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 .
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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 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
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.