Geiger-Muller tube. Illustration of a Geiger-Muller tube (blue) measuring radiation from a strontium-90 (Sr-90) beta radiation source (far left), with a scaler (bottom). An aluminium sheet is placed between the tube and the source. The Geiger-Muller tube is an instrument for detecting and measuring ionizing radiation. Strotium-90 emits beta particles, which are electrons. These cannot penetrate an aluminium sheet, and do not register on the scaler.