[en] Some examples of new phenomena expected at extreme values of neutron and proton number are discussed, including soft dipole modes and neutron-proton pairing. First results on the N = Z nucleus ⁶²Ga are presented in the context of the competition between T = 0 and 1 states in odd-odd, self-conjugate nuclei

[en] The characteristics of the SU(3) and 0(6) boson-fermion symmetries stemming from a U(6/12) group structure are discussed and compared with recent results of (n,γ) studies in the W-Pt region. The nuclei ¹⁸⁵W and ¹⁹⁵Pt are shown to represent the best empirical examples of the SU(3) and O(6) limits, respectively, and it is also shown that the consistent Q formalism can be extended to the odd A Hamiltonian to describe the transition between these two limits. Preliminary comparisons with the low lying structure of the odd Os nuclei are presented. The question of the empirical evidence for supersymmetry in this region is also discussed. 23 references

[en] The concept of symmetry in the Interacting Boson Model (IBM) description of even-even nuclei has proved to be one of the model's most important elements, because they provide benchmarks in the formulation of a unified description of a broad range of nuclei. The importance of the recently proposed symmetries in odd-even systems can thus be viewed in the same light, and their role in pointing to a simple prescription for the changing collective structure in odd A nuclei throughout a major shell is likely to prove even more essential, given the much greater complexity of the boson-fermion (IBFM) Hamiltonian. The group structure of a boson-fermion system is described by U/sup B/(6) x U/sup F/(m) where m specifies the number of states available to the odd fermion, and thus depends on the single particle space assumed. The ability to construct group chains corresponding to the symmetries SU(5), SU(3) or 0(6) depends on the value of m. Of the structures studied in detail to date, the case of m = 12 is the one with the broadest potential. The fermion is allowed to occupy orbits with j = 1/2, 3/2 and 5/2, so that the assumed single particle space corresponds to the negative parity states available to an odd neutron at the end of the N = 82-126 shell, namely, P/sub 1/2/, p/sub 3/2/ and f/sub 5/2/. The region of interest thus spans the W-Pt nuclei, and since one prerequisite for an odd-A symmetry is the existence of that same symmetry in the neighboring even-even core nucleus, the odd Pt nuclei around A = 196 offer the obvious testing ground for the 0(6) limit of U(6/12). The heavier even-even W nuclei, on the other hand, have the characteristics of an axial rotor, and hence the negative parity structure of the neighboring odd W isotopes offers the possibility to study the validity of the SU(3) limit. Given a definition and understanding of these two limits, the construction of a simple description of the transitional Os nuclei can be considered

[en] It is shown that the Interacting Boson Approximation can produce essentially identical results for M1 transitions in deformed even-even nuclei as can be obtained from the geometrical approach, and many comparisons with experiment are already available for the latter in the literature. Since no physical constants are involved in the IBA-1 formalism, it would be useful to use the similarity of the two approaches to obtain a physical description of the IBA constants. Use of the IBA-2 approach might also prove illuminating, since it should identify the contributions of the neutron and proton degrees of freedom, analogous to the method of Greiner

[en] An ARC (Average Resonance Capture) study of ¹⁸⁵W has been performed in order to identify the complete set of 1/2^-, 3/2^- levels below 1.5 MeV. The results have been compared to the Nilsson model and also to the SU(3) limit of the U(6/12) boson-fermion symmetry. Consideration of the level scheme of ¹⁸⁴W tests the evidence for supersymmetry in the tungsten nuclei

[en] The structure and characteristic properties and predictions of the IBA in deformed nuclei are reviewed, and compared with experiment, in particular for ¹⁶⁸Er. Overall, excellent agreement, with a minimum of free parameters (in effect, two, neglecting scale factors on energy differences), was obtained. A particularly surprising, and unavoidable, prediction is that of strong β → γ transitions, a feature characteristically absent in the geometrical model, but manifest empirically. Some discrepancies were also noted, principally for the K=4 excitation, and the detailed magnitudes of some specific B(E2) values. Considerable attention is paid to analyzing the structure of the IBA states and their relation to geometric models. The bandmixing formalism was studied to interpret both the aforementioned discrepancies and the origin of the β → γ transitions. The IBA states, extremely complex in the usual SU(5) basis, are transformed to the SU(3) basis, as is the interaction Hamiltonian. The IBA wave functions appear with much simplified structure in this way as does the structure of the associated B(E2) values. The nature of the symmetry breaking of SU(3) for actual deformed nuclei is seen to be predominantly ΔK=0 mixing. A modified, and more consistent, formalism for the IBA-1 is introduced which is simpler, has fewer free parameters (in effect, one, neglecting scale factors on energy differences), is in at least as good agreement with experiment as the earlier formalism, contains a special case of the 0(6) limit which corresponds to that known empirically, and appears to have a close relationship to the IBA-2. The new formalism facilitates the construction of contour plots of various observables (e.g., energy or B(E2) ratios) as functions of N and chi_Q which allow the parameter-free discussion of qualitative trajectories or systematics

[en] Angular correlations in ¹⁹⁶Pt have been measured using a new two detector correlation system at Brookhaven National Laboratory (BNL). The properties of ¹⁹⁶Pt can be well described in terms of the 0(6) limit of the IBA-1 formalism and the empirical determination of mixing ratios offers a further refinement to that comparison. The measured M1 components may provide information concerning the location of the isovector mode in the 0(6) region. In addition, known correlations can provide a test of the new correlation system. 3 figures

[en] The mechanism which determines the relative E2 strengths in the Interacting Boson Approximation is studied, and the structure of the E2 operator necessary to reproduce the empirical B(E2) values in deformed even-even nuclei in the rate earth region is investigated

[en] The first volume of the Annual Report for 1989/90 gives an overview of the Nuclear Structure Facility at Daresbury, its development and a selection of highlights of the year's programme. This volume is complementary, presenting brief specialist reports, submitted by the users, describing the progress and results of each individual proposal. The contents reflect the extremely successful year due in good measure to the performance of the tandem accelerator which provided a record number of hours with ''beam on target''. Reports are grouped in four sections: research into nuclear structure with contributions ordered in increasing Z numbers of the nuclei studied; investigations of nuclear reaction mechanisms; nuclear theory; accelerator operations and development plus experimental instrumentation and techniques. The appendix forms a concise summary of the work at the facility for the year. (author)

[en] It is shown that the systematics of the fragmentation of Nilsson strength in the Hf--Os region can be understood in terms of varying quadrupole and hexadecapole deformations. 2 figures