. A segmented Si-telescope and HPGe array, STARS-LIBERACE, was used to study the 156 Gd(p , t γ ) 154 Gd direct reaction by particle- γ coincidence spectroscopy. New cross sections with a 25 MeV proton beam are reported and compared to previous (p, t) and (t, p) studies. Furthermore, additional evidence for coexisting K π = 0 +1 , 2 +1 and 0 +2 , 2 +2 configurations at N = 90 is presented. Direct and indirect population patterns of the low-lying states are also explored. Review of the new and existing evidence favors an interpretation based on a configuration-dependent pairing interaction. The weakening of monopole pairing strength and an increase in quadrupole pairing strength could bring 2p-2h 0 + states below 2 Δ . This may account for a large number of the low-lying 0 + states observed in two-nucleon transfer reactions. A hypothesis for the origin of the 0 +2 and 0 +3 states is provided.

Two low-lying neutron-unbound excited states of O-24, populated by proton-knockout reactions on F-26, have been measured using the MoNA and LISA arrays in combination with the Sweeper Magnet at the Coupled Cyclotron Facility at the NSCL using invariant mass spectroscopy. The current measurement confirms the separate identity of two states with decay energies 0.51(5) MeV and 1.20(7) MeV, and provides support for theoretical model calculations, which predict a 2(+) first excited state and a 1(+) higher-energy state. The measured excitation energies for these states, 4.70(15) MeV for the 2(+) level and 5.39(16) MeV for the 1(+) level, are consistent with previous lower-resolution measurements, and are compared with five recent model predictions.

Using the high-resolution Q3D magnetic spectrograph at the Maier-Leibnitz Laboratory (MLL) Tandem accelerator in Munich, we studied 0+ excitations in the mercury isotopes 198Hg, 200Hg, and 202Hg after two-neutron transfer. We only observed 4-6 excited 0+ states per nucleus up to about 3-MeV excitation energy, far fewer than in other experiments of this (p, t) campaign. The results reveal a sharp drop in the number of low-lying 0+ states towards the 208Pb shell closure. We discuss the low-energy 0+ state density as a function of the valence nucleon number Nval. The 0+ excitation energies and the measured (p, t) transfer cross sections indicate a structural change throughout the Hg isotopes, with the most notable result being the peaking in the cross section of the low-lying excited 02+ state in 200Hg.

We used the high-resolution Q3D magnetic spectrograph at the Maier-Leibnitz Laboratory (MLL) Tandem accelerator in Munich to study Hg-200 and Hg-202 after two-neutron transfer. The results confirm the sharp drop in the number of low-lying 0(+) states towards the Pb-208 shell closure. In total, we assigned six 0(+) states in Hg-200 and four 0(+) states in Hg-202. The 0(+) excitation energies and the measured (p, t) transfer cross sections indicate a structural change throughout the Hg isotopes, with the most notable result being the peaking in the cross section of the 0(2)(+) state in Hg-200.

We studied 0(+) states in Hg-198 after the Hg-200(p, t) Hg-198 transfer reaction up to 3 MeV excitation energy. The experiment was performed using the high-resolution Q3D magnetic spectrograph at the Maier-Leibnitz Laboratory Tandem accelerator in Munich. In total, only four 0(+) states were observed in Hg-198, significantly fewer than in other experiments of the (p, t) transfer campaign. We discuss the low-energy 0(+) state density as a function of the valence nucleon number N-val and test if the 0(+) density can be used as a signature for the prolate-oblate shape-phase transition in the Hf-Hg region. DOI: 10.1103/PhysRevC.87.024318

Proton removal reactions from a secondary 22N beam were utilized to populate unbound states in neutron-rich carbon isotopes. Neutrons were measured with the Modular Neutron Array (MoNA) in coincidence with carbon fragments. A resonance with a decay energy of 76(14) keV was observed in the system C18+n corresponding to a state in 19C at an excitation energy of 653(95) keV. This resonance could correspond to the first 5/2+ state which was recently speculated to be unbound in order to describe 1n and 2n removal cross section measurements from 20C.

A search for the neutron-unbound nucleus 21C was performed via the single-proton removal reaction from a beam of 22N at 68 MeV/u. Neutrons were detected with the Modular Neutron Array (MoNA) in coincidence with 20C fragments. No evidence for a low-lying state was found, and the reconstructed C20+n decay-energy spectrum could be described with an s-wave line shape with a scattering length limit of |as|<2.8fm, consistent with shell-model predictions. A comparison with a renormalized zero-range three-body model suggests that 22C is bound by less than 70 keV.

A segmented Si telescope and HPGe array is used to study the 156 Gd(p, d-γ) 155 Gd direct reaction by d-γ and d-γ-γ coincidence measurements using 25-MeV protons. The present investigation is the first time that this N = 91 nucleus and the N = 90 region—which is known for a rapid change from vibrational to rotational character, several low-lying 0 + states in the even-even nuclei, and large Coriolis (ΔΩ = 1) plus ΔN = 2 mixing in the even-odd nuclei—have been studied by particle-γ coincidence following a direct reaction with light ions. Gamma-ray energies and branches, excitation energies, angular distributions, and cross sections are measured for states directly populated in the (p, d) reaction. A new low-energy doublet state at 592.46 keV (previously associated with the K = 0 ⊗ 3/2 - [521] bandhead) and several new γ-ray transitions (particularly for states with excitation energies > 1 MeV) are presented. Most notably, the previous ν 7/2 + [404] systematics at and around the N = 90 transition region are brought into question and reassigned as ν 5/2 + [402]. This reassignment makes the ν 1/2 + [400], ν 3/2 + [402], and ν 5/2 + [402] orbitals, which originate from the 3s 1/2 , 2d 3/2, and 2d 5/2 spherical states, respectively, responsible for the three largest cross sections to positive-parity states in the (p, d) 155 Gd direct reaction. These three steeply upsloping orbitals undergo ΔN = 2 mixing with their N = 6 orbital partners, which are oppositely sloped with respect to deformation. The presence of these steeply sloped and crossing orbitals near the Fermi surface could weaken the monopole pairing strength and increase the quadrupole pairing strength of neighboring even-even nuclei, which would bring ν 2p-2h 0 + states below 2Δ. Indeed, this could account for a large number of the low-lying 0 + states populated in the (p, t) 154 Gd direct reaction.

Excited states in Pt-192 and Pt-194 have been studied with high-energy resolution, in the (p, t) reaction, by using the Q3D magnetic spectrograph at the Munich MP tandem accelerator. Outgoing tritons were recorded at laboratory angles sensitive to the unique shape of the L = 0 angular distribution. Sets of previously unknown 0(+) states were identified up to an energy of similar to 3 MeV. The 0(+) states in Pt-192,Pt-194 are discussed in the context of the evolution of intruder states and shape coexistence in the light Pt isotopes.

The $g$ factor of the ${2}_{1}^{+}$ state of $^{172}\mathrm{Hf}$ was measured using the perturbed angular correlation technique in a static external magnetic field. The result, $g({2}_{1}^{+})=0.25(5)$, is discussed in relation to the systematics of the previously reported $g$ factors in the Hf isotopes and compared with the predictions of several models. An interesting outcome of the analysis presented in this paper is the agreement between the calculated $g$ factors within the interacting boson approximation (IBA) and the results of a large-scale shell model calculation. This agreement supports the emphasis in the IBA on the valence space. The undershooting of the empirical $g$ factors near midshell in both models suggests that they underestimate the role of the saturation of collectivity, which is explicitly incorporated into a phenomenological model that agrees better with the data.

Recently, ${0}^{+}$ excitations, especially in the rare-earth region, were studied extensively. We extend this work by studying the excited ${0}^{+}$ states in $^{170}\mathrm{Yb}$ using the $^{172}\mathrm{Yb}$$(p,t)$$^{170}\mathrm{Yb}$ reaction. Eighteen excited ${0}^{+}$ states, 14 of which are new, are observed up to an energy of $3.5$ MeV. The results are analyzed using the $\mathit{sd}$ and $\mathit{spdf}$ interacting boson models.

The lifetimes of the J(pi)=4(+), 6(+), 8(+), and 10(+) levels along the ground state band in Hf-168 were measured by means of the recoil distance Doppler shift (RDDS) method using the New Yale Plunger Device (NYPD) and the SPEEDY detection array at Wright Nuclear Structure Laboratory of Yale University. Excited states in Hf-168 were populated using the Sn-124(Ti-48,4n) fusion evaporation reaction. The new lifetime values are sufficiently precise to clearly prove the increase of quadrupole deformation as a function of angular momentum in the deformed nucleus Hf-168. The data agree with the predictions from the geometrical confined beta-soft (CBS) rotor model that involves centrifugal stretching in a soft potential.

The g factor of the 2(1)(+) state of Hf-172 was measured using the perturbed angular correlation technique in a static external magnetic field. The result, g(2(1)(+))=0.25(5), is discussed in relation to the systematics of the previously reported g factors in the Hf isotopes and compared with the predictions of several models. An interesting outcome of the analysis presented in this paper is the agreement between the calculated g factors within the interacting boson approximation (IBA) and the results of a large-scale shell model calculation. This agreement supports the emphasis in the IBA on the valence space. The undershooting of the empirical g factors near midshell in both models suggests that they underestimate the role of the saturation of collectivity, which is explicitly incorporated into a phenomenological model that agrees better with the data.

Lifetimes of prolate intruder states in 186Pb and 188Pb and oblate intruder states in 194Po have been determined through recoil distance Doppler-shift lifetime measurements. Deformation parameters of |β2|=0.29(5) and |β2| = 0.17(3) have been extracted from experimental B(E2) values for the prolate and the oblate bands, respectively. The present study addresses the phenomenon of shape coexistence typical for the nuclei near Z=82 and N=104, providing information on configuration mixing of intrinsic structures of the nuclei of interest. The results are compared with the available lifetime data and theoretical results for neutron-deficient Po, Pb, Hg and Pt nuclei. Furthermore, new self-consistent mean-field calculations have been carried out for 194Po.

The ratio of B(E2) strengths between the lowest lying members of the ground state band in Nd-144, B-4/2 = B(E2;4(1)(+) -> 2(1)(+))/B( E2;2(1)(+) -> 0(1)(+)), which is used to quantify collectivity in nonmagic nuclei, has been remeasured by means of Coulomb excitation. Ambiguities in literature values for this ratio have been resolved. The results are discussed in terms of the interplay of collective and single-particle degrees of freedom, and in the context of the formation of collective structures in the A = 140 mass region, and more generally near (sub) shell closures. In addition, the B-4/2 ratio of Sm-148 has been remeasured, and the quadrupole moment of the 4(1)(+) state is found to be on the order of 1 e b.

Femtosecond time-resolved small and wide angle x-ray diffuse scattering techniques are applied to investigate the ultrafast nucleation processes that occur during the ablation process in semiconducting materials. Following intense optical excitation, a transient liquid state of high compressibility characterized by large-amplitude density fluctuations is observed and the buildup of these fluctuations is measured in real time. Small-angle scattering measurements reveal snapshots of the spontaneous nucleation of nanoscale voids within a metastable liquid and support theoretical predictions of the ablation process.