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Geiger, Hans (Wilhelm)

particles alpha counter gas

[giy ger] (1882–1945) German physicist: invented a counter for charged nuclear particles.

Having studied electrical discharges through gases for his doctorate, Geiger moved from Erlangen in Germany to Manchester, where soon he began work under . Together they devised a counter for alpha-particles (1908), which consisted of a wire at high electric potential passing down the centre of a gas-filled tube. The charged alpha-particles cause the gas to ionize, and the gas briefly conducts a pulse of current which can be measured. They showed that alpha-particles have two units of charge, and Rutherford later established that they are helium nuclei. In 1909 Geiger and E Marsden (1889–1970) demonstrated that gold atoms in a gold leaf occasionally deflect alpha-particles through very large angles, and even directly back from the leaf. This observation led directly to Rutherford’s nuclear theory of the atom as like a small solar system rather than a solid sphere (1913). In 1910 Geiger and Rutherford found that two alpha-particles are emitted when uranium disintegrates. Work by Geiger and J M Nuttall (1890–1958) showed that there is a linear relation between the logarithm of the range of alpha-radiation and the radioactive time constant of the emitting atoms (the Geiger–Nuttall rule). Geiger took part in the identification of actinium-A (1910) and thorium-A (1911). Both are isotopes of element number 84, polonium.

Geiger served in the German artillery during the First World War. Following this he was head of the National Physicotechnical Institution in Berlin and in 1925 used his counter to confirm the effect by observing the scattered radiation and the recoil electron. Geiger became a professor at Kiel later that year and in 1928, together with W Müller, produced the modern form of the Geiger–Müller counter. In this, a metal tube acts as the negative cathode and contains argon at low pressure and a central wire anode. A window of thin mica or metal admits charged particles or ionizing radiation, which ionize the gas. The current pulse is amplified to operate a counter and produce an audible click. From 1936 he worked on cosmic rays, artificial radioactivity and nuclear fission. Geiger was ill during the Second World War, and died soon after losing his home and possessions in the Allied advance into Germany.

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