an array of 19 closely-packed germanium detectors surrounded by an active bismuth germanate (BGO) anti-coincidence shield. The instrument operates over the energy range 20 keV to 8 MeV with an energy resolution of 1–5 keV. A tungsten coded-aperture mask located 1.7 m from the detector array provides imaging over a 15° fully-coded field-of-view with an angular resolution of ∼3°. The point source narrow-line sensitivity is estimated to be 3–7×10−6 ph cm−2 s−1 over most of the range of the instrument (E>200 keV) for a 106 s observation. With its combination of high sensitivity, high spectral resolution and imaging, SPI will improve significantly over the performance of previous instruments such as HEAO-3, OSSE, and Comptel. It can be expected to take a major step forward in experimental studies in nuclear astrophysics. The ...

Recent measurements with a balloon borne Ge(Li) spectrometer in gamma-ray astronomy

Abstract A new high resolution γ-ray spectrometer for astrophysical measurements has been jointly developed at Saclay and Toulouse. The instrument consists of a large Ge(Li) crystal (140 cm 3 ) cooled to liquid nitrogen temperature and actively shielded by a sodium iodide crystal. Spectral resolution and effective area, are respectively 3.0 keV (fwhm) and 2.7 cm 2 at 1 MeV. Three flights were carried out in Brazil (Dec. 76, Feb. 77) with a stabilized gondola. The total background counting rate was about 1- counts/s in the 0.1–8 MeV energy range at float altitude (2.9 g cm −2 residual atmosphere, 12 GV rigidity cutoff). The sensitivity of the detector is about 10 −3 photons cm −2 s −1 over the above energy range for three hours of observation of a point source. Preliminary results are described.

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.