[en] The surprising absence of the N=34 subshell closure in the exotic nucleus ⁵⁶Ti is highlighted by examining the experimental single neutron energies along the N=28 isotopes, which are extracted from previously existing data on single-neutron transfer reactions on stable isotones and nuclear masses. The results point out the role of the monopole effect of the tensor force on the f_7/2-f_5/2 splitting and confirm the prediction of the reduction of this splitting in the Z>20 isotones. In addition, the importance of making similar measurements of the distribution of single neutron and single neutron hole strength in the exotic Z<20 N=28 isotones is discussed. Measurements of the (d,p) reaction and the neutron knockout reaction on the exotic nucleus ⁴⁶Ar have provided proof of principle for the necessary reactions, and advances presently taking place in detector technology will soon provide the tools necessary for reliably mapping the distribution of neutron strength in detail

[en] The evidence on single proton energies in the Si isotopes is traced from N=12 to N=28 using available experimental data on binding energies, single proton transfer reactions, radioactive decay, and direct reaction studies with neutron-rich exotic beams. In addition to demonstrating the value of having information on single particle energies over a long sequence of isotopes, the data point out a significant discrepancy between theoretical and experimental results on the size of the Z=14 subshell closure in ³⁴Si, which is located on the boundary of the island of inversion

[en] The neutron-rich radioactive isotopes ^28,29,30,31Na have been produced by nuclear fragmentation of ⁴⁸Ca and ⁴⁰Ar primary beams at intermediate energies. The energies and excitation cross sections to the lowest excited states were measured via intermediate-energy Coulomb excitation on a ¹⁹⁷Au target. Experimental results suggest a large degree of collectivity in ³¹Na. Intrinsic ground state and transition quadrupole moments in ^28,29,30,31Na are compared

[en] The structure of the ¹⁸O nucleus at excitation energies above the α decay threshold was studied using ¹⁴C+α resonance elastic scattering and (⁷Li,t) a-transfer reactions. A number of states with large α reduced widths have been observed, indicating that the a-cluster degree of freedom plays an important role in this N≠Z nucleus. A 0⁺ state with an α reduced width exceeding the single particle limit was identified at an excitation energy of 9.9±0.3 MeV. We discuss evidence that states of this kind are common in light nuclei and give possible explanations of this feature. Also, the astrophysical implications of the cluster structure of ¹⁸O is discussed.

[en] Systematic measurements of the first excited states in neutron-rich nuclei in the π(sd)-shell have been performed in the last two years using the technique of intermediate energy Coulomb excitation. These experiments were designed to study the evolution of nuclear shell structure as the neutron-dripline is approached and to provide experimental data which can be easily compared to nuclear model predictions. These measurements indicated a breakdown of the N=20 shell closure for ³²Mg and a weakening of the N=28 shell closure below ⁴⁸Ca, as well as a region of moderate collectivity around ^40,42S. However, this experimental technique is sensitive only to the electromagnetic response of the nucleus. To probe the neutron distributions we performed a series of proton scattering experiments in inverse kinematics on neutronrich sulfur and argon isotopes. Proton scattering at energies of about 30 MeV/nucleon is more sensitive to neutrons than to protons by about a factor of three. The combination of the proton scattering results with the Coulomb excitation measurements allows a determination of the ratio of the neutron transition matrix element M_n to the proton transition matrix element M_p. Deviations from the homogeneous collective model expectation M_n/M_p=N/Z can be indicative of isovector deformation predicted to occur for neutron-rich sulfur isotopes. Proton scattering experiments and Coulomb excitation experiments on the same isotopes have been performed at the NSCL with radioactive beams produced in the A1200 fragment separator and protons detected in an array of Si strip-Si-CsI telescopes

[en] In order to map out a possible mass and isospin dependence of the effective charges in the sd-shell, we performed an intermediate energy Coulomb excitation experiment with the semi-magic, proton-rich nucleus ³⁸Ca. The transition between the first excited state (2₁⁺, E*=2206 keV) and the ground state (0_gs⁺) was studied, and the reduced transition strength and the proton transition matrix element were deduced. The nucleus ³⁸Ca was chosen for this study, as it is at the upper mass end of the sd-shell, and will together with already existing high precision data for the lower mass end of the shell most clearly show a systematic trend in the effective charges. Our study also helps to fill the void of available high quality data for proton-rich nuclei in this mass region. We compare our data to theoretical shell model predictions with mass dependent, and mass independent effective charges, and to experimental values of the neutron-rich mirror nucleus ³⁸Ar

[en] Measurements of the N=28 isotones ⁴²Si, ⁴³P, and ⁴⁴S using one- and two-proton knockout reactions from the radioactive beam nuclei ⁴⁴S and ⁴⁶Ar are reported. The knockout reaction cross sections for populating ⁴²Si and ⁴³P and a 184 keV γ-ray observed in ⁴³P establish that the d_3/2 and s_1/2 proton orbits are nearly degenerate in these nuclei and that there is a substantial Z=14 subshell closure separating these two orbits from the d_5/2 orbit. The increase in the inclusive two-proton knockout cross section from ⁴²Si to ⁴⁴S demonstrates the importance of the availability of valence protons for determining the cross section. New calculations of the two-proton knockout reactions that include diffractive effects are presented. In addition, it is proposed that a search for the d_5/2 proton strength in ⁴³P via a higher statistics one-proton knockout experiment could help determine the size of the Z=14 closure

[en] Two-neutron knockout reactions from nuclei in the proximity of the proton dripline have been studied using intermediate-energy beams of neutron-deficient ³⁴Ar, ³⁰S, and ²⁶Si. The inclusive cross sections, and also the partial cross sections for the population of individual bound final states of the ³²Ar, ²⁸S and ²⁴Si knockout residues, have been determined using the combination of particle and γ-ray spectroscopy. Similar to the two-proton knockout mechanism on the neutron-rich side of the nuclear chart, these two-neutron removal reactions from already neutron-deficient nuclei are also shown to be consistent with a direct reaction mechanism

[en] The γ-ray spectroscopy of ²⁵Si and ²⁹S has been performed using single neutron knockout reactions with intermediate energy beams of the exotic isotopes ²⁶Si and ³⁰S. Two γ rays have been observed in ²⁵Si and three in ²⁹S. These are the first γ rays observed in these two isotopes. These two nuclei appear to be well deformed, and possible future intermediate-energy Coulomb excitation measurements would confirm their rotational nature.

[en] We have recently studied the structure of the neutron rich sulfur isotope ⁴⁰S by using elastic and inelastic proton scattering in inverse kinematics. Optical potential and folding model calculations are compared with the elastic and inelastic angular distributions. Using coupled-channel calculations, the β₂ value for the 2₁⁺ excited state is determined to be 0.35±0.05. The extracted value of M_n/M_p ratio indicates a small isovector contribution to the 2₁⁺ state of ⁴⁰S. The microscopic analysis of the data is compatible with the presence of a neutron skin for this nucleus