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There are three recent themes in nuclear structure that come together in an interesting and useful way via the concept of effective charges and the framework of the IBM. These three concepts are the importance of dynamical symmetries in describing nuclear structure and the benefits that accrue from their exploitation, secondly, the critical role of the p-n interaction in the onset and development of collectivity in nuclei, and, thirdly, the importance of the valence nucleons in determining structure and its evolution.

The concept and application of algebraic models to nuclear structure is surveyed with emphasis on interpretations in terms of traditional geometric models. Recent studies relating to the relationship between β and γ vibrations in deformed nuclei, to the rigidity or softness of axial asymmetry, to signature splitting in octupole excitations, to O(6) quantum numbers and multi-phonon excitations, and to pseudo-SU(3) are discussed.

The average p-n interaction energy of each of the last two protons with each of the last two neutrons can be computed from the double difference in binding energies, $$\delta {V_{pn}}(Z + 1,N + 1) = \{ [B(Z + 2,N + 2) - B(Z + 2,N)] - [B(Z,N + 2) - B(Z,N)]\} /4.$$ Summation of δV pn , over all valence nucleons yields V pn , the integrated valence p-n interaction, which is found to increase linearly with N p N n , early in a shell and then saturates as mid-shell is approached. This saturation in V pn is consistent with the saturation in collectivity near mid-shell as evidenced by experimental B(E2:0+→2+) values and thus provides empirical support for the N p N n scheme. Results for various shell and subshell regions are discussed.

It is shown that 196pt is an excellent empirical manifestation of the newly proposed, and heretofore unobserved, 0(6) symmetry of the Interacting Boson Approximation Model. Further, with this as a starting point, a new, simpler, interpretation is possible of the complex Pt-Os transition region as initiating an 0(6) → rotor (SU(3)) transition.

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.

The Interacting Boson Approximation (IBA) nuclear structure model can be expressed in terms of the U(6) group, and thereby leads to three dynamical symmetries (or group chains) corresponding to different nuclear coupling schemes and geometrical shapes. The status of the empirical evidence for these three symmetries is reviewed, along with brief comments on the possible existance of supersymmetries in nuclei.

A detailed level scheme of 196Pt has been constructed based on extensive (n,γ) measurements. Most of the states populated have Jπ=0+,1+,2+. A number of candidates for negative-parity states are discussed in terms of partially decoupled bands.

Unique parity levels in odd mass heavy nuclei arise, in each shell, from that isolated orbit lying amidst others of opposite parity. Their effective isolation leads to high purity and therefore such states offer an ideal testing ground for nuclear models.

The non-selectivity of the (n,γ) reaction provides a powerful tool for the study of those nuclear models of broad applicability whose characteristic excitations span a wide range of degrees of freedom. Several recent examples of this are discussed with particular emphasis on the recent discovery of the 0(6) limit of the Interacting Boson Model and of a new interpretation of the Pt-Os transition region which consequently emerges. Other topics considered include recent extensions of the Nilsson model to new regions of nucleus, excitation energy and complexity of states.

A broad overview is presented of the principal tests to date of the IBA, for both even and odd nuclei, and including the evidence concerning the possible existence of the recently proposed super-symmetries. Although some details are presented, the aim is rather to survey the principal characteristics of the model, to assess the overall quality and extent of the agreement with experiment, and to indicate where further testing would be most useful.

Low and high energy spectra from thermal neutron capture in artificial 237Np have been studied over the energy ranges 17 to 650 and 2600 to 5500 keV. Primary transitions from neutron capture in four resonances have been observed between about 4800 and 5400 keV. Using 12 MeV deuterons, (d,p) spectra at three angles have been observed with a magnetic spectrograph. A nuclear level scheme for 238Np has been constructed by combining the results of the above measurements with previous data from a study of the 242mAm α-decay.1)2) The Nilsson model has been used to interpret the level structure. We have been able to identify the eight rotational bands based on the configurations where the odd proton is either in the π 5/2 + [642↑] or π 5/2 − [523↓] orbital and the odd neutron in either the υ 1/2 + [631↓] or υ 5/2 + [622↑] orbital (Fig. 1). The experimental and theoretical (d,p) cross section patterns are very similar, but there are some systematic difference which have also been observed3) in odd-A nuclei. The Gallagher-Moszkowski4) rule is found to be valid for the four cases where we have observed both parallel and antiparallel coupled bands. Two bands have K = 0. Their band heads have spin 1 and the Newby odd-even shift5) has been determined in both cases. A new investigation of the 242mAm decay is presently being performed at Livermore while new (n,e−) and (n,γ) experiments, using the spectrometers BILL and GAMS at ILL/Grenoble, are under way.

The 108Pd(n,γ)109Pd reaction discloses a family of low-spin odd parity states. A Coriolis calculation reveals difficulties in rotation-aligned models for low-spin unfavored states.

It is shown that the energy gap in non-spherical even-even nuclei is rather sharply delineated in γ-ray spectra following average resonance neutron capture. The relation to other methods of determining the energy gap is also presented.

This review will survey the status of experimental tests of the IBA including a discussion of both IBA-1 and IBA-2. Overall, to date, the IBA has enjoyed enormous success, both in individual nuclei and as a general scheme for the unified treatment of entire regions. As the number of tests of the model has grown recently, the first significant discrepancies have also appeared and will be discussed in some detail. The review is limited to even-even nuclei. Even so, it is impossible to discuss all relevant work within the short space available. The growing pace of experimental work in this area is attested by the rather different contents of this review and an earlier one1 this year: the reader is, therefore, also referred to that review where it was possible to treat some of the relatively smaller number of tests in more detail and also for its survey of tests of the IBA for odd mass nuclei.