Introduction

The history of the phenomenological theory of neutrino interactions and weakinteractions in general, begins with the attempts to understand the physicsof nuclear radiation known as β-rays. These highlypenetrating and ionizing component of the radiation discovered by Becquerel[9] in 1896 were subsequently established to be electrons by doing manyexperiments in which their properties like charge, mass, and energy werestudied [8]. Since the energy of these β-rayelectrons was found to be in the range of a few MeV, they were believed tobe of nuclear origin in the light of the basic structure of the nucleusknown at that time [4]. It was assumed that the electrons are emitted in anuclear process called β-decay in which a nucleus inthe initial state goes to a final state by emitting an electron. The energydistribution of the β-ray electrons was found to becontinuous lying between me, the mass of theelectron, and a maximum energy Emax corresponding to theavailable energy in the nuclear β-decay, that is,Emax = Ei − Ef , where Ei andE f are the energies of the initial andfinal nuclear states. A typical continuous energy distribution for theelectrons from the β-decay of RaE is shown in Figure1.1 of Chapter 1. It was first thought that the electrons in theβ-decay process were emitted with a fixed energyEmax and suffered random losses in theirenergy due to secondary interactions with nuclear constituents as theytraveled through the nucleus before being observed leading to a continuousenergy distribution. However, the calorimetric heat measurements performedby Ellis et al. [15] and confirmed later by Meitner et al. [16] in theβ-decays of RaE, established that the electronsemitted in the process of the nuclear β-decay havean intrinsically continuous energy distribution. The continuous energydistribution of the electrons from the β-decay poseda difficult problem toward its theoretical interpretation in the context ofthe contemporary model of the nuclear structure and seemed to violate thelaw of conservation of energy.