In 1908 Hans Geiger, developed a technique for detecting alpha particles that would later be used in the Geiger-Müller tube. This counter was only capable of detecting alpha particles but in 1928 Geiger and Walther Müller developed the sealed Geiger-Müller tube which could detect more types of ionizing radiation and it became a practical radiation sensor. Once this was available, Geiger counter instruments could be produced relatively cheaply.