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Recent experimental results related to the quadrupole collectivity in neutron-rich carbon isotopes are analyzed in a phenomenological approach. fi(E2;21+ → 01+) transitions rates derived from lifetime measurements are interpreted in terms of the mixing of basic neutron and proton 2+ excitations. A seniority inspired scheme is used to describe the neutron component. The observed increase in collectivity can be explained with a corresponding increased role of proton excitations. This is likely due to the reduction of the proton p3/2-p1/2 spin-orbit splitting caused by the tensor and two-body spin-orbit components of the force between the protons and the added neutrons in the (d5/2 + S1/2) shells. [ABSTRACT FROM AUTHOR]

The geometry of the shears mechanism in nuclei is derived from the nuclear shell model. This is achieved by taking the limit of large angular momenta (classical limit) of shell-model matrix elements. [ABSTRACT FROM AUTHOR]

Over a period of several years we have performed three separate experiments at Lawrence Berkeley National Laboratory's 88-Inch Cyclotron in which 235U (thick target) was Coulomb-excited. The program involved standalone experiments with Gammmasphere and with the 8pi Spectrometer using 136Xe beams at 720 MeV, and a CHICO-Gammasphere experiment with a 40Ca beam at 184 MeV. In addition to extending the known negative-parity bands to high spin, we have assigned levels in some seven positive-parity bands which are in some cases (e.g., [631]l/2, [624]7/2, and [622]5/2) strongly populated by E3 excitation. The CHICO data have been analyzed to extract E2 and E3 matrix elements from the observed yields. Additionally, many M1 matrix elements could be extracted from the γ-ray branching ratios. A number of new features have emerged, including the unexpected attenuation of magnetic transitions between states of the same Nilsson multiplet, the breakdown of Coriolis staggering at high spin, and the effect of E3 collectivity on Coriolis interactions. [ABSTRACT FROM AUTHOR]

The superfluid tunneling model is applied to the calculation of ground-state-to-ground-state α decay in the even-even neutron-deficient Te-Ba nuclei. We show that there is a larger α-particle formation probability in nuclei of this region above 100Sn when compared to analogous nuclei above 208Pb. This is consistent with the expected systematic variation of the pair gap Δ as a function of mass number. The recent experimental data on the α decay of the N=Z nuclei 104Te and 108Xe are shown to leave open the possibility of enhanced α-particle formation involving nucleon correlations beyond the standard treatment of like-nucleon pairing, which is the mechanism suggested as underlying "superallowed" α decay. [ABSTRACT FROM AUTHOR]

The nature of the nuclear pairing condensates in heavy nuclei, specifically neutron-proton (spin-triplet), versus identical-particle (spin-singlet) pairing has been an active area of research for quite some time. In this work, we probe three candidates that should display spin-triplet, spin-singlet, and mixed-spin pairing. Using theoretical approaches such as the gradient method and symmetry-restoration techniques, we find the ground state of these nuclei in Hartree-Fock-Bogoliubov theory and compute ground state to ground state pair-transfer amplitudes to neighboring isotopes while simultaneously projecting to specific particle number and nuclear-spin values. We identify specific reactions for future experimental research that could shed light on spin-triplet and mixed-spin pairing. [ABSTRACT FROM AUTHOR]

Spectroscopic factors in 10Be, 11Be, and 12Be, extracted from (d,p), one-neutron knockout, and (p,d) reactions, are interpreted within the rotational model. Assuming that the ground state and first excited state of 11Be can be associated with the ½[220] and ½[101] Nilsson levels, the strong-coupling limit gives simple expressions that relate the amplitudes of these wave functions (in the spherical basis) with the measured cross sections and derived spectroscopic factors. We obtain good agreement with both the measured magnetic moment of the ground state in 11Be and the reaction data. [ABSTRACT FROM AUTHOR]

Spectroscopic factors, extracted from one-neutron knockout and Coulomb dissociation reactions, for transitions from the ground state of 33Mg to the ground-state rotational band in 32Mg, and from 32Mg to low-lying negative-parity states in 31Mg, are interpreted within the rotational model. Associating the ground state of 33Mg and the negative-parity states in 31Mg with the 3/2[321] Nilsson level, the strong coupling limit gives simple expressions that relate the amplitudes (Cjℓ) of this wave function with the measured cross sections and derived spectroscopic factors (Sjℓ). To obtain a consistent agreement with the data within this framework, we find that one requires a modified 3/2[321] wave function with an increased contribution from the spherical 2p3/2 orbit as compared to a standard Nilsson calculation. This is consistent with the findings of large-scale shell model calculations and can be traced to weak binding effects that lower the energy of low-ℓ orbitals. [ABSTRACT FROM AUTHOR]

Rotational and electromagnetic properties of the yrast band in 102Pd are analyzed in terms of a phenomenologi-cal phonon model that includes anharmonic terms. Both the moment of inertia and B(E2)'s are well reproduced by the model, providing an independent confirmation of the multiphonon picture recently proposed. The (empirical) dependence of the phonon-phonon interaction on the phonon frequency, in Ru, Pd, and Ru isotopes, follows the expectations from particle-vibration coupling. [ABSTRACT FROM AUTHOR]

Background: Competing interpretations of the results of a 30Mg (t,p)32Mg measurement populating the ground state and 0+2 state in 32Mg, both limited to a two-state mixing description, have left an open question regarding the nature of the 32Mg ground state. Purpose: Inspired by recent shell-model calculations, we explore the possibility of a consistent interpretation of the available data for the low-lying 0+ states in 32Mg by expanding the description from two-level to three-level mixing. Methods: A phenomenological three-level mixing model of unperturbed 0p0h, 2p2h, and 4p4h states is applied to describe both the excitation energies in 32Mg and the transfer reaction cross sections. Results: Within this approach, self-consistent solutions exist that provide good agreement with the available experimental information obtained from the 30Mg(t,p)32Mg reaction. Conclusion: The inclusion of the third state, namely the 4p4h configuration, resolves the "puzzle" that results from a two-levelmodel interpretation of the same data. In our analysis, the 32Mg ground state emerges naturally as dominated by intruder (2p2h and 4p4h) configurations, at the 95% level. [ABSTRACT FROM AUTHOR]

It is pointed out that the ground state of n neutrons and n protons in a single-j shell, interacting through an isoscalar (T=0) pairing force, is not paired, J=0, but rather spin aligned, J=n. This observation is explained in the context of a model of isoscalar P (J=1) pairs, which is mapped onto a system of p bosons, leading to an approximate analytic solution of the isoscalar-pairing limit in jj coupling. [ABSTRACT FROM AUTHOR]

The role of the pairing interaction in multi-quasiparticle isomeric states was examined by calculating β-decay half-lives with the superfluid tunneling model. We found that this model reproduces the experimental β-decay half-lives with an accuracy typical of current β-decay models. In spite of the simplicity of the model, we are able to demonstrate how the reduction of pairing in multi-quasiparticle isomers has a remarkable effect on β-decay half-lives. Taking this effect into account may be important for spin and parity assignments of β-decaying multi-quasiparticle isomers. [ABSTRACT FROM AUTHOR]

A measurement of the direct two-proton removal from 42Si has provided the first structural information on the N = 28 isotone 40Mg. The value for the inclusive cross section for two-proton removal from 42Si of 40+27-17 μb is significantly lower than that predicted by structure calculations using the recent SDPF-MU shell-model effective interaction combined with eikonal reaction theory. This observed discrepancy is consistent with the interpretation that only one of the predicted low-lying 0+ states in 40Mg is bound. A two-state mixing analysis describing two-proton knockout cross sections along N = 28 provides support for the interpretation of a prolate-deformed 40Mg ground state. [ABSTRACT FROM AUTHOR]

Establishing how and when large N/Z values require modified or new theoretical tools is a major quest in nuclear physics. Here we report the first measurement of the lifetime of the 2+I state in the near-dripline nucleus 20C. The deduced value of r2. = 9.8 ± 2.8(stat)+0.5-1.1syst) ps gives a reduced transition probability of B(E2; 2+I →. O+g) = 7.5 +3.0-1.7(stat)+1.0-0.4(stat) e2 fm4 in good agreement with a shell model calculation using isospin-dependent effective charges. [ABSTRACT FROM AUTHOR]

The superfluid tunneling model is applied to the calculation of half-lives of the observed a decays in N=84 isotones. Results of our calculations are compared to experimental data on the ground-state a decays along the isotonic chain from 144Nd to 159Re. Good agreement is found. The a decays of the known high-spin isomers in 155Lu, 156Hf, 157Ta, and 158W are also well reproduced, once a reduction in the pairing strength is taken into account. This includes reproduction of the main features of the recently observed fine structure from 155Lu(25/2-) and 156Hf(8+). Predictions for the α-decay fine structure of 157Ta(25/2-) and 158W(8+) high-spin isomers are presented. [ABSTRACT FROM AUTHOR]

The apparent splitting between orbitals that are spin-orbit partners can be substantially influenced by the effects of weak binding. In particular, such effects can account for the observed decrease in separation of the neutron 1p3/2 and 1p1/2 orbitals between the 41Ca and 35Si isotopes. This behavior has been the subject of recent experimental and theoretical works and cited as evidence for a proton "bubble" in 34Si causing an explicit weakening of the spin-orbit interaction. The results reported here suggest that the change in the separation between the 1p3/2 and 1p1/2 partners occurs dominantly because of the behavior of the energies of these 1p neutron states near zero binding. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

There is significant evidence supporting the existence of deformed ground states within the neutron-rich N≈20 neon, sodium, and magnesium isotopes that make up what is commonly called the "island of inversion." However, the rotational band structures, which are a characteristic fingerprint of a rigid nonspherical shape, have yet to be observed. In this work, we report on a measurement and analysis of the yrast (lowest lying) rotational band in 32Mg up to spin I=6+ produced in a two-step projectile fragmentation reaction and observed using the state-of-the-art γ-ray tracking detector array, GRETINA (γ-ray energy tracking in-beam nuclear array). Large-scale shell-model calculations using the SDPF-U-MIX effective interaction show excellent agreement with the new data. Moreover, a theoretical analysis of the spectrum of rotational states as a function of the pairing gap, together with cranked-shell-model calculations, provides intriguing evidence for a reduction in pairing correlations with increased angular momentum, also in line with the shell-model results. [ABSTRACT FROM AUTHOR]

We studied transition rates for the lowest 1/2+ and 5/2+ excited states of C17 through lifetime measurements with the GRETINA array using the recoil-distance method. The present measurements provide a model-independent determination of transition strengths giving the values of B(M1;1/2+→3/2g.s.+)=1.04-0.12+0.03×10-2µN² and B(M1;5/2+3/2g.s.+)=7.12-0.96+1.27×10-2µN². The quenched M1 transition strength for the 1/2+→3/2g.s.+ transition, with respect to the 5/2+3/2g.s.+ transition, has been confirmed with greater precision. The current data are compared to importance-truncated no-core shell model calculations addressing effects due to continuum and three-body forces. [ABSTRACT FROM AUTHOR]