In , particles ionize gas atoms. The tube contains a gas at low pressure. At one end of the tube is a very thin ”window” through which charged particles or gamma rays pass. Inside the tube is a copper cylinder with a negative charge. A rigid wire with a positive charge runs down the center of this cylinder. The voltage across the wire and cylinder is kept just below the point at which a spontaneous discharge, or spark, occurs. When a charged particle or gamma ray enters the tube , it ionizes a gas atom between the copper cylinder and ....