Beta-Decay
The experiments which established the existence of naturally and artificially radioactive emissions of negative and positive electrons by atomic nuclei and the critical calorimetric experiments which showed that part of the energy released during a β-decay process escapes in some yet undetected form were completed before 1933. In that year there was introduced the currently accepted picture of the β-process, in which Pauli's neutrino hypothesis occupies a central position. The assumption is that in the process a nucleon is transformed from a proton into a. neutron (or vice versa) with the simultaneous creation of a, positron (or negatron) and an (anti) neutrino. The latter particle is hypothesized to be the carrier of the missing energy and the failure to detect it after its emission is ascribed to its having no charge, probably no magnetic moment, and only small non-electromagnetic interactions with other particles.
More satisfyingly direct observation of the neutrino was undertaken by Crane and Halpern (C4) and Allen (A5). These investigators attempted to observe the recoils of nuclei from neutrino emissions. Allen's work seems the most nearly conclusive. He made his observations on Be', a nucleus suggested by Kan Chang Wang (K5) and many others since it is a light nucleus undergoing K capture and therefore having no electron emitted together with the neutrino. Allen found recoils somewhat too energetic to be due to the known y-ray and moreover could find no y-rays coincident with the recoils. Unfortunately, the quantitative aspects of the method are subject to relatively large corrections. Thus the present status of the neutrino can be summed up as follows: Although the detection of an individual neutrino has perhaps not yet been carried out in a completely decisive bway, the neutrino hypothesis seems to be the only one which can correlate the known facts, and it has probably done this, through the Fermi theory, completely enough to justify the assumption. A recounting of the successes and of the present limitations of the Fermi theory will occupy most of this review.