Domenico Pacini, uncredited pioneer of the discovery of cosmic rays A. de Angelis(1)

During a series of experiments performed between 1907 and 1911, the Italian physi-cist Domenico Pacini (Marino 1878 – Roma 1934), at that time researcher at the Central Bureau of Meteorology and Geodynamics in Roma, studied the origin of the radiation today called “cosmic rays”, the nature of which was unknown at that time. In his conclusive measurements in June 1911 at the Naval Academy in Livorno, and confirmed in Bracciano a couple of months later, Pacini, proposing a novel experi-mental technique, observed the radiation strength to decrease when going from the surface to a few meters underwater (both in the sea and in the lake), thus demon-strating that such radiation could not come from the Earth. Pacini’s conclusive experiment was performed, and the results published (in Italian), one year before the famous balloon experiment by Victor Hess, who found the ionization rate to increase with height. While Hess is today celebrated as the discoverer of cosmic rays, Pacini’s work was largely overlooked. Hess was awarded the Nobel Prize in Physics in 1936, two years after the death of Pacini, who had become a full pro-fessor of Experimental Physics at the University of Bari and the Director of th