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> The discovery of radioactivity Issue: 2003-3 Section: Science



Radioactivity is a phenomenon in which a nucleus of an unstable element, called radioactive, once disintegrated, emits subatomic particles and electromagnetic waves changing into a nucleus of another element of different chemical and physical properties.

The change takes the name of radioactive decadence and it can occur spontaneously (natural radioactivity), or through bombardment with radiations (artificial or induced radioactivity).

The discovery of the radioactivity was entirely casual ...

... in 1896 the physicist French Henri Becquerel was making some experiments on the fluorescence of Uranium salts exposed to light but, because of a long series of misty days, he was forced to interrupt them. Becquerel then put in a drawer some photographic plates, wound in black paper, together with a fragment of the mineral used for his research. After some days, developing the plates, he realised that they had veiled as if they had “taken light”. He tried to repeat the phenomenon with a virgin plate and he obtained the same result.

Becquerel understood, therefore, that Uranium was able to emit some bright radiations spontaneously (natural radioactivity), without being influenced by possible exposures to the solar light. Continuing to examine the phenomenon, the physicist also discovered that the Pechblend (a sort of mineral containing Uranium) had a great radioactive activity in comparison to its content of Uranium, and he deduced that there had to exist another element, able to impress the photographic plates.

Becquerel, subsequently, asked the two French chemists Marie and Pierre Curie, to seek the substance, which caused the phenomenon observed by him.

The couple, after some experiments, declared that the luminescence was due to the presence of two elements: Polonium, which, struck by solar rays and by rays of other nature, became luminescent, and Radium, which emitted radiations spontaneously because of the disintegration of some atoms.

It was 1898 when the discovery of Radium and Polonium entered history.

In a second time the radioactivity of Thorium was observed, of Actinium and of Radon. After these amazing results, in the following decades the phenomenon was widely studied by scientists all over the world.

Ernest Rutherford discovered in the radiations the presence of two components: α particles and β particles, the latter characterised by a penetrating power 100 times greater. In the following experiments, the radiation, exposed to electric and magnetic fields, revealed the existence of a third component of high energy: γ rays. It was shown then that these three types of radiation were constituted by nuclei of helium (α p.), by fast electrons (β p.) and by electromagnetic radiations (γ r.) of the same nature as X rays, but with a shorter wavelength.

It was noticed above all that radioactivity, unlike the greater part of physical and chemical phenomenona, was not influenced by changes of temperature, by pressure or by chemical reactions, but it is a peculiarity of atomic nuclei.

In 1911 Rutherford, after the experiments on the diffusion of α particles by metallic foils, conducted by H. Geiger and E. Marsden, supposed that the atom was composed of a central nucleus, in which the greatest part of the mass was concentrated, and of a certain number of electrons which orbited in space around the same nucleus.

Subsequently, in 1919, Rutherford discovered that the atomic nuclei could artificially be disintegrated through bombardment with α particles, thus transforming them in nuclei of different atoms. He succeeded in penetrating α particles, emitted by radioactive Polonium (Po), into a Nitrogen (N) atom, finding out that the latter emitted a proton, which struck against the fluorescent screen and provoked a very small spark of light.

The British physicist Sir James Chadwick, in 1932, ascertained that Beryllium, after being bombed with α particles, emitted a particle deprived of electric charge (with almost equal weight to that of a proton) which he called neutron, because it was electrically neutral.

In the same year, the English physicists Sir John Cockcroft and Ernest Walton discovered that a Lithium plate, struck with some high speed protons emitted α particles.

The French chemists Frèdèric Joliot and Irène Curie, in 1934, bombing Aluminium with α particles, succeeded in artificially creating the first radioactive substance: the Aluminium nuclei captured the particles and emitted neutrons changing into a Phosphorus isotope, which decayed emitting positrons soon after.

One year later Enrico Fermi verified that, through bombardment with neutrons, all the elements could be made artificially radioactive.

The result of this intense activity of research is the present knowledge of a lot of radioactive isotopes of various elements, which have become of vital importance in medicine, industry and agriculture.



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