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Event Types

The photon entering the telescope can be detected due to its interaction with the absorbing material of the detector. Three major types of interactions play a dominant role: photoelectric absorption, Compton scattering and pair production. In the photoelectric absorption process a photon undergoes an interaction with an absorber atom in which the photon completely disappears. In its place an energetic photoelectron is ejected by the atom, carrying away most of the original photon energy. The Compton scattering takes place between the incident gamma-ray photon and an electron in the absorbing material. The incoming photon is deflected and it transfers a portion of its energy to the electron. The energy transferred to the electron can vary from zero to a large fraction of the initial gamma-ray energy. In the pair production process the gamma-ray photon disappears and is replaced by an electron-positron pair. The positron will annihilate in the absorbing medium and two annihilation photons are normally produced as secondary products of the interaction. Depending on the size of the detector and on the energy of the incoming photon, a photon scattered in a Compton interaction can escape the detector, or undergo a second interaction. The pairs of 511 keV photons, produced by the annihilation of the positrons resulting from pair creation, can also produce other interactions or escape the detector.

Both ISGRI and PICsIT record the coordinates of each event registered in the corresponding layer, to build up an image. The anticoincidence VETO is used to reject background events.

The coded mask produces a shadowgram. Photons from the source and the background are distributed across the entire field of view, but cross-correlation techniques allow the full image to be reconstituted for the fully coded field of view (9 9 ) at each pointing. For the partially coded field of view (out to 29 29 ), special cleaning techniques must be applied to the data to properly reconstruct the image. The actual sky coverage in an observation of course depends on the dither pattern.

The on-board electronics classify registered events according to the activated layer and the number of events detected by a submodule practically simultaneously. Events detected by different submodules are treated as independent ones. There are five main events type:

In Fig.9 the efficiencies of the various detection techniques is shown.

Figure 9: IBIS sensitivities for the various detection techniques.
Image umf3


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