T. Glasmacher

Collectivity in 44S

Abstract The energy and reduced transition probability B(E2; 0g.s.+ → 2+) for the lowest excited state in the neutron-rich isotope 1644S28 were measured by intermediate-energy Coulomb excitation. The excitation energy is E(2+) = 1297(18) keV and the reduced transition probability is B(E2; 0g.s.+ → 21+) = 314(88) e2fm4. The experimental results are compared with self-consistent mean-field calculations and shell model calculations with empirical interactions. The shell model calculations indicate that the large B(E2) value in 44S is vibrational in origin, while the neighboring isotopes 40,42S are statically deformed.

Thirty-two-fold segmented germanium detectors to identify γ-rays from intermediate-energy exotic beams

Abstract Thirty-two-fold segmented coaxial high-purity large-volume germanium detectors for use in intermediate-energy radioactive-ion-beam experiments have been developed and tested. This high degree of segmentation will allow a precise localization of the point of photon interaction in the detector, thus allowing accurate doppler reconstruction of the energy of a γ-ray emitted in flight. In this article we report on the design of these detectors and their operational characteristics.

Role of intruder configurations in 26,28Ne and 30,32Mg

Abstract The energies and reduced transition probabilities B(E2;0g.s.+→2+) for the lowest Jπ=2+ excited states in the neutron-rich radioactive isotopes 26,28 Ne and 30,32 Mg were measured via intermediate energy Coulomb excitation. The data suggest that the interaction between coexisting 0ℏω (normal) and 2ℏω (intruder) configurations significantly perturbs the energy of the 21+ state in 28 Ne, while the data on 26 Ne and 30 Mg can be well understood in the context of 0ℏω configurations alone. Nilsson model calculations suggest that if these nuclei have static axially symmetric deformations, they are prolate.

Structure of 52,54Ti and shell closures in neutron-rich nuclei above 48Ca

Abstract The level structure of 5422Ti32 has been explored for the first time by combining β-decay measurements from fragmentation products with prompt γ-ray spectroscopy following deep inelastic reactions. The latter technique was also instrumental in tracing 52Ti30 to higher spin. The data provide new tests of effective interactions for full pf-shell calculations in neutron-rich nuclei above 48Ca. The data indicate the presence of a significant subshell gap at N=32 and comparisons between theory and experiment suggest an additional shell closure at N=34 in Ca and Ti isotopes.

Onset of intruder ground state in exotic Na isotopes and evolution of the N=20 shell gap

The onset of intruder ground states in Na isotopes is investigated by comparing experimental data and shell-model calculations. This onset is one of the consequences of the disappearance of the N=20 magic structure, and the Na isotopes are shown to play a special role in clarifying the change of this magic structure. Both the electromagnetic moments and the energy levels clearly indicate an onset of ground state intruder configurations at neutron number N=19 already, which arises only with a narrow N=20 shell gap in Na isotopes resulting from the spin-isospin dependence of the nucleon-nucleon interaction (as compared to a wider gap in stable nuclei like {sup 40}Ca). It is shown why the previous report based on the mass led to a wrong conclusion.

Statistical Multifragmentation in Central Au+Au Collisions at 35 MeV/u

Abstract Multifragment disintegrations, measured for central Au + Au collisions at E A = 35 MeV , are analyzed with the Statistical Multifragmentation Model. Charge distributions, mean fragment energies, and two-fragment correlation functions are well reproduced by the statistical breakup of a large, diluted and thermalized system slightly above the multifragmentation threshold.

Spectroscopy of the 21+ state in 22O and shell structure near the neutron drip line

Abstract The energy and electromagnetic matrix element B(E2↑) for the 21+ state in 22O have been determined via inelastic scattering of a beam of these radioactive nuclei from a 197Au target. These results provide strong evidence for the existence of the N=14 subshell closure in 22O. This demonstrates that the shell structure in 22O is similar to that of stable oxygen isotopes, even though this nucleus is only two neutrons away from the neutron drip line. These results are reproduced in the standard sd shell model as well as with a an expanded space which also includes the f7/2 and p3/2 orbitals.

Low-lying collective states in neutron-rich oxygen isotopes via proton scattering ☆

Abstract Proton elastic and inelastic scattering angular distributions to the 2+1 and the 3−1 states for the neutron-rich nucleus 20 O were measured with a secondary beam using the MUST silicon strip detector array. Data for 18 O were also obtained for comparison. A phenomenological analysis has been used to deduce the deformation parameters βp,p′ for the collective excitations. Matter and transition densities were generated from self-consistent QRPA calculations. DWBA calculations using microscopic optical potentials obtained with these densities and the JLM interaction are compared to the data. The isovector character of the 2+1 state in 20 O is confirmed and predictions are discussed for the properties of the heavier neutron-rich oxygen isotopes.

Z = 50 shell gap near 100Sn from intermediate-energy Coulomb excitations in even-mass 106-112Sn isotopes.

Rare isotope beams of neutron-deficient 106,108,110Sn from the fragmentation of 124Xe were employed in an intermediate-energy Coulomb excitation experiment. The measured B(E2,0(1)(+)-->2(1)(+)) values for 108Sn and 110Sn and the results obtained for the 106Sn show that the transition strengths for these nuclei are larger than predicted by current state-of-the-art shell-model calculations. This discrepancy might be explained by contributions of the protons from within the Z = 50 shell to the structure of low-energy excited states in this region.

'Magic' nucleus 42Si.

Nuclear shell structures—the distribution of the quantum states of individual protons and neutrons—provide one of our most important guides for understanding the stability of atomic nuclei. Nuclei with ‘magic numbers’ of protons and/or neutrons (corresponding to closed shells of strongly bound nucleons) are particularly stable. Whether the major shell closures and magic numbers change in very neutron-rich nuclei (potentially causing shape deformations) is a fundamental, and at present open, question. A unique opportunity to study these shell effects is offered by the 42Si nucleus, which has 28 neutrons—a magic number in stable nuclei—and 14 protons. This nucleus has a 12-neutron excess over the heaviest stable silicon nuclide, and has only one neutron fewer than the heaviest silicon nuclide observed so far. Here we report measurements of 42Si and two neighbouring nuclei using a technique involving one- and two-nucleon knockout from beams of exotic nuclei. We present strong evidence for a well-developed proton subshell closure at Z = 14 (14 protons), the near degeneracy of two different (s1/2 and d3/2) proton orbits in the vicinity of 42Si, and a nearly spherical shape for 42Si.