196 Pt has been investigated with numerous (n,y) techniques. The structure of the lowspin positive-parity states below the pairing gap shows excellent agreement with the predictions of the 0(6) limit of the interacting boson approximation model of Arima and Iachello.The pure harmonic vibrator and the quadrupoledeformed rotor have long provided two elegant nuclear-structure symmetries or limiting cases. Though few nuclei attain the idealized extremes, these limits are useful in part because their simple energy-level and branching-ratio predictions offer a framework from which deviations, and thereby the forces or interactions that produce them, are more easily identified. A class of nuclei exists toward the end of major shells for which neither limit is applicable. These nuclei are characterized, for example, by low-lying 2 2 + states and missing or much higher-lying excited 0 + levels. (The triaxial-rotor model has sometimes been invoked for such cases, but with varying success.) Our purpose here is to summarize a third limiting symmetry, recently proposed, 1 which may characterize such nuclei and, in particular, to propose that 196 Pt may be an excellent empirical manifestation of it.Recently Iachello and Arima have developed an interacting-boson approximation (IBA) 1 " 3 model in which the Hamiltonian is written in terms of interactions between bosons which can occupy L = 0 and L = 2 (s and d) states. This model can be phrased in the group theoretical language of SU(6) in terms of which three natural limits arise for which analytical solutions are obtainable. These limits correspond to three subgroups of SU(6), namely SU(5), 2 SU(3), 3 and 0(G). 1 ' 3 The first two correspond to an (anharmonic) vibrator and the quadrupole-deformed rotor (with degener-ate "23 + " and "2 y + " levels), respectively. Many examples of nuclei close to these two limits are well known. The third limit and its application to 196 Pt is the subject of this Letter.In the 0(6) limit the energies of collective states are given by 1
E(V,T,J)=\A(N-U){N + G + 4)where N is the number of bosons, defined as half the sum of the number of protons plus the number of neutrons away from the nearest respective closed shells (for 196 Pt, iV = 6); a=N,N-2,N -4,...,0, and r = 0,l,... ,o\ J takes on the values 2 X, 2X-2, 2X-3, ..., X + l, X, where X is a nonnegative integer defined by X = r -3v A for y A = 0,l,2,... . An example of a level scheme with iV = 6 is shown in Fig. 1. Each level can be uniquely identified by the quantum numbersThe wave functions of the collective levels may be expanded 1 " 3 in basis states characterized by their spin, rf-boson number n d , and the numbers of pairs and triplets of bosons coupled to spin zero. In this representation, states with identical J and r but different a are composed of identical nonvanishing basis states whose amplitudes are distributed in different (orthogonal) ways. States differing only in r consist of basis states differing inn d . Electromagnetic transitions follow the E2 selection rules 1 Acr = 0, A...
