1. Apparatus for counting radiation damage tracks in sheet material, comprising a base of electrical insulating material, a first electrode plate supported by said base and exposed on one side for contact with substantially the whole of an exposed side of a sheet having electrical discharge paths formed through it along heavy energy particle produced damage tracks, a second electrode for contact with a portion of an electrical discharge puncturable metal layer on a covered side of said sheet, an electrical resistance connected to said first electrode to increase an electrical charging period between said first electrode for sequential electrical discharging through each of said damage tracks, and an electrical impulse counter connected to said electrodes for counting electrical discharges through said damage tracks.

5. A method of counting radiation damage tracks in sheet material, comprising placing the sheet material, having damage tracks therein forming electrical discharge paths and a metal layer on one side between a first eLectrode in contact with the exposed side of said sheet and a second electrode in contact with said metal layer, impressing a voltage on the said electrode to cause electrical discharges sequentially through the material and generally along the radiation tracks, and counting the number of discharges, said metal layer being such that each electrical discharge taking place along a damage track punctures the metal layer at that damage track so that a second discharge will not occur along that damage track.