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For the first time in the history of high energy astronomy, a large CdTe gamma-ray camera is operating in space. ISGRI is the low-energy camera of the IBIS telescope on board the INTEGRAL satellite. This paper details its design and its in-flight behavior and performances. Having a sensitive area of 2621 cm$^2$ with a spatial resolution of 4.6 mm, a low threshold around 12 keV and an energy resolution of $\sim$ 8% at 60 keV, ISGRI shows absolutely no signs of degradation after 9 months in orbit. All aspects of its in-flight behavior and scientific performance are fully nominal, and in particular the observed background level confirms the expected sensitivity of 1 milliCrab for a 10$^6$s observation., Comment: INTEGRAL A&A special issue

A new generation of large area semiconductor based γ-ray camera for astrophysics space application has been developed through the γ-ray IBIS (Imager on Board INTEGRAL Satellite) telescope program, on board the ESA INTEGRAL satellite. This instrument features a coded-mask aperture imaging system, active and passive shields and two detector arrays. The first one (ISGRI : INTEGRAL Soft Gamma-Ray Imager) is made of 16384 CdTe detectors (4×4 mm large, 2 mm thick) operating at room temperature and will operate between 15 keV and 1 MeV. The second detector array (PICsIT : Pixelated CsI Telescope) is made of 4096 CsI scintillators (9.2×9.2 mm large, 30 mm high) coupled to silicon PIN photodiodes working between 150 keV and 10 MeV. IBIS combine broad band energy, spectroscopic capabilities and high spatial resolution imaging (12 arcmin) over a wide field of view (∼20°×20°, 50% sensitivity). A “flight-like” model of the detectors has been built, integrated and successfully tested. After a description of the IBIS imaging principle, we will report results obtained with the qualification model of the ISGRI camera.

A basic component called Polycell has been developed for the ISGRI CdTe γ-ray camera of the IBIS telescope on board the INTEGRAL satellite. This imager is an assembly of 16384 CdTe detectors (4×4 mm large, 2 mm thick) operating at room temperature. ISGRI covers the lower part (20 keV–1 MeV) of the IBIS energy range (20 keV–10 MeV). Detectors are arranged 4×4 on Polycells, where they are connected to their front-end electronics. The front-end electronics is based on radiation tolerant 4-channel analog–digital ASICs. The ASICs contain a low-noise charge-sensitive preamplifier (160e− rms; 1 pF load) and a pulse rise-time measurement in addition to the standard pulse-height measurement. This permits to compute a charge loss correction based on the charge drift time. The digital part includes the internal acquisition timing sequence as well as the dialogue with external electronics. After a description of the front-end electronics, we will present the improvements made on the new and final version of this hybrid mini CdTe γ-ray cameras. In conclusion, we will present the spectometric performance obtained owing to this modular device. A spectral resolution around 7% at 122 keV is obtained with this last generation of ASICs.

The gamma-ray telescope IBIS, on board the INTEGRAL satellite, features a coded-mask aperture, active and passive shields and two detector arrays. The first one (ISGRI) is an assembly of 16384 CdTe detectors (4×4 mm large, 2 mm thick) operating at room temperature. ISGRI covers the lower part (15 keV–1 MeV) of the IBIS energy range (15 keV–10 MeV). Detectors are arranged on polycells, each including 16 crystals, connected to their front-end electronics (ASICs). Each of the eight independent ISGRI modules are made of 128 polycells. The ASICs contain a low noise charge-sensitive preamplifier and feature pulse rise-time measurement in addition to the standard pulse height measurement. This permits to compute a charge loss correction based on the charge drift time. After application of this correction, a spectral resolution around 7.5% at 122 keV is obtained with the ASICs. Today, 16 polycells have been mounted on the first representative ISGRI module. This module has been interfaced with the entire ISGRI data-processing electronics and forms the engineering model of ISGRI. After a presentation of the scientific context, this paper describes the ISGRI design with particular emphasis on the detectors, the polycells performances and the module assembly.

The γ-ray imager telescope IBIS, on board the INTEGRAL satellite, features a coded mask aperture and two detector arrays. The first detector array (ISGRI) is an assembly of 16 384 CdTe detectors (4×4 mm 2 large, 2 mm thick) operating at room temperature under 100 V bias. ISGRI covers the lower part (20 keV–1 MeV) of the IBIS energy range (20 keV–10 MeV). The polycell is the basic component of the ISGRI detector array. It is made of 16 CdTe pixels and their front-end electronics. In order to improve the response of the instrument, a charge loss correction based on the charge-drift time is necessary. Therefore, the front-end electronics performs the rise-time measurement in addition to the standard pulse-height measure. On the other hand, the necessarily tight packaging and the limited available power requires the use of ASICs. These 4-channel analog–digital ASICs should be radiation resistant either through the use of a latch up free technology or by applying an appropriate layout design. The optimal operating temperature (around 0°C) is ensured under vacuum conditions by radiative cooling. This paper describes the ISGRI design with particular emphasis on the ASICs and polycells, and reports preliminary performance measurements.

A basic component, called Polycell, has been developed for the ISGRI (INTEGRAL Soft Gamma Ray Imager) CdTe camera on board the INTEGRAL (INTErnational Gamma-Ray Astrophysics Laboratory) satellite. Operating at room temperature, it covers the 20 keV -1 MeV energy range. It features a sub-ensemble of 16 CdTe detectors and their associated front end electronics. This electronics is based on 4-channel analog-digital ASICs. Their analog part features a low noise preamplifier, allowing a threshold below 20 keV and a pulse rise-time measurement which permits a charge loss correction. The digital part ensures the internal acquisition timing sequence as well as the dialogue with external electronics. Two versions of the ISGRI ASIC have been developed in a collaboration of two CEA microelectronics teams from CEA/DTA/LETI/DSYS and CEA/DSM/DAPNIA/SEI, respectively on a standard CMOS AMS process hardened against radiation by lay-out, and on a Silicon On Insulator process (DMILL MHS), the latter being latch-up free. This paper presents the ASIC and polycell architecture as well as experimental results obtained with polycells equipped with AMS ASICs.

The IMAGER gamma-ray telescope on board INTEGRAL features a coded aperture and a two-layer detector. The first of these layers, ISGRI, is a 16 384 pixel CdTe gamma-camera which will operate in the 20 keV–1 MeV energy range. The spectral performances of ISGRI at energies greater than 50 keV are obtained using a software charge-loss correction based on pulse rise-time measurements. At high energy, the study of the Compton interactions between the two IMAGER layers should ensure a good background rejection. This device will provide unprecedented performances in this energy range: a spatial resolution of 4 mm, a spectral resolution better than 7% at 122 keV and a broad band sensitivity at 3σ of about 10 −6 cm −2 s −1 keV −1 at 100 keV for a 10 5 s observation. Galactic X-ray binary sources, supernovae and active galactic nuclei will be prime targets for ISGRI.

The launch in December 1989, of SIGMA, one of the main devices aboard the GRANAT spacecraft, has provided high-energy astronomers for the first time with a telescope whose imaging properties in the soft γ-ray regime match those of instruments operating in the hard X-ray band. Having examined ∼ 300 celestial fields during more than two years of successful in-orbit operations, this coded-aperture telescope, sensitive to radiation in the energy range 35 keV to 1.3 MeV, has succeeded both in disentangling severe source confusion problems and in providing firm identifications and reliable spectra of sources at soft γ-ray wavelengths. A selection of the salient scientific results obtained so far is presented, with intent to emphasize the unique contribution of a coded-mask telescope able to perform accurate images of the sky in the soft γ-ray domain.

High detector background level is the major obstacle encountered in astronomical observations in the hard X- and γ-ray waveband. Progresses have been made in understanding and modelling the background in the space environment, particularly at LEO helped by valuable data obtained by instruments operating in such orbits. Telescope operating in a HEO is facing a different radiation environment, hence it is expected that its background will differ from what is known in the LEO. The measurements by the SIGMA telescope provide us the first opportunity to assess all aspects of the background of a soft γ-ray detector operating in a HEO, these include: the overall background level, its composition and stability, the effects of trapped radiation and solar activities, and the prominent background lines. Comparing to the backgrounds of detectors operating in LEOs, it shows that the HEO is a very competitive orbit for γ-ray observations from the point of view of the expected background level, and with the advantage of long and uninterrupted observations time and better sky coverage.

The French soft γ-ray coded mask imaging telescope on board Soviet spacecraft GRANAT has experienced some unexpected gain drifts during its operation in the space. This gain instability seriously affected its data analysis and scientific interpretation of the results. After a detailed study of its background spectra and the long term performance of the detector, we have identified several background features which enable us to perform the in flight energy calibrations and gain corrections for the observations. The precision of such an energy calibration is estimated to be in the order of 0.5%, which is accurate enough for all types of data analysis such as spectrum de-convolution, and summation of observation spectra. More importantly it makes the search of astrophysical line features in the spectrum possible.

The SIGMA telescope, the largest French scientific space-payload ever launched, is one of the main devices aboard the Soviet astronomy satellite GRANAT, successfully launched on December 1, 1989 from Baikonour, USSR. This high-energy space-telescope of unprecedented size, has been designed to produce high-resolution images of the hard X-ray and soft gamma-ray sky, in the energy range from 35 KeV to 1.3 MeV. After a comprehensive description of the instrument, a report is given on the most relevant characteristics of the telescope, including preliminary results from in-flight calibrations performed in the course of bright source observations.

Radioactivity induced by cosmic rays in the elements of space gamma-ray detectors was simulated under irradiation with 1.2 GeV protons. Radioactivity decay data from one minute to one month after irradiation were analyzed for Cd, Te, CSi, and BGO targets. In this paper we present the identification of unstable γ emitters isotopes activated in the samples, the temporal evolution of the 511 keV γ line and the calculation of some isotopes half life and yields of production through the evolution in time of their γ lines.

The present paper gives a preliminary description of the INTEGRAL Soft Gamma-Ray Imager (ISGRI), a CdTe layer, replacing the first CsI layer of the INTEGRAL IMAGER model payload proposed in phase A study. It provides a much lower threshold, around 20 keV instead of 80 keV, allowing a study of the hard X-ray part of the spectrum of any source within the entire field of view. In addition it offers significantly better spatial resolution, better point source location accuracy and improved imaging sensitivity. The sensitivity to partially-coded sources is significantly enhanced. The spectroscopic performance is also strongly improved allowing the search for low-energy nucleosynthesis lines (44Ti) as well as cyclotron lines.

The low energy gamma‐ray telescope SIGMA, aboard the GRANAT observatory, is now entering in his second year of life. It has provided during the last year, high angular resolution images of the sky in the energy domain 35 keV‐1.3 MeV. The data accumulated in different background conditions on series of astrophysical sites allow us to present the results on in‐flight performances of this instrument.

A scientific model of the wide-gap spark chamber which constitutes the central part of the Soviet-French high energy gamma-ray telescope “GAMMA 1” has been exposed to a tagged gamma-ray beam at DESY, Hamburg in 1976. The main purpose of these tests was to evaluate the angular resolution achievable with this type of detector at 100 MeV. The influence on the angular resolution of the spark-chamber plate material and thickness has been investigated. The methods currently used to reconstruct the photon direction have been compared. Using only the information recorded in the first spark-chamber gap following the photon materialization plate it was found that the three-dimensional angular resolution is about 2° at 100 MeV.

A gamma-ray telescope with a 850 cm2 multiplate spark chamber as a detector, has been used in an experiment of six balloon flights to investigate the Crab Nebula Pulsar 0532. The triggering signal is provided by a plastic scintillator-directional Čerenkov counter system. The spark chamber events are photographed by a stereo camera. The time of arrival of each event is recorded in UTC time with one millisecond accuracy, to search for a possible pulsed gamma emission from NP0532 above 50 MeV.

WE have investigated the gamma-ray emission from the Crab Nebula in a series of balloon flights of a multiplate spark chamber triggered by a Cerenkov detector-scintillator coincidence system1. The previously quoted gamma-ray energy threshold for pulsed emission from the Nebula and NP 0532 (refs. 1, 2) is ∼50 MeV; a detailed investigation of the telescope efficiency as a function of energy has now shown that the efficiency extends down to 20 MeV.

The ‘Gamma-1’ telescope has been developed through a collaboration of scientists in the USSR and France in order to conduct γ-ray astronomical observations within the energy range from 50 to 5000 MeV. The major characteristics of the telescope were established by Monte-Carlo simulations and calibrations made with the aid of electron and ‘tagged’ γ-ray beams produced by an accelerator, and these have been found to be as follows: the effective area for photons coming along the instrument's axis varies from about 50 cm2 at Eγ = 50 MeV to approximately 230 cm2 at Eγ ≥ 300 MeV; the angular resolution (half opening of the cone embracing 68% events) is equal to 2.7° at Eγ = 100 MeV, and 1.8° at Eγ = 300 MeV; the energy resolution (FWHM) varies from 70% to 35% as the energy of the detected photons increases from 100 to 550 MeV; the telescope's field-of-view at the half-sensitivity level is 300–450 square degrees depending upon the spectrum of the detected radiation, and the event selection logic. Proceeding from the thus obtained characteristics it is demonstrated that a point source producing a photon flux J (Eγ ≥ 100 MeV) = 3 × 10-7 cm-2 s-1, can be detected with a 5σ significance by observing it during 106 s at the level of the Cygnus background, and a source having intensity J (Eγ ≥ 100 MeV) = 10-6 cm-2 s-1 can be detected to within a mean square positional accuracy of about 15′.

The gamma-ray emission from the Crab Nebula and NP-0532 has been investigated in a series of balloon flights of a multiplate Spark Chamber triggered by a Cerenkov detector-scintillator coincidence system (Parlier et al., 1973; Leray et al., 1972; Vasseur et al., 1971), having a gamma-ray energy threshold for detection at 20 MeV. Figure 1 shows a schematic view of the gamma-ray telescope; Figure 2 the view of the telescope without its thermal protection.

The telescope ‘Gamma-1’ is designed to investigate cosmic gamma rays in the energy range from 50 MeV to 5000 MeV. The geometrical sensitive area of the telescope amounts to 1500 cm2, the angular resolution in each direction is equal to 1.2° at the energy 300 MeV and is about 20′ when including a coded mask in the telescope, the energy resolution changes from 70% at 100 MeV to 35% at 550 MeV. The characteristics of the telescope and its systems have been determined by the Monte-Carlo method as well as by accelerator calibrations. Discrete sources at the intensity level of 10−7 quanta cm−2 s−1 may be recorded in a year of observations with the gamma-ray telescope ‘Gamma-1’ with a source location accuracy of ≈ 10 arc min.

A SEARCH for pulsed gamma ray emission from NP 0532 has been carried out with a balloon-borne experiment using a spark chamber as the central detector. This pulsar, situated in the Crab Nebula, is already known to pulsate in the radio, optical and X-ray range1–5.