Resonance phenomena in the scattering of α -particles by some light nuclei

The early experiments of Chadwick and Bieler (1921) and Rutherford and Chadwick (1927) showed that it was possible, in the case of light nuclei, to observe deviations from the classical-coulomb scattering laws by using theα -particles from radioactive sources. The departure from the classical scattering was found to be most marked at large energies, the ratio of the actual to the classical scattering appearing to increase very rapidly with energy for the most energetic particles used. Later experiments by Riezler (1932, 1935) extended these early observations, but also showed that in one case, the large angle (150°) scattering by carbon, the departure from classical scattering does not become uniformly greater for increasing energy of theα -particle, for largeα -particle energies, but shows a marked maximum at 5 MeV. This result was subsequently interpreted by Wentzel (1934) as a resonance phenomena. In the experiments of Riezler the resolution with respect to energy of the scattered particles was rather low. In addition the scattering was not observed at a sufficiently large and closely spaced series of energies to decide whether or not the ratio to classical scattering did increase continuously with increasingα -particle energy, for very fast particles, in cases other than carbon. In the case of carbon itself, the maximum in the scattering ratio was rather broad (about 1 MeV) and it was not possible in view of the low resolution of the experiments to decide the real width of the resonance maximum. Since the experiments described here were carried out, observations of the scattering ofα -particles in oxygen and neon have been published by Brubaker (1938). In both these cases resonance phenomena are in evidence, but even in these experiments the resolution with respect to energy was not very high and the spacing of the observations not as close as is desirable.