History of radioactivity

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It was in 1896, in the experiments of Henri Bacquerel, that the atom that he possessed a complex structure. It was during these experiments that the radioactivity of uranium salts was discovered. This discovery was marked by a whole series of fortuitous circumstances; but the history of science knows many cases where discoveries of great importance went unnoticed, quite simply because they exceeded the level of development of the natural sciences at the time, and that we did not have suitable means to grasp them. all the scope.
At the end of the 19th century, the problem of atomic structure was already on the agenda of chemistry, and therefore a fortuitous observation, among many others, could be detected as primordial, since it allowed to tackle this problem experimentally.
In any case, the radioactivity would have been discovered, because the development of experimental physics, the implementation in physico-chemical research of photographic techniques and spectral analysis could not fail sooner or later to highlight these natural sources of unusual radiation.
The merits of the first researchers of radioactivity H. Becquerel, P.Curie, M. Sklodovska-Curie and their collaborators are most important. they crossed the threshold of a new and mysterious world, of the world of unstable atoms, and they blazed a path which was to lead towards atomic energetics and towards deep theoretical overviews.
It is probable that the descriptions of the properties of the radiation discovered by Wilhem Roentgen aroused in Bacquerel a great interest in the phenomenon of fluorescense. Becquerel subjected to the action of solar radiation various salts, placed on a photographic plate carefully wrapped in a black paper impervious to solar rays. The purpose of these experiments was to determine whether fluorescent substances excited by light would not be able to emit rays similar to those discovered by Roentgen. Uranium salts showed that this was really so. However, once, by mistake, crystals of a uranium salt K2UO2 (SO4) 2 remained in contact for several days with a sealed envelope containing a photographic plate, sheltered from solar rays, the plate was however veiled. The obvious conclusion was that the uranium salts were the source of a penetrating ray, capable of acting on the photographic emulsion. An even more intense effect was observed in an experiment involving a uranium ore. In 1898 Schmidt discovered the radioactivity of thorium salts. During this same year, P.Curie and M.Sklodovska tackle the study of this strange phenomenon by experimenting with the pitchblende of Saint-Joachimsthal (Austria). Gradually separating the radioactive substances from the impurities with which they were mixed, the researchers noted that the intensity of the radiations emitted became all the stronger as the purification became more thorough.

  • In July 1898, they let it be known that they had discovered a new radioactive element. It was the Polonium Po, so named in honor of Poland, the homeland of Mr Sklodovska. In November of the same year, they resumed their work, setting out in search of yet another element which must have been particularly radioactive. This mysterious element allowed itself to be precipitated with barium sulphate, but the ore analyzed only contained traces of it. In December 1898, this new element was isolated and named radium Ra, it was very difficult to isolate it, because of its very low concentration. It was not until 1902 that we succeeded in determining with precision its atomic mass equal to 225.9
    The research carried out by Mr Sklodovska-Curie and his pupils subsequently made it possible to isolate two other elements, also radioactive, actinium and protactinium. The subsequent study of the radiation emitted by these elements showed that it is not homogeneous and consists of radiation and fluxes of alpha and beta particles.
    Beta rays are a stream of electrons whose speeds, close to that of light, are distributed fairly uniformly in a certain speed spectrum. In other words, in any electron beam emitted by a radioactive substance, we observe particles with different energy states, therefore fast particles and slow particles.

The electron was discovered and studied by J.J.Thomson in 1897 during the study of electric discharges in gases. The level of knowledge of physics was already in conformity, at that time, with the progress of knowledge concerning the internal structure of the atom.
Alpha rays are made up of electropositive particles with a mass four times that of the hydrogen atom. The charge of an alpha particle has a value that is twice that of an electron. Alpha particles move at a speed of around 20,000 km / s and strongly ionize the air. When the particles collide with molecules of oxygen or nitrogen, they cause electrons to shoot out, so that their journey is extended by a tail of charged molecules known as gas ions. A single alpha particle manages to produce along its trajectory nearly 200,000 pairs of gaseous ions and as a result it quickly loses its energy and is stopped, after having traveled in the air a path of the order of 7 to 20 cm. Even a sheet of paper can stop alpha particles. It is the alpha particles that take away most of the energy released in radioactive decays.

The study of the interaction of alpha particles with the nuclei of other atoms was made particularly easy by the invention of a very simple device, the Wilson chamber:

Wilson's chamber represents a cylindrical vessel inside which one can achieve at a given moment an atmosphere of supersaturated water vapor, the chamber is filled with water vapor at a saturation pressure, then a sudden expansion is produced: the temperature of the water vapor then drops and the vapor becomes supersaturated. When alpha particles or electrons pass through a volume of supersaturated water vapor, ionization of the water molecules or gas molecules present in the chamber occurs. The gas ions play the role of counter-condensation on which tiny droplets of water form. A charged particle leaves behind a hazy trail that can be easily photographed.

  • One of the most remarkable properties of alpha particles is their ability to transform into helium atoms. All it takes is for an alpha particle to capture two electrons which neutralize its double positive charge and transform it into a helium atom. We can therefore deduce that alpha particles are nothing other than nuclei of helium atoms.

Such a conclusion would suggest that the phenomenon of radioactivity would be reduced in short to a disintegration and a transmutation of atoms. So, the atom of radium, having an atomic mass of 226, emitting an alpha particle in the atomic mass is equal to 4, gives rise to helium and another element.
Analysis of the products of radioactive decay made it possible to show that the decay of Ra, U, Ac and Th is accompanied by the formation of many other radioactive elements. These elements form families in which each element is obtained from the one that precedes it in the row and gives rise to the one that follows it.
To be able to realize the nature of the differences in the internal structures of the atoms of innumerable decay products, it is necessary to understand the periodic system of the elements and to establish a relation between the physical properties of the atoms and their position. in Mendeleev's table.

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