The EIDOS gamma-ray Imager has been proposed in response to the INTEGRAL Announcement of Opportunity issued on the 1st July 1994. Through a combination of different detector technologies, it provides high-quality imaging of celestial

gamma-ray sources in the energy range between 20 keV and 10 MeV combined with high continuum sensitivity, high sensitivity to both narrow and broadened gammaray lines and good spectral resolution. The instrument also offers sensitive polarimetry of gamma-ray sources. The scientific objectives of this instrument are vast and range from studies of themost compact Galactic objects through to the structure of the Galaxy and on to active galactic nuclei. Careful attention has been taken to ensure that the design

of the EIDOS Imager is compatible with the mission requirement for a Galactic Plane Survey

Di Cocco, G.

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IL NUOVO CIMENTO VOL. 20 C, N. 5 Settembre-Ottobre 1997The Imager on INTEGRAL ()()G. DI COCCOfor of the INTEGRAL Imager CollaborationIstituto TESRE/CNR - via P. Gobetti 101, 40129 Bologna, Italy(ricevuto il 4 Aprile 1996; approvato il 24 Maggio 1996)Summary. — The EIDOS gamma-ray Imager has been proposed in response to theINTEGRAL Announcement of Opportunity issued on the 1st July 1994. Through acombination of different detector technologies, it provides high-quality imaging of ce-lestial gamma-ray sources in the energy range between 20 keV and 10 MeV combinedwith high continuum sensitivity, high sensitivity to both narrow and broadened gamma-ray lines and good spectral resolution. The instrument also offers sensitive polarimetryof gamma-ray sources. The scientific objectives of this instrument are vast and rangefrom studies of the most compact Galactic objects through to the structure of the Galaxyand on to active galactic nuclei. Careful attention has been taken to ensure that the de-sign of the EIDOS Imager is compatible with the mission requirement for a GalacticPlane Survey.PACS 96.40 – Cosmic rays.PACS 95.55.Ka – X- and -ray telescopes and instrumentation.PACS 98.70.Qy – X-ray sources.PACS 01.30.Cc – Conference proceedings.1. – IntroductionThe intimate relationship that -rays have with nuclear and particle processes pro-vides a direct means to explore the most energetic phenomena which occur in natureand enables some of the most fundamental questions of physics and astrophysics to beaddressed. Particle collisions, radioactivity, matter/antimatter annihilation and nuclearexcitation are all directly related to the production of -rays with unique and powerfullydiagnostic emission characteristics. Precise information is provided on the detailed geom-etry, energetics and physics of the underlying systems that power celestial sources. Un-like most other wavebands, -rays are not secondary reprocessed photons and are thus() Paper presented at the VII Cosmic Physics National Conference, Rimini, October 26-28, 1994.() The author of this paper has agreed to not receive the proofs for correction.G Societa` Italiana di Fisica 783784 G. DI COCCOable to probe these fundamental driving forces directly. This attribute is further aided bythe extreme penetrating power of the -ray photons.INTEGRAL consists of two main instruments operating in the -ray range, one opti-mized for imaging and the other for spectroscopy. The payload comprises also two mon-itors, one operating at X-ray energies and the other in the optical window. The overallINTEGRAL payload has been carefully designed to address the imaging, accurate posi-tioning and spectroscopy of celestial objects. As in most other wavebands, the full mea-surement capability of fine- and large-scale imaging together with fine and broad-bandspectroscopy is not technically possible within a single instrument. Two separate and co-aligned instruments, which may be regarded as effectively one telescope, are necessaryto provide the full diagnostic information. The Imager and Spectrometer have been con-ceived and designed as a fully complementary pair.EIDOS will provide the complement of fine imaging, source identification, polarime-try and maintains spectral sensitivity to broadened lines and the continuum. The Spec-trometer will provide narrow-line sensitivity and maintains an extended source imagingcapability. In conjunction with the Spectrometer, EIDOS will employ these powerful di-agnostic capabilities to a range of celestial objects of all classes ranging from the mostcompact galactic systems, through violent explosive events, to the structure of the Galaxyand on out to extragalactic objects and then cosmological implications. The major ad-vance in observing capability, when applied for the first time to the direct study of nuclearand particle processes within celestial objects, offers the chance of a new and radicallyaltered perception of the underlying physics. As in other cases when “new” astronom-ical windows have been opened onto the Universe, important discoveries will be made,fundamental questions can be answered.2. – Scientific objectives and capabilitiesThe scientific objectives of INTEGRAL include galactic and extragalactic astro-physics.The Galactic Centre and Bulge house some of the most active objects in the hard–X-ray and soft–-ray band, one of the few windows available for observations through theintervening gas and dust. EIDOS will be able to provide a high-quality continuum andspectral line measurement in a moderate exposure of 104 s, locating the emission comingfrom the various transient and persistent sources discovered by previous experiments,with a precision better than 1 arcmin (see table I).The study of compact objects with EIDOS is an important objective of the mission. Vir-tually all types of compact objects are significant sources of high-energy emission rangingfrom white dwarfs (Cataclysmic Variables) to neutron stars (isolated and in binary sys-tems) and black holes (transients). The fine imaging capability of EIDOS will ensureaccurate source location and identification and the high sensitivity will allow any broad-line spectral features, hard tails and the underlying continuum emission associated with aparticular source to be studied in great detail.The observation of the -ray lines produced by the nuclear transitions from the de-excitation of decay products and the annihilation of positrons produced in various nucleo-synthesis sites, such as supernovae (present and historical) is another important scientificobjective of EIDOS. For instance, the 44Ti mean life of 78 years is sufficiently long thatthe associated lines (0.068, 0.078 and 1.156 MeV) from recent supernovae will appear as“hot spots” when the Galactic Plane is imaged by EIDOS. The Galactic Plane constitutesTHE IMAGER ON INTEGRAL 785TABLE I. – Key EIDOS performance parameters.Energy range 20 keV–10 MeVDetection area 2500 cm2Spectral resolution (E=E FWHM) 6% at 100 keV, 6.5% at 1 MeVAngular resolution (FWHM) 160 at 100 keV, 190 at 1 MeVPoint-source location (20 source) 10Field of view 9 square degrees (fully coded)150 square degrees (1/2 coded)Continuum sensitivity 2.310 7 ph cm 2 s 1 keV 1 at 100 keV6.210 8 ph cm 2 s 1 keV 1 at 1 MeVLine sensitivity 4.110 6 ph cm 2 s 1 at 100 keV2.110 5 ph cm 2 s 1 at 1 MeV (broad line)Polarimetry sensitivity 10 mCrab,   1Timing accuracy 0.1 msthe core program of INTEGRAL which will aim to the study of the -ray emission in the26Al line at 1.8 MeV which the Imager will map at arcminute level.The capability of simultaneous detections of optical and -ray emission from -raybursts in the field of view of EIDOS, with the Optical Monitor, will allow the identificationof the bursts which fall in the partially coded field of view (several per year).The study of extragalactic objects (Seyfert, Quasars, BL Lacs, other galaxies, and clus-ters) is one of the most important scientific objectives of EIDOS. -rays carry away thedominant fraction of the emitted energy from active galactic nuclei, and the deep broad-band sensitivity level associated with the imaging capability of the Imager will increaseour knowledge of the mechanisms responsible for the emission, of the physical conditionof the objects, and of the identification of the sources, and will offer the possibility to de-termine the low-energy -ray luminosity function and the logN - logS, and to evaluate thecontribution of AGNs to the Cosmic Diffuse Background at these energies.3. – The ImagerEIDOS is based very closely on the extensive studies carried out for the Imager duringthe INTEGRAL Phase-A [1]. It employs a coded mask in conjuction with an activelyshielded, three-layer position-sensitive -ray detector.The detector is highly modular and consists of three planes (two of which are identi-cal) and each plane consists of 24 triangular modules. Within each module there are 120detector cells (in the front plane these are “hexacells”, each containing 6 distinct pieces ofCdTe). The modularity is important in making the development, construction, qualifica-tion and testing of a complex system praticable. The stack of three planes is surroundedby a BGO scintillator veto shield (also modular) which is operated in anticoincidence withthe CsI detector elements. The entrance aperture has a tantalum tube collimator to re-duce the effects of the diffuse -ray background.The coded mask is made from 1.5 cm thick tungsten elements sandwiched betweencarbon-fibre–reinforced plastic honeycomb panels. This provides >70% opacity even inthe difficult region between 2 and 3 MeV. The mask element size has been chosen to be15.67 mm (across flats) so that, with a distance of 3.25 m between the Imager detector andthe mask, the telescope will have an angular resolution of 190 FWHM, and by centroiding,786 G. DI COCCOFig. 1. – Continuum sensitivity of EIDOS compared with what achieved by other instruments work-ing in the same energy range.source positions may be located to typically 10. The size of the mask has been chosen toprovide a fully coded field of view of 9 square degrees and exposes 150 square degrees ofthe sky with more than 50% coding.4. – Detector performanceThe key performance parameters (detection efficiency, background noise, and sensi-tivity) of EIDOS have been estimated using a combination of Monte Carlo modelling andlaboratory tests.The use of CdTe and CsI ensures a detection efficiency always greater than 50%throughout the whole energy range from 20 keV up to 8 MeV. The energy resolution isaround 6% both at 100 keV (with CdTe) and at 1 MeV (with CsI). Particular care has beengiven to the minimization of background noise. Several techniques have been envisaged:the tantalum collimator and the reduction down to 2 mm of the top CdTe plane for energiesless than 200 keV; identification of multiple events attainable with the 3d discrete detec-tion element structure which also gives the possibility of reconstructing the kinematic ofthe Compton events.THE IMAGER ON INTEGRAL 787Figure 1 shows the continuum sensitivity curve of EIDOS compared to what has beenachieved by other instruments operating in the same energy range. It is important to notethe broad-band capability of EIDOS, which enables to cover the range between the low-energy limit of SIGMA up to the upper energy range of OSSE. The resulting sensitivityis a few times better than OSSE but with an angular resolution that is of the order of whatachieved by SIGMA which has a sensitivity a factor of 30 worse.REFERENCES[1] INTEGRAL COLLABORATION, Phase-A Study, INTEGRAL Report on the Phase-A Study,ESA SCI(93)1 (1993).

The Imager on INTEGRAL

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The Imager on INTEGRAL

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