Johannes Wilhelm Geiger (Hans)
           (born: September 30, 1882 / died: September 24, 1945)
Hans Geiger, was born in Neustadt-an-der-Haardt,  Germany.  Geiger was awarded the Ph.D. by the University of Erlangen in 1906. Being one of the most valuable collaborators of Ernest Rutherford, Geiger worked in Manchester England with   Rutherford from 1906 to 1912. Eventually, in 1911, they devised the first version of the Geiger counter to count the number of alpha particles and other ionising radiation. With the aid of other radiation detectors, he used his counter in early experiments that led to the identification of the alpha particles as the nucleus of the helium atom. They also demonstrated that alpha-particles had two units of charge. It was also observed that occasionally alpha-particles are deflected through large angles when they strike a thin leaf of gold or silver. This scattering experiment was essential in leading to Rutherford's nuclear theory of the atom, made in 1912, that in any atom, the nucleus occupies a very small volume at the centre.
Many theories of radioactivity were also found and demonstrated by Geiger. In 1910, with Rutherford, they showed that two alpha-particles are emitted in the radioactive decay of uranium and in 1912, with J. M. Nuttal, they proved that this is caused by two uranium isotopes. The Geiger-Nuttall rule of 1911, states that the relationship is linear between the logarithm of the range of alpha-particles and the radioactive time constant, which is involved in the rate of decay of emitting nucleus. 
Geiger returned to Germany in 1914. During World War I, he served as an artillery officer in German Army. With Walther Bothe, Geiger devised the technique of coincidence counting and used it in 1924 to clarify the detail of the Compton effect. In the next year, at the University of Kiel, where he was offered a professional appointments, he and Walther Müller improved the sensitivity, performance, and durability of the particle counter which Geiger made before. Named the Geiger- Müller counter (picture shown on the left) in the present-day, the improved device detects not only alpha particles but other types of ionising radiation such as beta particles (electrons) and ionising electromagnetic photon.

Geiger also participated in Germany's abortive attempt to develop an atomic bomb during World War II. He died in Berlin on September 24, 1945.

    This is a typical geiger counter.  It has an ionizing tube with a high voltage potential supply.  Below are some typical geiger tubes.