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Despite recent contentions, it is shown that, for the many known empirical quantities in 196 Pt that can distinguish O(6) from U(5), the data overwhelmingly favor an O(6) description and are in substantial disagreement with the U(5) picture. The origins of the differences between the O(6) and U(5) predictions are discussed.

Absolute differential cross sections for strong $L=0 (t,p)$ transitions on targets of $^{46,\phantom{\rule{0ex}{0ex}}48,\phantom{\rule{0ex}{0ex}}50}\mathrm{Ti}$, $^{50,\phantom{\rule{0ex}{0ex}}52,\phantom{\rule{0ex}{0ex}}54}\mathrm{Cr}$, and $^{54,\phantom{\rule{0ex}{0ex}}56,\phantom{\rule{0ex}{0ex}}58}\mathrm{Fe}$ have been measured. The bombarding energy was 13 MeV and reaction angles of 12.5 and 20\ifmmode^\circ\else\textdegree\fi{} were used. The data are compared with model predictions, with special emphasis on the pairing-vibrational model with isospin. The latter model correctly predicts the dependence on neutron number of the ($t,p$) cross sections but fails, as do all other models, to predict the steady cross-section decrease with increasing $Z$. It is suggested that the latter effect is caused by a slow change in the ground-state correlations.

The 182W(t, p)184W reaction has been studied at 20 MeV. The outgoing protons were detected in an Elbek broad range magnetic spectrograph. Absolute cross sections and angular distributions were measured. Evidence for inelastic effects in the reaction mechanism was observed for the first excited 2+ state, the 3− level at 1221 keV and the 5+ state at 1295 keV. The 0+ level at 1002 keV was populated with ≈ 2% the ground state cross section. A 4+ level at 1536 keV was observed with ≈ 50% the ground state cross section. Calculations of the absolute (t, p) cross sections to this and other states with known structure resulted in excellent agreement with the measured values.

The (τ, α) and (d, t) reactions have been used to investigate the positive-parity intrinsic excitations in 179, 181, 183, 185 W stemming form the i 13 2 spherical shell-model state. The 13 2 + members of the rotational bands built on each of these orbitals are expected to be populated strongly in neutron-transfer reactions due to the near-unity values of the C j = 13 2 coefficients. These states were populated in the (τ, α) reaction which favours transfer of high angular momentum. The assignments of l -values were based on the measured ratio of (τ, α) to (d, t) cross sections populating the same state. The Nilsson model, with the inclusion of pairing and Coriolis mixing, was unable to account for the experimental results, namely the observation of all the expected strength localized in only two or four states below 2 MeV of excitation. Much better agreement with experiment was obtained if the extended Nilsson model including a hexadecapole deformation of e 4 ≈ +0.06 was used as a starting point for a Coriolis-coupling calculation.

Energy levels of 182Wand 184W have been investigated by (d, p), (d, t) and (τ, α) reactions on enriched 183W targets. In the deuteron induced reactions, the reaction products at 60°, 90° and 125° were magnetically analysed and approximate l-transfer values were deduced from the cross-section ratios at these angles. The (τ, α) measurements provided important complementary data locating several 182W states populated with l = 6 transfer. The two-neutron configurations identified include 9 2 [624], 1 2 [510] Kπ = 4− and 5−, 3 2 [512], 1 2 [510] Kπ = 2+ and 1+, and tentatively 1 2 [521], 1 2 [510] Kπ = 1+ in 182W, and 11 2 [615], 1 2 [510] Kπ = 5− and (6−), 7 2 [503], 1 2 [510] Kπ = 3+ and (4+), and 3 2 [512], 1 2 [510] Kπ = 2+ and (1+) in 184W. The results also provide valuable insight into the microscopic structure of the γ-vibrations and higher-lying Kπ = 2+ excitations in these nuclei and made possible a detailed test of the predictions of microscopic calculations. Our analysis indicates that the distinct difference in the observed collective character of the higher-lying Kπ = 2+ excitations in the two nuclei is associated with the N = 108 energy gap in the Nilsson diagram.

In the context of the consistent Q formalism of the interacting boson approximation, which contains only one essential parameter, denoted chi, it is possible to investigate the limits of allowable boson numbers N for well deformed nuclei. This is done by examining the agreement with experiment of certain E2 branching ratios as a function of N. The results show that, in general, the best agreement is obtained for Nroughly-equalN/sub IBA/, where N/sub IBA/ is the usual interacting boson approximation value defined as the sum of half the number of valence protons and neutrons; moreover, significant discrepancies with experiment are obtained if N

The cross section ratios σ (4 g + )/ σ (2 g + ) and σ (2 γ + )/ σ (2 g + ) are nearly an order of magnitude larger in the 182 W(t,p) 184 W reaction than for other measured cases of the (t,p) or (p,t) reactions in the rare earth and actinide deformed regions. On both relative and absolute scales the 2 g + state of 184 W is anomalously weakly populated in 182 W(t,p) while the 4 g + and 2 γ + states are enhanced relative to the ground-state transition.

The lifetimes of the lowest 2/sup +/ and 4/sup +/ levels can distinguish between the O(6) and U(5) dynamical symmetries of the interacting boson model. The data for the A = 130 nuclei confirm the proposed O(6) character of these nuclei.

Previous (d,t) and (τ,α) cross sections to 13 2 + states in odd-mass tungsten nuclei have been interpreted in terms of moderate hexadecapole deformations (ϵ4≈0.06) while a recent Coulomb excitation measurement has suggested a much larger value for 182W. We have calculated the pickup spectra and show that in fact the data can be successfully interpreted with a wide range of ϵ4 values from ≈0.06 to ≈0.16.

The g factor of the ${2}_{1}^{+}$ state in $^{142}\mathrm{Ba}$ has been remeasured with higher accuracy. The new result, when combined with recently measured values of ${T}_{1/2}$ for this state and with g${(2}_{1}^{+}$) values in $^{144}$,146Ba, clearly indicates that in this region, the proton-neutron interacting boson model gives a much better description of magnetic moments than the hydrodynamical model with the Greiner corrections.

The excitation energies of low lying levels in $F$-spin multiplets are compared. An overall constancy of these energies is found, which indicates that the interacting boson model Hamiltonian has rather constant parameters in the multiplet.

Over the recent years the nuclear structure around the N=50 shell closure, which is very pronounced in the strontium and zirconium isotopes, has been the subject of extensive experimental and theoretical work1–5). On the proton side Z=38 and Z=40 provide fairly closed sub-shells1). In the strontium isotopes the lg9/2 neutron shell is closed at 88Sr, supplying relatively pure neutron-hole and neutron-particle states with large spectroscopic factors in 87Sr and 89Sr, as well as core-coupled states. The mass region is thus ideally suited to examine the transition from a correlated to an uncorrelated (chaotic?) excitational behavior6,7). These two types are characterized e.g. by the density of excited states, the transition strengths, and the spectroscopic factors observed in transfer reactions.

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.

Four new neutron-rich, fission-product nuclei have been identified at the on-line mass separator TRISTAN at Brookhaven National Laboratory. Their half-lives have been measured to be, for /sup 156/Pm, T/sub 1/2/ = 28.2 +- 1.1 sec; for /sup 159/Sm, T/sub 1/2/ = 15 +- 2 sec; for /sup 160/Sm, T/sub 1/2/ = 8.7 +- 1.4 sec; and for /sup 161/Eu, T/sub 1/2/ = 27 +- 3 sec. These nuclei are of particular importance in nucleosynthesis calculations, since they lie in the region of the rare-earth maximum in r-process abundances. A comparison of these and other new half-lives with recent calculations discloses a characteristic pattern of discrepancies.

Recently it has been shownl that a measure of the average number of interactions of each valence nucleon with those of the other type given by the parameter P = NpNn/(Np+Nn) [where Np(Nn) are the number of valence protons (neutrons)] provides a general and physically meaningful explanation for the development of collectivity and the onset of deformation in medium and heavy nuclei. For example, as shown in Fig. 1, the energy ratio \({E_{{4^+}_1}}/{E_{{2^+}_1}}\) plotted versus P exhibits remarkedly similar behavior independent of mass region and, in all cases indicates a transition to deformation \(({E_{{4^+}_1}}/{E_{{2^+}_1}}\approx 3.0)\) when P ≈ 4–5. This rule provides a necessary condition for deformation that the nuclear ground state cannot be deformed for medium or heavy nuclei if the number of valence nucleons of either type is less than 4. Furthermore, this result was seen as a manifestation of the competition between the like nucleon pairing and p-n quadrupole interactions since the pairing strength, which is about 1 MeV and which supports sphericity, is approximately off-set by the quadrupole deformation-driving term when 4–5 valence p-n interactions (each with strength ≈ 200 keV) are present.

The data for five mass regions from $A=100\ensuremath{-}200$ are plotted against the valence-nucleon product ${N}_{p}{N}_{n}$. They are found to display a remarkable similarity, heretofore unrecognized, which suggests a unified interpretation in terms of the strength of the proton-neutron interaction and which also leads to a better understanding of the $A=190$ region.

The TRISTAN research program is briefly outlined. The discussion then turns to the measurements of magnetic moments of excited states in neutron‐rich fission product nuclei using the perturbed angular correlation method. The results for 2+1 states are analyzed in light of the systematics of magnetic moments in the A=70–200 region, and in relation to collective nuclear models.

It is shown that an underlying SU(3) structure of the interacting boron approximation is exhibited by nuclei near neutron number 104 in the rare earth region. This is particularly evident in Yb and Hf and to a lesser extent in the heavy Er and light W nuclei. Although the structure and mixing effects are complex, this region seems to represent the closest approach to the SU(3) limit yet observed in heavy nuclei.

A new semiempirical method that significantly simplifies atomic mass systematics and which provides a method for making mass predictions by linear interpolation is discussed in the context of the nuclear valence space. In certain regions complicated patterns of mass systematics in traditional plots versus Z, N, or isospin are consolidated and transformed into linear ones extending over long isotopic and isotonic sequences.

The nuclei 155Gd, 161Dy and 163Dy have been investigated by means of the (n, γ), (n, e) and transfer reactions. Level schemes have been constructed up to about 1.5 MeV, which include spin and parity assignements for many levels. New rotatinal bands based on single particle states (Nilsson) as well as on vibrational states have been identified. The results are discussed.

Absolute γ‐intensities have been measured for the strong transitions in the decay chain 98Rb→98Sr→98Zr and 98Nb→98Mo and found ≊4 times higher than those currently adopted. Shape coexistence is established in 98Y with a Jπ=1− ‘‘spherical’’ ground state and with a previously reported deformed band with band‐head at 495.7 keV. A fast first‐forbidden β‐transition is observed between 98Yg and 98Zrg, which alongside with the previously reported 96Yg→96Zrg decay forms a new region, other than 208Pb and 16O, of such fast transitions.

A universal, physically intuitive description of collectivity in heavy nuclei is obtained in terms of the parameter P=${N}_{p}$${N}_{n}$/(${N}_{p}$+${N}_{n}$) which gives the average number of interactions of each valence nucleon with those of the other type. Regardless of mass region, deformation sets in when P=${P}_{\mathrm{crit}\mathrm{\ensuremath{\approxeq}}4--}$5. This, in turn, gives a rule that medium-mass and heavy nuclei with effective ${N}_{p}$ or ${N}_{n}$\ensuremath{\le}4 cannot be deformed. An interpretation in terms of the monopole and quadrupole p-n interactions is discussed.

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.

Electromagnetic transition probabilities and static moments of excited nuclear states are known to be good probes of nuclear structure. Therefore, a systematic analysis of the large amount of existing experimental data for these observables is expected to provide valuable information about the respective isotopes. It is the purpose of this talk to show that a combined analysis of static magnetic moments of 2 1 + states and B(E2) transition probabilities for even-even nuclei can be used to obtain effective numbers of valence nucleons. This kind of information is of particular interest in cases where subshell closures are found. For example, it is well known1 that for the transitional nuclei in the A=150 region the Z=64 subshell is active when the number of neutrons N

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.

The decay of the low‐lying states of 178Hf was investigated using: (1) High‐resolution curved crystal spectrometry of the secondary γ‐rays using the GAMS‐1 and GAMS 2/3 facilities at the ILL, (2) Measurements of the secondary (n,e−) transitions using the Electron Spectrometer BILL at the ILL, (3) Measurements of the primary γ‐transitions following thermal neutron capture with the pair‐spectrometer at the ILL and (4) Average Resonance Capture (ARC) measurements at the neutron energies of 2 keV and 24 keV, using the tailored beam facilities at BNL.

It is shown that a recently proposed method that reproduces the saturation in {ital B}({ital E}2;0{sub 1}{sup +}{r arrow}2{sub 1}{sup +}) values in deformed nuclei also improves interacting boson model predictions for {gamma}-band energy systematics near midshell. The method is based on an explicit recognition of the (Nilsson) orbit dependence of the valence {ital p}-{ital n} interaction and is, in effect, a refinement to the {ital N}{sub {ital p}}N{sub {ital n}} scheme which linearizes the behavior of certain observables.

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.

The low-spin β-decay of 96,98Y has been reinvestigated to obtain detailed information on the fast first-forbidden transitions to the ground states of 96,98Zr. A considerably more detailed decay scheme of 96Y confirms the general features of a previous study. The spin of the ground state of 98Y has been measured to be 0- from γγ(Θ) angular correlations in 98Sr→98Y decay and from Q β measurements by β∓γ coincidences in the decay of 98Y→98Zr. From β-decay data on 90–100Y nuclei, mixing of the “spherical” and “collective” 0+ states is deduced, and the strength of the mixing matrix elements is found to vary greatly across the Zr nuclei.

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.

Although the IBA‐1 contains no solutions corresponding to a rigid triaxial shape, it does contain an effective asymmetry arising from zero point motion in a γ‐soft potential leading to a non‐zero mean or rms γ. In the Consistent Q Formalism (CQF) of the IBA, most results of a calculation depend only on one parameter χ. A relation will be established between χ and the effective asymmetry parameter γ. The relation between the asymmetry occurring naturally in IBA‐1 and the triaxiality arising from the introduction of cubic terms into the IBA Hamiltonian will be discussed. It will be shown that γ‐band energy staggering is a particularly sensitive indicator of the degree of γ rigidity. Finally, an extensive new region of 0(6) like Xe and Ba nuclei near A=130 will be discussed. Their remarkable similarity to Pt will be explored. Deviations from the strict 0(6) limit can be described in terms of the interplay of soft and rigid axial asymmetry and calculations will be presented that interpret the Xe, Ba and Pt isotopes in this way.

In this reply, we emphasize the importance of the results of Bijker and Dieperink, and point out several additional implications and applications of their approach, particularly with regard to the role of finite boson number in the interacting boson approximation.

The low-lying energy levels of 106 Ru have been investigated with the 104 Ru(t, p) 106 Ru reaction at 15 MeV bombarding energy. Spins and parities were obtained from the measured angular distributions. The energy levels are discussed in terms of the phonon and rotational models and 106 Ru is found to be of transitional character between these limits.