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Background: The $r$ process is thought to be a dominant mechanism for the production of medium mass and heavy nuclei. Despite extensive study, neither the process nor its site, and, consequently, the origin of these elements, is well understood. One of the principal reasons is the lack of adequate knowledge of neutron capture cross sections in neutron-rich nuclei, especially up to a few hundred keV. Existing statistical model calculations differ widely and are unreliable more than a few nucleons beyond stability.Purpose: To provide a new, more reliable empirical method to estimate neutron capture cross sections.Method: To use an entirely empirical approach by exploiting a newly discovered correlation between two-neutron separation energies and neutron capture cross sections.Results: It is shown that there is a compact correlation between neutron capture cross sections and ${S}_{2n}$ for ${S}_{2n}$ values from 10 to 16 MeV, encompassing nuclei in five mass regions comprising seven classes of nuclei from $A\ensuremath{\approx}110$ through the actinides over Maxwellian energy distributions from $kT=5$ to 100 keV. As a consequence, many unknown cross sections can be predicted by interpolation, with accuracies on the order of $\ifmmode\pm\else\textpm\fi{}25%$. In other cases, the cross sections can still be predicted with greatly improved accuracy compared to current models. Finally, it is shown that, in very neutron-rich nuclei, measurements of ${S}_{2n}$ can replace more difficult or impossible neutron capture measurements to estimate $r$-process abundances.Conclusions: A new approach to estimate neutron capture cross section allows much improved predictions of these key ingredients into $r$-process calculations, perhaps providing an enhanced ability to model this process and to better define its site(s).

Reactions on atomic nuclei play a key role in understanding environments as diverse as hydrostatic stellar interiors, nuclear reactors, and explosions that occur in the death throes of a massive st...

Over the last decade, the Low-Energy Nuclear Physics (LENP) and Nuclear Astrophysics (NAP) communities have increasingly organized themselves in order to take a coherent approach to resolving the challenges they face. As a result, there is a high level of optimism in view of the unprecedented opportunities for substantial progress. In preparation of the 2015 US Nuclear Science Long Range Plan (LRP), the two American Physical Society Division of Nuclear Physics town meetings on LENP and NAP were held jointly on August 21–23, 2014, at Texas A&M, College Station, in Texas. These meetings were co-organized to take advantage of the strong synergy between the two fields. The present White Paper attempts to communicate the sense of great anticipation and enthusiasm that came out of these meetings. A unanimously endorsed set of joint resolutions condensed from the individual recommendations of the two town meetings were agreed upon. The present LENP White Paper discusses the above and summarizes in detail for each of the sub-fields within low-energy nuclear physics, the major accomplishments since the last LRP, the compelling near-term and long-term scientific opportunities plus the resources needed to achieve these goals, along with the scientific impact on, and interdisciplinary connections to, other fields.

A short perspective on the career and scientific achievements of Francesco Iachello is presented in the context of the power and beauty of looking at many-body systems from the perspective of their regularities and their structural evolution with nucleon number, including the emergence of and the role of symmetries and simple models in their description.A short perspective on the career and scientific achievements of Francesco Iachello is presented in the context of the power and beauty of looking at many-body systems from the perspective of their regularities and their structural evolution with nucleon number, including the emergence of and the role of symmetries and simple models in their description.

In 1956 A. Kerman explained the observed energy levels of the lowest K = 1/2 and K = 3/2 bands in 183W as resulting from rotational perturbations due to the effects of the Coriolis force. Following from that seminal paper, in this work we consider Kerman’s problem when one of the Nilsson single-particle levels involved is a resonant state. This is the simplest model of Coriolis mixing that allows for analytical expressions derived in first-order perturbation theory, without losing the main physical ingredients of the continuum effects. We present results for the perturbed solution of the ground state band energies as a function of the state width, which suggest a general behavior of both increased moment of inertia and staggering.

Peter von Brentano“Das Gegenteil eines Fehlers ist ein (anderer) Fehler!” (Bernard von Brentano aus “Theodor Chindler”) [“The opposite of an error is an error!”]; “Sie mussen zugeben” [“You must ad...

The U(5), SU(3), and O(6) symmetries of the Interacting Boson Model (IBM) have been traditionally placed at the vertices of the symmetry triangle, while an O(5) symmetry is known to hold along the U(5)–O(6) side of the triangle. We construct [1] for the first time a symmetry line in the interior of the triangle, along which the SU(3) symmetry is preserved. This is achieved by using the contraction of the SU(3) algebra to the algebra of the rigid rotator in the large boson number limit of the IBM. The line extends from the SU(3) vertex to near the critical line of the first order shape/phase transition separating the spherical and prolate deformed phases. It lies within the Alhassid–Whelan arc of regularity, the unique valley of regularity connecting the SU(3) and U(5) vertices amidst chaotic regions, thus providing an explanation for its existence.

Nuclei are complex objects yet display remarkable simplicities and regular patterns. The study of these and their origins has long been one of the twin pillars of nuclear structure research. We will discuss the behavior of atomic nuclei from this point of view. A key element will be the advantages of looking at the same data from different perspectives and of inter-relating these perspectives.

Background: Microscopic calculations of heavy nuclei face considerable difficulties due to the sizes of the matrices that need to be solved. Various approximation schemes have been invoked, for example by truncating the spaces, imposing seniority limits, or appealing to various symmetry schemes such as pseudo-SU(3). This paper proposes a new symmetry scheme also based on SU(3). This proxy-SU(3) can be applied to well-deformed nuclei, is simple to use, and can yield analytic predictions. Purpose: To present the new scheme and its microscopic motivation, and to test it using a Nilsson model calculation with the original shell model orbits and with the new proxy set. Method: We invoke an approximate, analytic, treatment of the Nilsson model, that allows the above vetting and yet is also transparent in understanding the approximations involved in the new proxy-SU(3). Results: It is found that the new scheme yields a Nilsson diagram for well-deformed nuclei that is very close to the original Nilsson diagram. The specific levels of approximation in the new scheme are also shown, for each major shell. Conclusions: The new proxy-SU(3) scheme is a good approximation to the full set of orbits in a major shell. Being able to replace a complex shell model calculation with a symmetry-based description now opens up the possibility to predict many properties of nuclei analytically and often in a parameter-free way. The new scheme works best for heavier nuclei, precisely where full microscopic calculations are most challenged. Some cases in which the new scheme can be used, often analytically, to make specific predictions, are shown in a subsequent paper., 24 pages, 6 figures, 15 tables

Using a new approximate analytic parameter-free proxy-SU(3) scheme, we make simple predictions of shape observables for deformed nuclei, namely gamma and beta deformation variables, the global feature of prolate dominance and the locus of the prolate-oblate shape transition. The predictions are compared with empirical results., Comment: 14 pages, 6 figures, 4 tables

Background: A characteristic observable of nuclear collective motion is the relative $B(E2)$ values from the $\ensuremath{\gamma}$ band to the ground band in even-even deformed nuclei. The Alaga rules provide an idealized set of benchmarks for these observables. However, deviations from the Alaga rules are universally observed and have been traditionally and successfully interpreted in terms of parameterized $\ensuremath{\gamma}$-band--ground-band bandmixing. An alternate approach, partial dynamical symmetries, has no bandmixing whatsoever and is parameter free, yet mimics closely the effects of bandmixing, due solely to the effects of finite valence nucleon number.Purpose: To investigate the relation between these two seemingly contradictory approaches to understand how they can produce such similar results.Method: To derive approximate relations between the two formalisms.Results: A consistent relationship is found linking bandmixing to finite valence nucleon number effects on interband $\ensuremath{\gamma}$ to ground-band $B(E2)$ values.Conclusions: Two disparate approaches to one of the iconic characteristics of deformed nuclei are shown to be intimately related. Moreover, a systematic difference in their predictions also emerges naturally from the derivation. The qualitative linkage of valence nucleon number and the separation of vibrational and rotational degrees of freedom has long been assumed but never before explicitly demonstrated through complementary models.

Ani Aprahamian (on right of figure), the Frank M. Freimann Professor of Physics at the University of Notre Dame in the United States, was named the new director of the A. Alikhanyan National Labora...

The SU(3) symmetry realized by J. P. Elliott in the sd nuclear shell is destroyed in heavier shells by the strong spin-orbit interaction. However, the SU(3) symmetry has been used for the description of heavy nuclei in terms of bosons in the framework of the Interacting Boson Approximation, as well as in terms of fermions using the pseudo-SU(3) approximation. We introduce a new fermionic approximation, called the proxy-SU(3), and we discuss how some of its novel predictions come out as a consequence of the short range of the nucleon-nucleon interaction and the Pauli principle., Comment: 6 pages, 2 tables, to appear in the proceedings of the 26th Annual Symposium of the Hellenic Nuclear Physics Society (HNPS2017), Anavyssos, Greece, 9-10 June 2017, ed. Ch. Tsabaris, R. Vlastou, M. Kokkoris, and D. Patiris

Using a new approximate analytic parameter-free proxy-SU(3) scheme, we make predictions of shape observables for actinides and superheavy elements, namely beta and gamma deformation variables, and compare these with predictions by relativistic and non-relativistic mean-field theories. Furthermore, we make predictions for B(E2) transition rates of deformed nuclei and compare these with existing data and predictions of other theoretical approaches., 6 pages, 3 figures, to appear in the proceedings of the 26th Annual Symposium of the Hellenic Nuclear Physics Society (HNPS2017), Anavyssos, Greece, 9-10 June 2017, ed. Ch. Tsabaris, R. Vlastou, M. Kokkoris, and D. Patiris. arXiv admin note: text overlap with arXiv:1711.09201, arXiv:1711.08596

Excited states in the deformed nucleus 168Er have been studied with high energy resolution in the (p, t) reaction, with the Munich Q3D spectrograph. A large number of excited 0+ states (25) and 2+ states (64) have been assigned up to 4.0-MeV excitation energy. This allows detailed investigations along two directions of current interest: first, an extension of microscopic model interpretations into the region of medium level density above the pairing gap; second, a first analysis of the statistical fluctuation (order/chaos) properties of pure sequences of levels, in one deformed nucleus. Predictions of two models (the quasiparticle-phonon model and the projected shell model) are compared to the data, and it is concluded that, in both cases, mixing of more configurations is required in the wave functions.

The B(E2) excitation strength of the first excited 2(+) state in even-even nuclei should directly correlate with the size of the valence space and maximize at mid-shell. A previously found saturation of B(E2) strengths in well-deformed rotors at mid-shell is tested through high-precision measurements of the lifetimes of the lowest-lying 2(+) states of the Hf-168 and W-174 rare earth isotopes. Measurements were performed using fast LaBr3 scintillation detectors. Combined with the recently remeasured B(E2; 2(1)(+) -> 0(1)(+)) values for Hf and W isotopes the new data remove discrepancies observed in the differentials of B(E2) values for these isotopes.

© 2016 American Physical Society. Background: The giant pairing vibration (GPV), a correlated two-nucleon mode in the second shell above the Fermi surface, has long been predicted and expected to be strongly populated in two-nucleon transfer cross sections similar to those of the normal pairing vibration. Recent experiments have provided evidence for this mode in C14,15 but, despite sensitive studies, it has not been definitively identified in either Sn or Pb nuclei where pairing correlations are known to play a crucial role. Purpose: Our aim is to test whether features inherent to the mixing of bound and unbound levels might account for this and to study the effect in a simple and intuitively clear approach. Method: We study the mixing of unbound levels in a set of toy models that capture the essential physics of the GPV, along with a more realistic calculation including distorted-wave Born approximation transfer amplitudes. Results: The calculated (relative) cross section to populate a simulated GPV state is effectively low, compared to the case of bound levels with no widths Conclusions: The mixing turns out to be only a minor contributor to the weak population. Rather, the main reason is the melting of the GPV peak due to the width it acquires from the low orbital angular momentum single-particle states playing a dominant role in two-nucleon transfer amplitudes. This effect, in addition to a severe Q-value mismatch, may account for the elusive nature of this mode in (t,p) and (p,t) reactions.

Signatures of criticality in the evolution of the nuclear ground-state shapes across the NxZ plane are discussed. Attention is paid to specific data indicating sudden structural changes in various isotopic and isotonic chains of medium-mass and heavy even-even nuclei, as well as to diverse theoretical aspects of the models used to describe these changes. The interacting boson model and the geometric collective model, in particular, are discussed in detail, the former providing global predictions for the evolution of collective observables in nuclei between closed shells and the latter yielding a parameter-efficient description of nuclei at the critical points of shape transitions. Some issues related to the mechanism of first- and second-order quantum phase transitions in general many-body systems are also outlined.

This overview discusses the evolution of collectivity in atomic nuclei, with particular focus on the rapidly developing field of quantum phase transitions in the nuclear shape, and on trajectories of structural evolution in the N / Z plane. Particular stress is put on the interplay of a nucleon-based description of the driving mechanisms in the emergence of coherence and collectivity in nuclei with a more macroscopic perspective in terms of nuclear shapes and the symmetries and quantum numbers of the many-body system taken as a whole.

Recently, the idea of quantum phase transitions has been applied to equilibrium shape changes in finite atomic nuclei. An introduction to and discussion of this concept, both from the macroscopic perspective and from a microscopic approach, in terms of shell structure and residual interactions is presented. Following this, the quantitative predictions of the critical point symmetries (CPS) X(5) and E(5) are discussed in some detail and compared to the data in a number of nuclei and mass regions. Any successful new model paradigm soon generates modifications to improve the predictions and, likewise, often spawns related models inspired by the original ansatz. This is eminently true of the CPS and we make an effort to briefly discuss this rapidly advancing area in terms of a number of recent modifications to the CPS as well as related models that are couched in a geometrical perspective.

Statistical measures of chaos have long been used in the study of chaotic dynamics in the framework of the interacting boson model. The use of large number of bosons renders additional studies of chaos possible, that can provide a direct comparison with similar classical studies of chaos. We intend to provide complete quantum chaotic dynamics at zero angular momentum in the vicinity of the arc of regularity and link the results of the study of chaos using statistical measures with those of the study of chaos using classical measures. Statistical measures of chaos are applied on the spectrum and the transition intensities of 0+ states in the framework of the interacting boson model. The energy dependence of chaos is provided for the first time using statistical measures of chaos. The position of the arc of regularity was also found to be stable in the limit of large boson numbers. The results of the study of chaos using statistical measures are consistent with previous studies using classical measures of chaos, as well as with studies using statistical measures of chaos, but for small number of bosons and states with angular momentum greater than 2., 13 pages, 8 figures

Background: A recent survey of well-deformed rare earth nuclei showed that B(E2) values from the γ band to the ground band could be explained rather well by a parameter-free description in terms of a partial dynamical symmetry (PDS). Purpose: Our purpose in this paper is to extend this study to deformed and transitional nuclei in the actinide and A ~ 100 regions to determine if the success of the PDS description is general in medium- and heavy-mass nuclei and to investigate further where it breaks down. Method: As with the previous study we study the empirical relative B(E2 : γ to ground) values in comparison to a pure rotor (Alaga) model and to the SU(3) PDS. Results: The data for the actinides, albeit sparser than in the rare-earth region, are reasonably well accounted for by the PDS but with systematic discrepancies. For the Mo isotopes, the PDS improves on the Alaga rules but largely fails to account for the data. Conclusions: As in the rare earths, the parameter-free PDS gives improved predictions compared to the Alaga rules for the actinides. The differences between the PDS predictions and the data are shown to point directly to specific mixing effects. Finally, inmore » the Mo isotopes, their transitional character is directly seen in the large deviations of the B(E2) values from the PDS in the direction of the selection rules of the vibrator.« less

At the PF1B cold neutron beam line at the Institut Laue Langevin, the EXILL array consisting of EXOGAM, GASP and ILL-Clover detectors was used to perform (n,$\gamma$) measurements at very high coincidence rates. About ten different reactions were measured in autumn 2012 using a highly collimated cold neutron beam. In spring 2013, the EXOGAM array was combined with 16 LaBr3(Ce) scintillators in the EXILL&FATIMA campaign for the measurement of lifetimes using the generalised centroid difference method. We report on the properties of the set-ups and present first results from both campaigns.

Atomic nuclei exhibit phase transitions as a function of the number of their constituent protons and neutrons. These phase transitions are not of the usual thermodynamic type, but rather they are quantum phase transitions in the equilibrium shape and structure of the ground and low-lying states. This realization has sparked a new area of research, focusing on the concept of 'critical-point symmetries', which describe the structure of nuclei at phase-transitional points. Both macroscopic (geometric or algebraic) and microscopic (shell-model) aspects of these phase transitions and of changes in structure with proton and neutron number in nuclei are discussed, along with an interpretation in terms of simple Landau theory. Finally, some alternative scenarios and schematic models for different classes of nuclei based on other simple potentials are briefly summarized.

The evolution of the structure of the gamma-vibrational band head 2+ γ in well-deformed rare-earth nuclei is studied within the framework of RPA calculations using a quasiboson approximation based on the Nilsson model. The obtained evolution of the quasiparticle structure of the gamma-vibrational band head is shown to correlate well with the evolution of the same nuclei within the IBA symmetry triangle when described in the extended consistent-Q formalism. An empirical relation is presented that links the IBA parameter χ with the quasiparticle structure of the gamma-vibrational band head.

A (p, t) study of eight nuclei in the rare earth region identified 96 0 + states (67 new). Their density at low energy is used to corroborate a fundamental property of quantum phase transitional behavior and to provide a new signature of nuclei near the critical point.

States in 162 Dy have been populated by ( n , γ ) , ( n , e − ) , and ( α , 2 n ) reactions in order to characterize a complete set of low K bands below 2 MeV. The ( n , γ ) and ( n , e − ) measurements were made using high precision curved crystal and β − spectrometers. Gamma–gamma coincidence and DCO measurements were made following the ( α , 2 n ) reaction using an array of Compton-suppressed Ge detectors. Using all these data and employing the Ritz combination principle, a level scheme was developed which represents one of the most complete and extensive spectra of levels available for comparison with nuclear models. The observed positive and negative parity bands are discussed in terms of collective quadrupole and octupole vibrational excitations as well as 2 quasi-particle excitations.

Measuring the neutron-induced fission cross-sections of short-lived nuclei represents an experimental challenge due to target activity and the low intensity of neutron beams. One way to alleviate the problems inherent in the direct measurement is to use the surrogate method, where one measures the decay probability of the same compound nucleus formed using a charged beam and a stable target. The decay probability of the compound nucleus is then used to estimate the neutron-induced cross-section. As an extension to the surrogate method, we introduce a new method of reporting the fission probabilities of two compound nuclei as a ratio, which has the advantage of removing most of the systematic uncertainties. The ratio method was checked in a known case, the 236U(n, f)/238U(n, f) cross-section ratio, which turned out to be the same as the probability ratio of P(236U(d, pf))/P(238U(d, pf)). As an application, the 237U(n, f)/235U(n, f) cross-section ratio was inferred, on the basis of the measured P(238U(d, d'f))/P(236U(d, d'f)) probability ratio.

Lifetimes for decays linking near-yrast states in 107Cd have been measured using the recoil distance method (RDM). The nucleus of interest was populated via the 98Mo(12C,3n)107Cd fusion–evaporation reaction at an incident beam energy of 60 MeV. From the measured lifetimes, transition probabilities have been deduced and compared with the theoretical B(E2) values for limiting cases of harmonic vibrational and axially deformed rotational systems. Our initial results suggest a rotor-like behaviour for the structure based on the unnatural-parity, h11/2 orbital in 107Cd, providing further evidence for the role of this 'shape-polarizing' orbital in stabilizing the nuclear deformation in the A ~ 100 transitional region.

Recent theoretical and experimental work focusing on nuclei in spherical-deformed transition regions has sparked considerable interest in phase transitional nuclei. A review of searches and experimental results on critical point nuclei in the A = 130 and A = 150 regions is presented.

The triaxial strongly deformed (TSD) bands in 167Lu were populated by the 123Sb(48Ca, 4n) reaction with a beam energy of 203 MeV. Gamma rays, requiring five fold or more in prompt coincidence, were detected with the Gammasphere spectrometer. Of particular interests are TSD bands 1 and 2 which have previously been interpreted as zero phonon and one phonon wobbling bands, respectively. Using the Doppler shift attenuation method (DSAM), a preliminary transition quadrupole moment of 6.9+0.3−0.3 eb was extracted for the TSD1 band. Data analysis continues for TSD2 which is considerably more weakly populated.

The puzzling nature of 0 + excitations, especially in transitional and deformed nuclei, has attracted new attention. Following a recent experiment studying 158 Gd, we investigated 15 additional nuclei in and near the rare earth region. Our aim was to map out the energies of as many 0 + states as possible. A series of (p, t) experiments was performed at the Q3D magnetic spectrograph at the University of Munich MP tandem accelerator laboratory. These reactions are particularly sensitive to 0 + states. Outgoing tritons were recorded at various lab angles, and their relative cross sections are compared to those calculated using the distorted wave Bom approximation (DWBA). The L = 0 angular distribution is easily discerned. New 0 + states are found in each of these nuclei, and their systematics are discussed.

Low-energy excited states of 210 Ra and 208 Ra were investigated at the Wright Nuclear Structure Laboratory of Yale University. Fusion evaporation recoils were selected using the gas-filled spectrometer, SASSYER. Delayed γ-rays, following isomeric decays, were detected at the focal plane of SASSYER with a small array of HPGe detectors. Transitions following the proposed J π = 8 + isomers were observed, and the half-lives measured. The experiments are discussed and results compared to expectations from the seniority scheme.

The atomic nucleus is a complex, quantum mechanical system that is of fundamental importance in our quest to understand the world around us, to comprehend how the universe developed to where it is today, and as a tool for a variety of applications from energy to medicine. The nucleus, central to each atom, comprises over 99.9% of the mass of the atom, yet occupies less than a trillionth of its volume. The smallness of the atomic nucleus relative to every-day objects is astounding. If the nucleus were the size of a football, then the atom would be as large across as London. A Frisbee, in relative size, would be as large as our entire solar system. Despite this incredibly small size, the task of nuclear science is to manipulate atomic nuclei on a femto-scale, make new forms of nuclear matter by rearranging its constituent pieces, and come to a thorough understanding of its inner workings.

Shape/phase transitions in low-energy nuclear spectra, the new critical point symmetries E(5) and X(5), and their empirical realization have recently been the subject of many experimental and theoretical investigations. With a set of polar coordinates, the precise location of the critical phase transition region and of X(5)-type nuclei can be mapped in the interacting boson model symmetry triangle. An empirical mapping of the symmetry triangle for the N=82–104 rare-earth nuclei is also obtained.

The Wright Nuclear Structure Laboratory has recently acquired a gas-filled recoil separator previously used at Berkeley National Laboratory for heavy-element synthesis. The separator will be used to separate reaction recoils from primary beam particles and fission products following target bombardment. Commissioning of the separator has recently been completed, and the structure of 203 Rn investigated.

B(E2;0+1→2+1) values for the unstable, neutron-rich nuclei 132,134Te were determined through Coulomb excitation, in inverse kinematics, of accelerated beams of these nuclei. The systematics of measured B(E2) values from the ground state to the first excited state have been extended to the N=82 shell closure in the Te nuclei and have been compared with the predictions of different theories. The measurements were performed at the Holifield Radioactive Ion Beam Facility (HRIBF) using the GRAFIK detector. The success of this approach, which couples a 5.7% efficient through-well NaI(Tl) γ-ray detector with thin foil microchannel plate beam detectors, also demonstrates the feasibility for Coulomb excitation studies of neutron-rich nuclei even further from the valley of beta stability, both at present-generation ISOL facilities and at the proposed Rare Isotope Accelerator.

Symmetries in nature offer very simple descriptions of complex systems. Partial Dynamical Symmetries (PDS) can considerably broaden their relevance. To present the first extensive test of a PDS for nuclei, we compare an SU(3) PDS to gamma to ground band $B(E2)$ values for 47 deformed nuclei. The parameter-free PDS is found to be quite successful, but with characteristic discrepancies, suggesting that symmetry remnants are more pervasive than heretofore realized. Furthermore, the SU(3) PDS gives new insights into collective models (e.g., interacting boson approximation). If these reproduce the PDS, they reflect finite size effects, while differences from the PDS point to SU(3) configuration mixing.

Experiments on exotic nuclei, in both “near” and “far” regions off stability, are discussed from the complementary standpoints of improvements in experimental techniques, on the one hand, and the exploitation of simple signatures of structure on the other.

Nuclear structure in exotic nuclei is likely to be quite different than in the nuclei we have been able to access near the valley of stability. With the development of advanced radioactive-beam facilities, we will, for the first time, have access to long isotopic chains of nuclei. A key facet of exotic nuclei therefore will be the study of the evolution of structure with N, Z, and A to an extent never before imagined. Yet, the beam intensities will be very low (relative to stable beams); hence, we will need to extract more physics from less data. We will discuss several aspects of structural evolution and its elucidation in exotic nuclei.

The advent of radioactive nuclear beams (RNBs) has led to a rapid growth in the study of exotic nuclei. Already a number of major discoveries have been made. Examples are halo nuclei, mapping the bounds of nuclear existence, assessing the fragility of magicity, producing special nuclei such as 100Sn and 48Ni, and measuring key reaction rates of astrophysic interest. The growth in this field in the next decade will be enormous. With advanced RNB facilities being planned or under construction, more and more exotic nuclei will become accessible. One of the most interesting opportunities will be the study of the evolution of nuclear structure along extended iso-chains of nuclei. A prime example is Ni, where four magic numbers (20, 28, 40, and 50) and five major shells will be accessible. Structural evolution will be discussed from several standpoints, both theoretical and experimental, with emphasis on methods to obtain a maximum of information on new nuclei from the sparse data that will be available at the extremes of accessible N/Z ratios.

The role of highly efficient detector systems and highly efficient signatures of structure in the study of exotic nuclei is discussed. The discovery of “critical point symmetries” in the study of phase transitional behavior in nuclei, with applications to unknown regions, is also discussed.

Enhanced proton-neutron interactions occur in heavy nuclei along a trajectory of approximately equal numbers of valence protons and neutrons. This is also closely aligned with the trajectory of the saturation of quadrupole deformation. The origin of these enhanced p-n interactions is discussed in terms of spatial overlaps of proton and neutron wave functions that are orbit-dependent. It is suggested for the first time that nuclear collectivity is driven by synchronized filling of protons and neutrons with orbitals having parallel spins, identical orbital and total angular momenta projections, belonging to adjacent major shells and differing by one quantum of excitation along the z-axis. These results may lead to a new approach to symmetry-based theoretical calculations for heavy nuclei., 6 pages, 4 figures

The lifetime of the 4 + 2 level in 152 Sm has been measured by the recoil distance method using Coulomb excitation. The result, 12±2 ps, gives a B(E2 :4 + 2 →2 + 2 ) value of 204 W.u., which is much smaller than the literature value. This removes an anomaly among collective transitions and a serious discrepancy with existing calculations. This result is interpreted by exploiting the concept of Q-invariants. The extensive data on 152 Sm is used to establish for the first time the validity of certain approximate expressions for the Q-invariants. Both exact and approximate expressions are then shown to reflect and support recent discussions of phase coexistence and β -softness in 152 Sm by applying them to the yrast states, and for the first time, to an excited family of levels based on the 0 + 2 level as well.