Event Types

The photon which entered 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 absorbing atom in which the photon completely disappears. In its place an energetic photo electron is ejected by the atom. The photo electron carries off 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. 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 this 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.

In SPI the detector array is used to recover the total energy of an incoming photon. This contributes to raise the full-energy peak and to reduce the Compton continuum. Unfortunately it also degrades spatial information as it is not possible, in general, to determine which detector was hit first.

All signals produced from the detection plane are also directed towards the Pulse Shape Discriminator (PSD) System (see more details in Section ).

All events detected by SPI are classified in accordance to the number of detectors that respond to an incoming photon. The event detected by only one detector is called a single event. Multiple events are detected by several detectors and the value of multiplicity corresponds to the number of the detectors hit.