An imaging telescope for soft gamma ray astronomy

Position sensitive multiwire proportional counters for X-ray astronomy

“…In order to produce images via spatial modulation with good efficiency, a coded mask is alternatively used to modulate the incoming beam, coupled with a position sensitive detector (Dicke, 1968;Carter et aL, 1981 ;Althouse et al, 1987;Grindlay et al, 1986). Use of masks based on Uniformly Redundant Arrays (URA) (Fenimore, 1978) leads to a very clean X-ray image with no side lobes or ghosting effects.…”

“…The detection plane is an array of 9 position sensitive NaI(T1) bars each of dimension of 5.8 cm x 55 cm (5 cm thick), surrounded by 13 anti-coincidence bars, having the same physical dimensions but without positional resolution, and shielded on top with a plastic scintillator. At a distance of 3.5 m from the detection plane is placed a 9 x 7 Hadamard mask, made out of tungsten elements with dimensions 6.7 cm x 7 cm x 2 cm thick, 90~o opaque at 1 MeV photons (Carter et aL, 1981;Villa et al, 1987). The basic mask pattern, four times replicated (twice in each direction) has the same area of the detector in order to have a full imaging of the sky in a field of view of 9 x 11 deg.…”

“…In order to produce position sensitivity in the direction along the wires, the process of current division is applied to the signals from either end of each resistive anode wire (Charpak et aL, 1963;Auriemma et aL, 1974). The position of interaction of a photon along the wire is given by X/L = (V~ -Vz)/(V 1 + Vz) (Equation (1)), where L is the length of the anode and 1/1 and 1/2 are the signals derived from the two ends of the wire.…”