A coded mask.
A position detector.
Active and passive shielding.
Operating subsystems : memory,
computer, electronics, etc.
The main elements of SIGMA telescope are :
A coded mask.
A position detector.
Active and passive shielding.
Operating subsystems : memory,
computer, electronics, etc.
Coded mask
The coded mask is a
rectangle of absorbing elements of tungsten settled in a
pseudo-random pattern determined by a mathematical procedure. It permits to
reconstruct images of the sky.
The mask elements are 1.5 cm
thick and their size is 9.5 x 9.5 mm. The dimensions of the coded mask are 27.8
x 29.7 cm. The coded mask is held in place at a distance of 2.5 m above the
position detector by an aluminium tube.
Position detector
The position detector is
based on the Anger scintillation gamma camera principle used in nuclear
medicine.
It consists of a fragile sodium
iodide crystal (1.25 cm thick and 57 cm diameter) mounted on an optical glass
disk and analysed by 61 hexagonal photomultiplier tubes. The active surface
area of the position detector is 794 square centimeters.
Electronics associated with the
detector gives the position of the interaction point of a gamma photon with the
sensitive sodium iodide crystal and its energy.
Active shielding
The active shielding
eliminates the background noise induced by the charged particles. It consists
of 31 cesium iodide crystals analysed by 62 photomultiplier tubes, arrayed
around the detector.
The principle is the one of the
anticoïncidence : an event is ignored when it is detected simultaneously by
both the active shielding and the position detector.
The active cesium iodide
anticoïncidence shield surrounding the position detector is also used to
detect gamma-ray bursts.
Passive shielding
The background noise is also
eliminated by limiting the field of view of the telescope with passive
shielding, which is constituated of an association of three metals (0.5 mm
lead, 0.1 mm tantalum and 0.4 mm tin).
The passive shielding is wrapped
around the aluminium tube and reduces the flux of photons coming from all other
directions in the sky outside the field of view of the telescope.
A plastic scintillator is placed
in the telescope aperture to eliminate the charged particles without adversely
affecting the indispensable transparency to those photons coming from sources in
the field of view.
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