[en] As the Palais de la Decouverte (in Paris) is the sole scientific vulgarization establishment in the world to operate an actual particle accelerator able to provoke different types of nuclear reactions, the author recalls some historical aspects of the concerned department since the creation of the 'Radioactivity - Atom synthesis' department in 1937. He recalls the experiments which were then performed, the installation of the particle accelerator in 1964 and its renewal. He describes what's going on in this accelerator. He gives an overview of the difficulties faced after it has been decided to move it, of the works which had to be performed, and of radiation protection measures

[en] This article draws the story of the Wilson chamber. In the second half of the 19. century, a general trend among scientists in Great-Britain was still the observation and the imitation of nature, this romantic approach was launched by Goethe 50 years before. In 1895, Wilson as a young physicist tried to reproduce fog in laboratory. His challenge was to reduce the role of dust particles in the Aitken method: the expansion of wet-saturated air loaded with dust particles creates fog. In 1896, at the Cavendish laboratory, he discovered that ions could be very efficient 'dust particles' to condensate vapor and in 1898 he wrote that the expansion method is probably one of the most sensitive method to detect radiation emitted by radioactive materials. The Wilson chamber got a full success only from the twenties and was intensively used for 30 years in particles and nuclear physics laboratories. This detector allowed the experimental discovery of the positron, the muon and strange particles. (A.C.)

[en] One hundred years after Roentgen discovered X-rays, an American team performed the X-radiography of a finger by unrolling an adhesive tape in a high-vacuum chamber. In fact as early as 1930 a Russian team showed that light can be observed when an adhesive tape is quickly removed from a pane of glass. In the eighties it was shown that the snatching of electrons from an unrolling adhesive tape generate high voltages that trigger micro electrical discharges that raise the temperature of the air to 4000 Celsius degrees, radio waves are also emitted. (A.C.)

[en] The article begins with a biography of Pakistani physicist Abdus Salam who received the Nobel prize of physics in 1979 for his work concerning the unification between electromagnetic and weak nuclear interactions. Abdus Salam dedicated his whole career to the study of symmetry and invariance principles, he defined the basis of the supersymmetry concept. The second part of the article deals in a very pedagogical manner with symmetry breaking and renormalisation within the electroweak theory. (A.C.)

[en] This article draws the history of the understanding of radioactivity by the scientific community. The different speculations made by scientists to explain this new phenomenon are reviewed. We see how the study, the interpretation, the collecting of results from relevant experiments, helped physicists to build a consistent and global view of atomic structure and radioactivity. (A.C.)

[en] 100 years ago, Albert Einstein discovered that an excited atom can decay into a lower energy level by emitting a photon not only spontaneously but also if the atom is stimulated by a photon with an adequate energy. The photon is emitted in the same direction as the incoming photon, which makes an amplification effect that led to the discovery of lasers many years later. Einstein showed that stimulated emission was necessary to have the the blackbody's spectral density match the Planck law instead of the Wien law. The Wien law being valid at high frequencies, Einstein deduced that stimulated emission prevails over spontaneous emission in the domain of low frequencies. (A.C.)

[en] The stunning invention of the Wilson chamber in 1911 stayed almost ignored by the physicist community for more than a decade although it permitted the visualization of the track of an alpha particle as soon as 1911. The detection principle is based on the expansion of a saturated vapor that allows the visualization of a ionizing particle motion through the formation of droplets around the ions created all along the particle path. The original device was too slow (only one expansion per minute) to be used for the detection of rare events unless to operate it for months or years. A decisive step was made in the beginning of the twenties by the Japanese physicist Takeo Shimizu, working at the Cavendish Laboratory. He managed to automate the Wilson chamber and to equip it with 2 mirrors and a camera. This new generation allowed 5 expansion per second and the knowledge of the particle track orientation in space thanks to a simultaneous photography following 2 perpendicular directions. A rate of 4 shots a minute was achieved. The Wilson chamber was constantly improved during its 30 years long career and was only supplanted by the bubble chamber in the fifties. (A.C.)

[en] This article describes the saga of the neutrino, from the suggestion of its existence in 1931 by W.Pauli to the latest theories to explain the shortage of solar neutrinos via the experimental proof of its existence. The various nuclear reactions on which were based the different neutrino detection experiments performed since 1968 such as Homestake, Kamiokande, SAGE, Super-Kamiokande and Gallex are presented. In summer 1998 the Super-Kamiokande team announced to have observed a neutrino oscillation. The community of physicists is waiting for confirmation hoping that the recently-launched SNO (Sudbury neutrino observatory) and the future Kamland will definitely solve the riddle of solar neutrinos. (A.C.)