The
discovery of artificial radioactivity in 1934 was made within the laboratoire
Curie of the Institut du Radium thanks to the research of Irène and Frédéric
Joliot-Curie. The Musée Curie possesses in its collections the reconstructions
of the material used by the couple of scientists during their research on the
chemical separation of the first artificial radionuclides: a Geiger-Müller
counter and some very special glassware. All of these objects are presented in
the C12 showcase of the museum.
The Geiger-Müller counter
The Geiger-Müller counter is a measuring device for detecting ionizing radiations, of which radioactivity is a part. Its principle was invented in 1908 by E. Rutherford and H. Geiger, but it was not until 1928 that Walther Müller improved it and made it fully functional. It was therefore a relatively recent instrument when the Joliot couple began to use it in 1932-1933.
Its principle: a cylindrical metal tube, filled with gas under low pressure, acts as a chamber in which a wire is stretched. A voltage of about 1000 volts is established between the wire and the wall of the tube. When radiation enters the chamber, it ionizes the gas by tearing away some electrons. These are attracted by the wire, which produces electrical pulses. These are attracted by the wire, which produces electrical pulses in return. They are amplified and then counted by the experimenter with a visual signal (a digit or a needle) or a sound (in the more modern counters).
Extremely sensitive, this kind of device is capable of recording the passage of a single ionizing particle! Unfortunately, it can not discriminate the different types of radiation. The museum’s copy is made up of a transistor amplifier, a brass Geiger-Müller tube and a mechanical numerator that displays the number of “tops” received by the Geiger tube.
This is a reconstitution that is not identical to the original, Frédéric Joliot having donated it to the Science Museum in London. Materials, components and dimensions are not fully respected. It is in fact an object created in 1964 by Pierre Savel for the 30 years of the discovery, allowing redoing the manipulation in the presence of public.
The chemical separation of the first artificial
radionuclides
Although physicists of formation, the couple of scientists had to quickly imagine chemical methods in order to prove the existence of radioactive elements not existing in nature but created by men. Two in particular will focus their attention: radiophosphorus and radionitrogen. The two glass instruments presented in the showcase are replicas of those they used in January and February 1934. The firt glassware allowed them to show that radiophosphorus was formed when an aluminium foil was bombarded with α rays.
Indeed, a piece of aluminum (Al) previously irradiated by the α-rays of a Polonium source is placed in the tube where it is attacked and dissolved by a solution of hydrochloric acid (HCl).
The hydrogen produced during the reaction transforms the new formed radioactive phosphorus into a volatile element of formula PH3. The active gas is collected over a water tank in a thin-walled tube.
By approaching this tube of the Geiger counter, it is possible to detect the emission of β+-rays, proof of the presence of radioactive phosphorus that did not originally exist.
This also allows them to establish that its half-life is 3 minutes and 15 seconds. The other glassware was used to detect the radioazote created in boron. A sample of boron nitride (BN) irradiated by the α-rays of Polonium is placed in the left tube containing a little soda, then heated.
The nitrogen N is released and is collected in the right tube, placed in a Dewar vase. The Geiger counter serves again to show the existence of a newly created radioactive nitrogen. Its half-life is 14 minutes.
These chemical means developed by the Joliot-Curie have provided irrefutable proof both of the existence of the natural transmutation as discovered by Ernest Rutherford in 1919, but also that of the so-called artificial radioactivity.
The main difficulty to circumvent was the little “lifetime” of these elements: the chemical method to isolate the radioazote thus took no more than6 minutes; for the radiophosphorus, less than 3! It was this work in particular that earned them the Chemistry Nobel Prize in 1935.
Conception : Musée Curie
Photographies 2016 : Xavier Reverdy-Théveniaud
Photogrpahies anciennes : Musée Curie ; col. ACJC
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