Introduction

The interacting boson model-2 originated from the necessity to give to the collective nuclear states, as described by the interacting boson model-1, a microscopic foundation rooted in the spherical shell model (Mayer, 1949; Haxel, Jensen and Suess, 1949). It was originally introduced by Otsuka, Talmi and ourselves (Arima et al., 1977; Otsuka et al., 1978) following earlier ideas of Talmi, who had previously introduced the concept of generalized fermion seniority on which the interacting boson modei-2 is based (Talmi, 1971; Shlomo and Talmi, 1972; Talmi, 1973).

In the interacting boson model-2, the bosons have a direct physical interpretation as correlated pairs of particles with JP = 0+ and JP = 2+. This physical interpretation yields, on the one hand, the possibility of constructing a microscopic theory of collective states based on the spherical shell model (to be discussed in a subsequent book) and, on the other hand, a richer algebraic structure which can be exploited to study nuclear properties. In Part II of this book we describe this richer algebraic structure, which has produced a large number of interesting results, the most notable being the discovery of low-lying collective modes in which protons and neutrons move out of phase (Bohle et al., 1984).

Since, contrary to the case of the interacting boson model-1, all operators here have a direct microscopic counterpart, we shall occasionally point out this connection and use it as input in the choice of parameters.