Abstract: A PET radiation detector includes a pair of amplifying gaps 3,21 to enhance the electron yield from a TMAE gas, BaF.sub.2 gamma detector, together with a gate electrode 17 to inhibit passage of charge to the detector electrodes and reverse passage of ions to the crystal. A further reverse-biased gap may be positioned adjacent the crystal to prevent charge build-up thereon. Shield electrodes 23,25 prevent gate switching signals causing spurious responses in the detector circuit.

Abstract: In a method of sensing the position of a neutral particle, the particle is received in one of two spaced parallel cathode arrays, each of which comprises a plurality of metal strips arranged adjacent and edge to edge, the strips in one array being orthogonal to the strips of the other array, and the metal of which the strips are formed being such that a neutral particle is converted to a fast electron which escapes from the cathode. Between the two cathode arrays is an anode array consisting of parallel wires held at a known electrical potential and surrounded by a gas. The fast electron is converted to an avalanche of electrons and positive ions; the presence of the ions induces an electrical charge in at least one strip of each cathode array; and the presence of the charges is sensed to determine an orthogonal position.