Paul Ulrich Villard

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Paul Ulrich Villard
Paul Villard.jpg
Born(1860-09-28)28 September 1860
Died13 January 1934(1934-01-13) (aged 73)
Known forDiscoverer of Gamma Rays

Paul Ulrich Villard (28 September 1860 13 January 1934) was a French chemist and physicist. He discovered gamma rays in 1900 while studying the radiation emanating from radium.


Early research

Villard was born in Saint-Germain-au-Mont-d'Or, Rhône. He graduated from the École Normale Supérieure in 1881 and taught in several Lycées, ending with a Lycée in Montpellier. He would maintain a laboratory position at the Ecole Normale Supérieure until his retirement. At the time when he discovered what we now call gamma rays, Villard was working in the chemistry department of the École Normale Supérieure rue d'Ulm, Paris.

Villard is also credited with the discovery of argon hydrate. He spent the early part of his career (1888–1896) focusing on similar compounds at high pressure.

Discovery of gamma rays

Villard investigated the radiation from radium salts that escaped from a narrow aperture in a shielded container onto a photographic plate, through a thin layer of lead that was known to stop alpha rays. He was able to show that the remaining radiation consisted of a second and third type of rays. One of those was deflected by a magnetic field (as were the familiar "canal rays") and could be identified with Rutherford's beta rays. The last type was a very penetrating kind of radiation which had not been identified before.

Villard was a modest man and he did not suggest a specific name for the type of radiation he had discovered. In 1903, it was Ernest Rutherford who proposed to call Villard's rays gamma rays because they were far more penetrating than the alpha rays and beta rays which he himself had already differentiated and named (in 1899) on the basis of their respective penetrating powers. The name stuck.

Later work

Taking an X-ray image with early Crookes tube apparatus in 1896. The Crookes tube is visible in the centre. The standing man is viewing his hand with a fluoroscope screen. This was a shortcut method for setting up the tube.No precautions against radiation exposure are being taken. Crookes tube xray experiment.jpg
Taking an X-ray image with early Crookes tube apparatus in 1896. The Crookes tube is visible in the centre. The standing man is viewing his hand with a fluoroscope screen. This was a shortcut method for setting up the tube.No precautions against radiation exposure are being taken.

Villard spent much time perfecting safer and more accurate methods of radiation dosimetry, which had been done very crudely up until then (typically by evaluating the quality of the image of the experimenter's hand produced on a photographic plate). In 1908, Villard pioneered the use of an ionization chamber for the dosimetry of ionizing radiation. He defined a unit of kerma which was later renamed the roentgen. [1]

Retirement and death

When Villard retired, he left Paris. He died in Bayonne, France, on January 13th, 1934.

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Gamma ray Energetic electromagnetic radiation arising from radioactive decay of atomic nuclei

A gamma ray, also known as gamma radiation, is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves and so imparts the highest photon energy. Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900 while studying radiation emitted by radium. In 1903, Ernest Rutherford named this radiation gamma rays based on their relatively strong penetration of matter; in 1900 he had already named two less penetrating types of decay radiation alpha rays and beta rays in ascending order of penetrating power.

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or 4
indicating a helium ion with a +2 charge. Once the ion gains electrons from its environment, the alpha particle becomes a normal helium atom 4

Roentgen (unit)

The roentgen or röntgen is a legacy unit of measurement for the exposure of X-rays and gamma rays, and is defined as the electric charge freed by such radiation in a specified volume of air divided by the mass of that air . In 1928, it was adopted as the first international measurement quantity for ionising radiation to be defined for radiation protection, as it was then the most easily replicated method of measuring air ionization by using ion chambers. It is named after the German physicist Wilhelm Röntgen, who discovered X-rays.


  1. Clarke, R.H.; J. Valentin (2009). "The History of ICRP and the Evolution of its Policies" (PDF). Annals of the ICRP. ICRP Publication 109. 39 (1): 75–110. doi:10.1016/j.icrp.2009.07.009 . Retrieved 12 May 2012.