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Publications and source records attributed to Alberto De Gregorio.
When the Moon is at quarter, its terminator generally does not appear to be orthogonal to a straight line connecting the Moon with the Sun. A purely geometric, intuitive interpretation of this at first sight puzzling observation is suggested here.
We go through Bohr's talk about complementary features of quantum theory at the Volta Conference in September 1927, by collating a manuscript that Bohr wrote in Como with the unpublished stenographic report of his talk. We conclude - also with the help of some unpublished letters - that Bohr gave a very concise speech in September. The formulation of his ideas became fully developed only between the fifth Solvay Conference, in Brussels in October, and early 1928. The unpublished stenographic reports of the Solvay Conference suggest that we reconsider the role that discussions with his colleagues possibly had on Bohr's final presentation of the complementary sides of atomic physics in his 1928 papers.
We outline Fermi's early attitude towards old quantum physics. We sketch out the context from which his interest for quantum physics arose, and we deal with his work on quantum statistics. We also go through the first two courses on theoretical physics he held in Rome, and his 1928 book on atomic physics.
The coming into light of the neutron is discussed. It is remarked that some experiments had already suggested that the penetrating radiation from beryllium had an electromagnetic component, before Joliot-Curies suggested the beryllium radiation consisted of high-energy gamma rays. Joliot-Curies' idea also relied on the many Compton electron tracks in the Wilson chamber. In 1920, both Harkins and Rutherford foresaw the existence of a particle of mass 1 and zero charge, consisting of one proton and one electron, though they did not give any name to it. In 1932, the neutron was finally observed by Chadwick. However, the question whether considering the neutron as a fundamental or a compound particle still bothered the physicists and Chadwick himself for more than one year. Instead, as soon as Majorana heard about Joliot-Curies' experiments he guessed the existence of a "neutral proton." In 1933 he published some "corrections" to Heisenberg's theory. A very simple analysis shows that Heisenberg's original version of exchange interactions and Majorana's version clearly reflect, respectively, that the nucleus contained "electrons" and that the nucleus was free from electrons. The emission of electrons from the nucleus still needed to be accounted for. Fermi's theory of beta-decay settled the question. Thanks to exchange interactions and beta-decay theory, quantum mechanics could now be applied to the nucleus, which could be considered free from electrons. Also the neutron could be considered free from electrons, and treated as a fundamental particle.
Dawning neutron physics was more complex than one might expect. The chance that the neutron comprised a proton and an electron was diffusely taken into account after the discovery of the neutron. Moreover, uncertainties persisted about the composition of beryllium radiation until it was realized that the latter comprised both neutrons and gamma-rays. The interaction of neutrons with matter and nuclei was soon investigated. Both a spatial symmetry, a symmetry of charge, and a symmetry in the nuclear reactions soon emerged. The relation of negative beta-decay to the neutron abundance in nuclei was moreover reviewed. Positive beta-radioactivity induced by alpha-particles was eventually announced, having been foreseen some weeks before. Accelerated deutons and protons shortly afterwards revealed to be efficient in inducing radioactivity. The physics institute in Rome got ready to start research on neutrons, but apparently it only planned to go through alpha-induced radioactivity, at first. If so, it is then plausible that some new results achieved by foreign laboratories eventually bent Fermi to neutrons. Fermi's discovery of neutron-induced radioactivity is reviewed with regard to investigations then current, once more showing simplicity as a distinctive trait of Fermi's way of doing physics.
When Enrico Fermi discovered slow neutrons, he accounted for their great efficiency in inducing radioactivity by merely mentioning the well-known scattering cross-section between neutrons and protons. He did not refer to capture cross-section, at that early stage. It is put forward that a thermodynamic approach to neutron-proton radiative capture then widely debated might underlie his early accounts. Fermi had already met with a similar approach, and repeatedly used it.
This paper deals with the Physics Institute of via Panisperna in Rome, getting ready for investigation on neutron physics before Fermi's discovery of neutron-induced radioactivity. The importance of nuclear research had been acknowledged in the Physics Institute in Rome since 1929. The Institute had been directed towards nuclear physics since then, but from the experimental point of view, still in 1933, it was not yet engaged in nuclear experiments on account of the lack of adequate supplies. An adjustment of the equipment and supplies was undertaken, so that strong radioactive sources, Geiger-Mueller counters and Wilson chambers were finally available at the end of 1933, thanks largely to Rasetti's efforts.
At the "Domus Galilaeana" in Pisa, many original documents and records are kept, which belong to the scientific activity carried out by Enrico Fermi until 1938. I compared those documentary sources with the supported evidences, the personal recollections, concerning the discovery that hydrogenated substances increase the radioactivity induced by neutrons: such a comparison leads to the conclusion that the discovery occurred on October 20th 1934, i.e., two days before the date that all the accounts that have been supported so far report. That suggests that any historical study to come, and regarding the experiments carried out by Fermi and his group on neutrons, cannot neglect to analyse carefully the accounts regarding those experiments and to compare them with the archive records.
In October 1934 Fermi discovered that neutrons became particularly effective in rendering the elements radioactive after being slowed down by hydrogenous substances. His was described as an absolutely unpredictable scientific discovery. In this paper I report and discuss the knowledge about neutrons properties already available the previous year, which throws a new light onto a discovery too often depicted as "accidental".