Hans Geiger counts particles

The Geiger counter is fundamental to popular entertainment, especially in sci-fi films in which a scientist carrying a device the size of a shoebox waves a wand over some object and a crackling noise is heard. Everyone looks at one another in an ominous way: it’s (cue the startling music) radiation!

The device takes its name from its principal inventor, Johannes Wilhelm Geiger, born 30 September 1882 in Neustadt, Germany. His father, Wilhelm Ludwig Geiger, was a professor at the University of Erlangen, and young Hans earned a PhD there in 1906, studying physics and mathematics.

Geiger’s thesis was on the behaviour of gases, following on from the work of 19th century British physicist John Dalton, whose research into gases led to findings that became known as Dalton’s Atomic Theory.

The work earned Geiger a fellowship that brought him to Britain and the University of Manchester, where he joined a team of researchers led by New Zealand-born physicist Ernest Rutherford, winner of the 1908 Nobel prize for chemistry.

Rutherford was studying the findings of British scientist JJ Thomson, who in the late 1890s theorised that the atom, rather than being a solid piece of matter, was a bounded region of positive charge occupied by negative charges, which he believed to be electrons – a theory of atomic structure that became known as the “plum pudding” model.

Beginning in 1908, in what became known as “the gold foil experiments”, Rutherford set out to examine the effect of x-rays on various materials and the particles emitted by uranium and its compounds.

Working with Geiger and a student from New Zealand, Ernest Marsden, he sent beams of particles from a radioactive source onto and through a thin layer of gold foil and onto a zinc sulfide screen that would show a small flash of light when hit by a particle. They then had to try to count each flash of light given off by the screen.

To make it easier to observe the results of their experiments, Geiger built a device that could automatically detect individual alpha particles. This helped them identify the nucleus of the atom and led to Rutherford’s largely correct planetary model of the atom.

Geiger returned to Germany in 1912 as head of the National Institute of Science and Technology in Berlin, where he continued his studies on atomic structure and radiation counting until World War I intervened in 1914. He then served as an artillery officer in the German army.

Marc Shampo, writing in the December 2011 edition of the journal Mayo Clinic Proceedings, says Geiger took his first teaching position in 1925 at the University of Kiel, in northern Germany. He and his first PhD student, Walther Muller, improved the sensitivity, performance, and durability of Geiger’s counter, which became known as the “Geiger-Muller counter”.

“It could detect not only alpha particles but also beta particles (electrons) and ionising photons,” Shampo says.

An article in the 2007 edition of the US-based National Radiation Instrument Catalogue describes the workings of the Geiger-Muller counter. It “typically consists of a cylinder, called a Geiger tube, capped with a thin material such as mica through which radiation can penetrate. The cylinder is filled with inert gas and connected to a voltage source. Down the middle of the cylinder runs a wire or needle that functions as a positive electrode; the cylinder wall is a negative electrode. 

“When a particle of radiation penetrates the cap of the tube, it knocks a charged particle off a gas atom inside, thus ionizing it,” the article continues. “This displaced particle then heads, depending on its charge, either toward the positively charged wire or the negatively charged cylinder wall. This sets in motion an ionising avalanche: More and more particles get knocked off gas atoms as the first one rushes toward the electrode attracting it. This flurry of particles creates an electrical pulse when it hits the electrode, which is then amplified so it can be observed, usually as a clicking sound. The number of clicks generated per second by a Geiger counter indicates the intensity of the radiation it is measuring.”

Modern versions of the Geiger counter use the halogen tube invented in 1947 by Sidney Liebson, the article adds.

During World War II, Geiger had a role in Germany’s attempt to develop an atomic bomb but in June 1945 he fled Berlin and then to Potsdam, where he died on 24 September 1945.