A Geiger tube is a sealed metal tube that contains argon gas at low pressure. Alpha and beta particles and gamma radiation enter the tube through a thin mica window. The tube wall is connected to the negative potential and there is a metal rod in the tube which is connected to the positive terminal. As ionising radiation passes through the tube ions are created and discharged at the electrodes. The ions create more ions so in a short time many ions are created. A voltage pulse across the circuit is recorded as a single count.The count rate is the number of counts per given time. The background radiation count rate needs to be subtracted from the count rate. If a sheet of material is placed between the Geiger counter and the source the absorption can be compared.