Avalanche counter and encoder system for counting and mapping

Abstract

A parallel plate counter utilizes avalanche event counting over a large area with the ability to locate radioactive sources in two dimensions. One novel embodiment comprises a gas-filled chamber formed by a stretched stainless steel window cathode spaced from a flat semiconductive anode surface between which a high voltage is applied. When a beta ray, for example, enters the chamber, an ionization event occurs and the avalanche effect multiplies the event and results in charge collection on the anode surface for a limited period of time before the charge leaks away. An encoder system, comprising a symmetrical array of planar conductive surfaces separated from the anode by a dielectric material, couples charge currents the amplitude of which define the relative position of the ionization event. A number of preferred encoder system embodiments are disclosed including a novel matrix or grid pattern of electrical paths connected to voltage dividers and charge sensitive integrating amplifiers. The amplitude of coupled current delivered to the amplifiers defines the location of the event, the spatial resolution for a given signal-to-noise ratio can be controlled by changing the number of such amplifiers.