System for mapping radioactive specimens

An apparatus for detecting radioactive sources on a test specimen, the apparatus comprising:

• a counter having a gas filled chamber, said chamber being formed by an electrically conductive planar window and a parallel semiconductive surface spaced from said window, and adapted for having an electric field imposed within said chamber by a voltage differential between said window and said semiconductive surface;
• an encoder surface spaced from said semiconductive surface and having geometrically arrayed elements thereon for receiving an electrical charge induced on said elements by an ion avalanche occurring within said chamber in response to entry of a radioactive particle into said chamber;
• a dielectric layer between said semiconductive surface and said encoder surface, said semiconductive surface forming a coating on one side of said layer and said arrayed elements forming a coating on the opposite side of said layer;
• means for coupling each said element to a row wire and to a column wire which are electrically isolated from each other whereby a preselected fraction of coupled charge current is transferred to each wire defining an element, the amplitude of the respective transferred charge fraction depending upon the location of the charge current relative to the element;
• a plurality of charge sensitive integrating amplifiers, one such amplifier being connected to each row wire, respectively, and one such amplifier being connected to each column wire, respectively;
• a plurality of sample and hold circuits, each of such circuits being connected to a respective one of said amplifiers for selectively sampling and holding an analog signal created by said induced electrical charge;
• means controlling said sample and hold circuits for selectively holding said analog signal when the summed output of all said sample and hold circuits exceeds a predefined threshhold level;
• and
• means for converting each said analog signal into a corresponding digital signal and for calculating the location of each corresponding induced electrical charge on said encoder surface based on the relative amplitude of each analog signal on respective row wires and column wires.