Geiger-Muller tube. Illustration of a Geiger-Muller tube (blue) measuring radiation from a Co-60 (Cobalt-60 isotope) radiation source (far left), with a scaler (bottom). A block of lead is placed between the tube and the source. The Geiger-Muller tube is an instrument for detecting and measuring ionizing radiation. Cobalt-60 emits both beta particles (electrons) and gamma rays. Beta particles cannot pass through the lead block, and do not register on the scaler, but gamma rays are able to penetrate it and are detected.