Mass Measurement of Upper fp-Shell N=Z-2 and N=Z-1 Nuclei and the Importance of Three-Nucleon Force along the N=Z Line.

Using a novel method of isochronous mass spectrometry, the masses of ^{62}Ge, ^{64}As, ^{66}Se, and ^{70}Kr are measured for the first time, and the masses of ^{58}Zn, ^{61}Ga, ^{63}Ge, ^{65}As, ^{67}Se, ^{71}Kr, and ^{75}Sr are redetermined with improved accuracy. The new masses allow us to derive residual proton-neutron interactions (δV_{pn}) in the N=Z nuclei, which are found to decrease (increase) with increasing mass A for even-even (odd-odd) nuclei beyond Z=28. This bifurcation of δV_{pn} cannot be reproduced by the available mass models, nor is it consistent with expectations of a pseudo-SU(4) symmetry restoration in the fp shell. We performed ab initio calculations with a chiral three-nucleon force (3NF) included, which indicate the enhancement of the T=1 pn pairing over the T=0 pn pairing in this mass region, leading to the opposite evolving trends of δV_{pn} in even-even and odd-odd nuclei.

Evidence for partial dynamical symmetries in atomic nuclei.

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.

SU(3) quasidynamical symmetry underlying the Alhassid-Whelan arc of regularity.

The first example of an empirically manifested quasidynamical symmetry trajectory in the interior of the symmetry triangle of the interacting boson approximation model is identified for large boson numbers. Along this curve, extending from SU(3) to near the critical line of the first order phase transition, spectra exhibit nearly the same degeneracies that characterize the low energy levels of SU(3). This trajectory also lies close to the Alhassid-Whelan arc of regularity, the unique interior region of regular behavior connecting the SU(3) and U(5) vertices, thus offering a possible symmetry-based interpretation of that narrow zone of regularity amidst regions of more chaotic spectra.

Discovery of 229Rn and the structure of the heaviest Rn and Ra isotopes from Penning-trap mass measurements.

The masses of the neutron-rich radon isotopes {223-229}Rn have been determined for the first time, using the ISOLTRAP setup at CERN ISOLDE. In addition, this experiment marks the first discovery of a new nuclide, 229Rn, by Penning-trap mass measurement. The new, high-accuracy data allow a fine examination of the mass surface, via the valence-nucleon interaction deltaV{pn}. The results reveal intriguing behavior, possibly reflecting either a N=134 subshell closure or an octupolar deformation in this region.

Schottky mass measurement of the 208Hg isotope: implication for the proton-neutron interaction strength around doubly magic 208Pb.

Time-resolved Schottky mass spectrometry has been applied to uranium projectile fragments which yielded the mass value for the 208Hg (Z=80, N=128) isotope. The mass excess value of ME=-13 265(31) keV has been obtained, which has been used to determine the proton-neutron interaction strength in 210Pb, as a double difference of atomic masses. The results show a dramatic variation of the strength for lead isotopes when crossing the N=126 neutron shell closure, thus confirming the empirical predictions that this interaction strength is sensitive to the overlap of the wave functions of the last valence neutrons and protons.

Enhanced sensitivity of nuclear binding energies to collective structure.

We have studied calculated collective contributions to nuclear binding and separation energies and find that there is a deeper and much more sensitive link to nuclear structure than previously recognized or expected, especially near midshell in medium mass and heavy nuclei. As a consequence, measured masses may help understand the structure of well-deformed nuclei (e.g., intrinsic excitations). Conversely, future structure calculations must consider their implications for binding energies.

Unified description of 0+ states in a large class of nuclear collective models.

A remarkably simple regularity in the energies of 0+ states in a broad class of collective models is discussed. A single formula for all 0+ states in flat-bottomed infinite potentials that depends only on the number of dimensions and a simpler expression applicable to all three interacting boson approximation symmetries in the large N(B) limit are presented. Finally, a connection between the energy expression for 0+ states given by the X5 model and the predictions of the interacting boson approximation near the critical point of the first order phase transition is explored.

Simple empirical order parameter for a first-order quantum phase transition in atomic nuclei.

A simple, empirical, easy-to-measure effective order parameter of a first-order phase transition in atomic nuclei is presented, namely, the ratio of the energies of the first excited 6+ and 0+ states, distinguishing between first- and second-order transitions, and taking on a special value in the critical region, as data in Nd-Dy show. In the large NB limit of the interacting boson approximation model, a repeating degeneracy between alternate yrast and successive 0+ states is found in the critical region around the line of a first-order phase transition, pointing to a possible underlying symmetry.

Empirical proton-neutron interactions and nuclear density functional theory: global, regional, and local comparisons.

Calculations of nuclear masses, using nuclear density functional theory, are presented for even-even nuclei spanning the nuclear chart. The resulting binding energy differences can be interpreted in terms of valence proton-neutron interactions. These are compared globally, regionally, and locally with empirical values. Overall, excellent agreement is obtained. Discrepancies highlight neglected degrees of freedom and can point to improved density functionals.

Direct empirical correlation between proton-neutron interaction strengths and the growth of collectivity in nuclei.

A direct correlation between experimental values of proton-neutron interaction strengths and experimental measures of the growth of collectivity in nuclei is found. In particular, differences in the p-n interaction strengths and differences in growth rates of collectivity in particle-particle (or hole-hole) and particle-hole regions are found to correspond.

Breakdown of K selection in 178Hf.

Coulomb activation of the four quasiparticle Kpi=16+ 178Hf isomer (t1/2=31 y) has led to the measurement of a set of Elamda matrix elements coupling the isomer band to the ground band. The present data combined with earlier 178 Hf Coulomb excitation data have probed the components in the wave functions and revealed the onset and saturation of K mixing in low-K bands, whereas the mixing is negligible in the high-K bands. The implications can be applied to other quadrupole-deformed nuclei.

Proton-neutron interactions and the new atomic masses.

Double differences of masses can be used to isolate specific nucleonic interactions. With the new 2003 mass tabulation a significant increase in the number of empirical average proton-neutron interactions of the last nucleons can be extracted. It is shown that they exhibit dramatic and distinctive patterns, especially near doubly magic nuclei, that these patterns can be interpreted with a simple ansatz based on overlaps of proton and neutron orbits, and that the trends in p-n interactions across entire shells can be understood if they are correlated with the fractional shell filling. It is shown how these empirical interactions can be sensitive to changes in shell structure in exotic nuclei. Finally, these results are used to suggest criteria for future mass measurements with new exotic beam facilities.

Experimental confirmation of the Alhassid-Whelan arc of regularity.

Evidence is presented to show that a group of nuclei, spanning a range of structures, corresponds to a previously proposed isolated region of regular behavior between vibrational and rotational structures that was never before observed empirically. Nuclei predicted to show such regular spectra correspond to Hamiltonian parameters that lie amidst those giving more chaotic spectra. We identify a key observable that has a one-to-one correspondence to this arc of regularity and which therefore provides both an empirical signature for it and a clue to its underlying nature.

Alternative interpretation of sharply rising E0 strengths in transitional regions.

It is shown that strong 0(+)(2)-->0(+)(1) E0 transitions provide a clear signature of phase transitional behavior in finite nuclei. Calculations using the interacting-boson approximation (IBA) show that these transition strengths exhibit a dramatic and robust increase in spherical-deformed shape transition regions, that this rise matches well the existing data, that the predictions of these E0 transitions remain large in deformed nuclei, that they arise from the specific d-boson coherence in the wave functions, and do not necessarily require the explicit mixing of normal and intruder configurations from different IBA spaces.

Signature for vibrational to rotational evolution along the yrast line.

We present a simple method for discerning the evolution from vibrational to rotational structure in nuclei as a function of spin. The prescription is applied to the yrast cascades in the A approximately 110 region and a clear transition from vibrational to rotational motion is found.

Triple point of nuclear deformations.

We show that the second-order phase transition between spherical and deformed shapes of atomic nuclei is an isolated point following from the Landau theory of phase transitions. This point can occur only at the junction of two or more first-order phase transitions which explains why it is associated with one special type of structure and requires the recently proposed first-order phase transition between prolate and oblate nuclear shapes. Finally, we suggest the first empirical example of a nucleus located at the isolated triple-point.

B(E2) values in 150Nd and the critical point symmetry X(5).

Lifetimes of states in 150Nd were measured using the recoil distance method following Coulomb excitation of 150Nd by a 132 MeV 32S beam. The experiment was performed at the Yale Tandem accelerator, employing the SPEEDY gamma-ray detector array and the New Yale Plunger Device. Reduced transition probabilities in 150Nd are compared to the predictions of the critical point symmetry X(5) of the phase/shape transition that occurs for the N = 90 rare earth isotones. Very good agreement was observed between the parameter-free (apart from scale) X(5) predictions and the low-spin level scheme of 150Nd, revealing this as the best case thus far for the realization of the X(5) symmetry.

Quantum phase transition for gamma-soft nuclei.

We examine a quantum phase transition in gamma-soft nuclei, where the O(6) limit is simultaneously a dynamical symmetry of the U(6) group of the interacting boson model and a critical point of a prolate-oblate phase transition. This is the only example of phase transitional behavior that can be described analytically for a finite s,d boson system.

Empirical realization of a critical point description in atomic nuclei.

It is shown that (152)Sm and other N = 90 isotones are the first empirical manifestation of the newly predicted analytic description of nuclei at the critical point of a vibrator to axial rotor phase transition.

Phonons and Phase Transitions in Finite Nuclei.

The nature and evolution of collectivity and coherence in nuclei is one of the most fundamental issues in nuclear structure and its evolution with N and Z. Despite many experiments, the nature of nuclear vibrational modes in deformed nuclei and the nature of nuclear phase/shape transitions are not at all understood. We discuss new experiments on phonon and multi-phonon states in the rare earth nuclei and on new evidence for phase coexistence in Sm that relates to the possible existence of phase transitional behavior in finite nuclei.