The Overall Design

The SPI instrument is a coded mask spectrometer designed with an hexagonal geometry to maximize its compactness. An overall cut-out view of the instrument is given in Figure 1.

Figure 1: Left: A cut-away view of the SPI instrument. The mask, plastic scintillator, camera and ACS subsystems are highlighted. Right: view of the coded mask (top) and detector plane (bottom).

The detector plane of the instrument consists of 19 cooled, hexagonal shaped, high purity Germanium detectors, providing a total area of about 500 cm . In Figure 2 the detector numbering is shown. An Anti-Coincidence Shield (ACS) defines the instrument FoV by providing veto pulses for photons and particles coming from the sides. A plastic scintillator (PSAC), placed underneath the coded mask detects charged particles coming through the FoV and also provides veto signal. The sensitivity of the instrument is limited by the background due to the primary and secondary cosmic ray particles and the cosmic background radiation.

Each photon that is absorbed in a Germanium detector will give a pulse that is sent to the electronics. The electronics register the incoming pulses, reject those simultaneous with veto signals, and tags good photons with its energy, arrival time and type of event (see Section 3.4 for the discussion on the possible event types). These data are then sent to the ground. All photons detected by the detectors are summed into background spectra and are sent to the ground every 30 minutes.