A h c 4 r m d A stiirlv nf the I-ek in 1aEr I.)..______., i n i l i i l l w remnrA _ _ r I..__ nn in ... 1OR1 __ hsc hpen I __.._" m m d m s A dThe original neutron caplure data have been supplemented with dah from a new series of measurements taken with improved statislics, derived from cuNedcrystal measurements of selecled portions of lhe y-ray spectrum as well as from a measurement of y-rays in the range 13W keV < E, < 2300 keV using an intrinsic Ge spectrometer. These new data have led to the establishment of an impmved y-ray line lis1 which, in tum, has led lo more definitive mullipolarily information from exisling conversion electron data. Using thex data, employing the Ritz combinalion principle in conjunction wilh knowledge of the localion of levels in l@Er derived from a series at panicle transfer experiments, a comprehensive level scheme was constructed where 128 levels are arranged into 33 rotalional bands as well as four isolated bandheads. This detailed information a b u l the levels, their radiative decay, their spin-parity assignments and their grouping into rotational bands conslilutes an ideal experimenlally-ba& testing gmund againsl which the prediclions of theoretical models of low-lying collcctive acitations in heav nuclei can be compared. NUCLEAR REACTIONS 167Er(n.y)1"Er, E. lhermal; measured E.,* I.,. '@Er; deduced levels, radiative decay, transilion multipolarilies, J" values.Curved-cryslal spectameten, intrinsic Ge speclmmeter.

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...

The interacting boson approximation has been used to calculate the positive parity states of ' 'Er below 2 MeV. A simple parametrization has been used which corresponds to a description close to the SU(3) limit of the model. The predictions have been compared with new results available from a recent experimental study. Particular attention has been paid to the absolute magnitude of predicted B(E2) values in assessing the significance of discrepancies between theory and experiment. Estimates of absolute strengths have also been obtained for M 1 transitions in the scheme. The calculations correctly reproduce the complete sequence of E = 0+ and 2+ bands below the pairing gap and provide an excellent overall description of their decay properties. Most importantly, the predicted dominance of the decay from the P to the y band, over that to the ground state band, which represents a fundamental characteristic of the model in this region, is reflected by the experimental data. The reproduction of this feature from a band mixing approach in the framework of the geometrical description is also discussed, and the connection between the two descriptions is explored. NUCLEAR STRUCTURE Interacting boson approximation applied to ' 'Er. Calculated levels, B(E2), branching ratios. Single particle estimates of B(M1), B(E2). Comparison with experimental and bandmixing formalism in rotational model.

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