How did Hans Geiger discover the atomic nucleus?

Answer 1

Hans Geiger, by name of Johannes Wilhelm Geiger, was born in Neustadt-an-der-Haardt, German, on September 30, 1882. Being a German nuclear physicist, Geiger was the inventor of the Geiger counter which was a detector for radioactivity.

Geiger was awarded the Ph.D. by the University of Erlangen in 1906. Being one of the most valuable collaborator 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 thy 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.

Answer 2

Hans Geiger, along with Ernest Marsden, conducted the famous gold foil experiment under the guidance of Ernest Rutherford in 1909. They bombarded thin gold foil with alpha particles and observed that some particles scattered at large angles, suggesting a concentrated positive charge at the center of the atom, later identified as the atomic nucleus. This discovery led to the development of the nuclear model of the atom.

Answer 3

In the Geiger-Marsden gold foil experiment, a stream of alpha particles were fired at a sheet of gold foil, one atom thick. This setup was surrounded by a zinc sulphide screen which would scintillate if hit by an alpha particle.

At the time, the prevelant model of the atom was known as the 'plum pudding' model, which suggested that an atom was a sphere of uniform density with electrons randomly distributed within, like the raisins in a plum pudding. With this model the alpha particles should only have been deflected by a small amount at most.

When the experiment was carried out, most alpha particles streamed straight through the foil, as expected (the gaps between gold atoms were quite large and so alpha particles could fit through easily). Some were deflected through small angles and an even smaller number were deflected through angles of greater than 90 degrees. This was totally unexpected.

Ernest Rutherford did some calculations and worked out that the only way this could happen was if a large amount of the mass and charge of the atom was concentrated very highly in the centre of the atom (a nucleus) and surrounded by a lot of empty space and a few electrons.

Answer 4

Hans geiger invented a device that could detect alpha particles