On the Earlier and More Recent History of the Neutrino
The continuous energy spectrum of beta rays, discovered by J. Chadwick in 1914,’ immediately raised difficult problems of theoretical interpretation. Was it to be ascribed directly to the primary electrons emitted by the radioactive nucleus, or to secondary processes? The first view, which turned out to be the correct one, was advocated by C. D. Ellis,2 the second by L. Meitner.3 The latter appealed to the fact, known from alpha and gamma rays, that nuclei possess discrete energy levels. She focused the discussion on the discrete energies of the electrons which are likewise observed in many beta-radioactive nuclei. Ellis was able to interpret these as electrons ejected from the outer shells by monochromatic nuclear gamma rays by internal conversion, and to relate them to the observed X-ray lines. According to L. Meitner’s theory however, at least one of the electrons of discrete energy was a genuine primary electron from the nucleus, which could then likewise eject other secondary electrons of lower energies from the outer shells.4 This postulated primary electron of discrete energy could however never be detected. Moreover there are beta-radioactive nuclei, such as RaE, which do not emit gamma rays and in which moreover the electrons of discrete energy are completely absent.
More difficult was the decision of the alternative S-, T- versus V-, A- interaction in beta decay. For a long time there was a hold-up here on account of wrongly evaluated recoil measurements in He6. The first correct indication in favour of the alternative (V, A) was given by the angular correlation between electron and neutrino in A35,50 determined by recoiled experiments. In agreement with this there was also the result of an elegant experiment by M. Goldhaber, L. Grodzins and A. W Sunyar." This experiment enables the screw direction of the emitted neutrino to be deduced directly from the sense of the circular polarization of the gamma rays emitted after capture of an electron from the inner atomic sheIls, by means of resonance scatter- ing of the gamma rays at daughter nuclei. The experiment with Eu 152 gave an L-neutrino. According to the results of the other experiments, already mentioned, this corresponds to the alternative (V, A). Further confirmation was soon forthcoming for this (e. g. by new recoil experiments on He6) so that it can now be regarded as weIl assured.