<P> Observations of the energy levels of molecules by Franco Rasetti in 1929 were inconsistent with the nuclear spin expected from proton--electron hypothesis . Molecular Raman spectroscopy of dinitrogen (N) showed that transitions originating from even - numbered rotational levels are more intense than those from odd levels, hence the even levels are more populated . According to quantum mechanics and the Pauli exclusion principle, the spin of the N nucleus is therefore an integer multiple of ħ (the reduced Planck constant). Yet both protons and electrons carry an intrinsic spin of 1⁄2 ħ, and there is no way to arrange an odd number (14 protons + 7 electrons = 21) of spins ± 1⁄2 ħ to give a spin that is an integer multiple of ħ . </P> <P> The Klein paradox, discovered by Oskar Klein in 1928, presented further quantum mechanical objections to the notion of an electron confined within a nucleus . Derived from the Dirac equation, this clear and precise paradox showed that a high - energy electron approaching a potential barrier has a high probability of passing through the barrier, or escaping, by transforming to a particle of negative mass . Apparently, an electron could not be confined within a nucleus by any potential well . The meaning of this paradox was intensely debated at the time . </P> <P> By about 1930 it was generally recognized that it was difficult to reconcile the proton--electron model for nuclei with the Heisenberg uncertainty relation of quantum mechanics . This relation, Δx ⋅ Δp ≥ 1⁄2ħ, implies that an electron confined to a region the size of an atomic nucleus has an expected kinetic energy of 10--100 MeV . This energy is larger than the observed energy of beta particles emitted from the nucleus . The expected electron energy is also larger than the binding energy of nucleons, which Aston and others had shown to be less than 9 MeV per nucleon . </P> <P> While all these considerations did not "prove" an electron could not exist in the nucleus, they were challenging for physicists to interpret . In his 1931 textbook, Gamow summarized all these contradictions . Adding to the confusion in interpretation, the continuous energy distribution of beta decay electrons seemed to indicate that energy was not conserved by this "nuclear electrons" process . Indeed, Bohr, Gamow, Heisenberg and others considered the possibility that the laws of quantum mechanics were not applicable inside the nucleus . Such considerations were apparently reasonable, inasmuch as the laws of quantum mechanics had so recently overturned the laws of classical mechanics . The contradictions were mysterious and troublesome, until it was realized that there are no electrons in the nucleus . </P>

Who provided evidence that atoms contain subatomic particles