When the Rome group reached uranium, they had a problem: the radioactivity of natural uranium was almost as great as that of their neutron source. What they observed was a complex mixture of half-lives. Following the displacement law, they checked for the presence of lead, bismuth, radium, actinium, thorium and protactinium (skipping the elements whose chemical properties were unknown), and (correctly) found no indication of any of them. Fermi noted three types of reactions were caused by neutron irradiation: emission of an alpha particle (n, α); proton emission (n, p); and gamma emission (n, γ). Invariably, the new isotopes decayed by beta emission, which caused elements to move up the periodic table.