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The Neutrino

Wolfgang Pauli's name is inseparable from his pioneering hypothesis of the existence of the neutrino, which was confirmed by experiment only after 25 years. The starting point for Pauli was the continuous energy spectrum of beta rays, which could not be interpreted theoretically. Niels Bohr attempted it with the hypothesis of the restricted validity of the principle of energy conservation, which Pauli could not accept because the principle of the conservation of energy had proved itself in all fields of physics and its proposition seemed to be plausible. In this critical situation Pauli hit on a desperate way out: he developed the idea that during beta decay, apart from the electron a further, but electrically neutral particle is emitted in such a way that the sum of the energies of both particles is constant.
On 4th December 1930, Pauli wrote his famous letter to the "Dear Radioactive Ladies and Gentlemen" who had gathered in Tübingen. In it, he sketched out his idea and inquired how things stood with the experimental proof. But he considered his idea to be too immature to be published.
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Solvay

Participants of the Congress of Solvay in Bruxelles, 1933
The Italian nuclear physicist Enrico Fermi took up Pauli's idea and on its basis developed a theory of beta decay. Fermi also coined the term "neutrino", after Pauli had spoken of "neutron", but the latter designation was reserved for the heavy component of the atomic nucleus discovered in 1932.
Not until October 1933 at the 7th Solvay Conference in Brussels did Pauli dare to present his hypothesis in public. It then took a further 23 years before the experimental proof of the existence of the neutrino succeeded.
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