Introduction

The interacting boson model-1 originated from early ideas of Feshbach and Iachello (Iachello, 1969; Feshbach and Iachello, 1973, 1974), who in 1969 described some properties of light nuclei in terms of interacting bosons, and from the work of Janssen, Jolos and Dönau (1974), who in 1974 suggested a description of collective quadrupole states in nuclei in terms of a SU(6) group. The latter description was subsequently cast into a different mathematical form by us (Arima and Iachello, 1975) with the introduction of an s-boson, which made the SU(6), or rather U(6), structure more apparent. The success of this phenomenological approach to the structure of nuclei has led to major developments in the understanding of nuclear structure.

The major new development was the realization that the bosons could be interpreted as nucleon pairs (Arima et al., 1977) in much the same way as Cooper pairs in the electron gas (Cooper, 1956). This provided a framework for a microscopic description of collective quadrupole states in nuclei and stimulated a large number of theoretical investigations. An immediate consequence of this interpretation was that, since one expected both neutron and proton pairs, one was led to consider a model with two types of bosons, proton bosons and neutron bosons. In order to make the distinction between proton and neutron bosons more apparent, the resulting model was called the interacting boson model-2, while the original version retained the name of interacting boson model-1.