H. Scheit

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.

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.

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.

Proton scattering on the radioactive nucleus 20O and the 0gs+→21+ transition in the neutron-rich oxygen isotopes

Abstract Elastic and inelastic proton scattering to the 21+ state of the single closed shell radioactive nucleus 20 O have been measured in inverse kinematics with a beam energy of 30 MeV/u. The matrix element determined for the 0gs+→21+ transition in this work is compared with the corresponding electromagnetic matrix element to determine the ratio of the neutron and proton multipole matrix elements for this transition, Mn/Mp=2.9(4), which is quite different from the value Mn/Mp=N/Z=1.5 that would be expected for a purely isoscalar transition. A comparison of this result to a corresponding result for 18 O suggests that the strength of the core polarizing interactions between the valence neutrons and the core protons and neutrons is changing significantly with mass.

A position sensitive high-efficiency NaI(Tl) photon-detection system for use with intermediate energy radioactive ion beams

Abstract We have built an array of position-sensitive NaI(Tl) photon detectors for use in radioactive beam experiments at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University. The array consists of 38 cylindrical NaI(Tl) detectors oriented co-axially in three concentric rings around the particle-beam axis. Each crystal has a diameter of 5xa0cm, is 17xa0cm long and is read out by two photomultiplier tubes, one at each end of the crystal. Because of the position-dependent attenuation of the scintillation light in the crystal it is possible to determine both the photon energy and the interaction point of the photon in the detector. This photon detection system is very well suited for the investigation of exotic nuclei which, at the NSCL, are produced in fragmentation reactions. Exotic beams produced by this method have high velocities and potentially low particle rates. Both of these properties are addressed in the described setup. The Doppler shift due to the large beam velocities is corrected on an event-by-event basis using the known interaction positions of the photons in the detector. Low particle rates can be accommodated with particle–γ coincidence measurements and the high efficiency of the array.

Exploring Neutron-Rich Oxygen Isotopes with MoNA

The Modular Neutron Array (MoNA) was used in conjunction with a large‐gap dipole magnet (Sweeper) to measure neutron‐unbound states in oxygen isotopes close to the neutron dripline. While no excited states were observed in 24O, a resonance at 45(2) keV above the neutron separation energy was observed in 23O.

Inelastic proton scattering on the radioactive nuclei 18Ne and 20O in inverse kinematics

Elastic and inelastic scattering to the 21+ state of the single closed shell radioactive nuclei 18Ne and 20O have been measured in inverse kinematics with a beam energies of 30 MeV/u. The matrix element determined for the 0gs+→21+ transitions in these reactions are compared with the corresponding electromagnetic matrix elements to determine Mn/Mp, the ratio of the neutron and proton multipole matrix elements. A comparison between the Mn/Mp values in 20O and 18O suggests that the matrix element ratio is increasing as the neutron number increases. The Mn/Mp result for 18Ne is consistent with one obtained from a comparison of electromagnetic data from the mirror nuclei 18O and 18Ne.

Two-proton decay of the first excited state of 17Ne

The first excited state of 17Ne has been populated via intermediate energy Coulomb excitation with a radioactive beam of 17Ne on a 197Au target to search for two-proton decay, which is in competition with the γ-decay back to the ground state of 17Ne. The reconstructed invariant mass spectrum of the outgoing 15O in coincidence with two protons shows evidence for a two-proton transition from the first excited state in 17Ne as well as for transitions from higher excited states in 17Ne, which will decay via the emission of two sequential protons.

Intermediate-energy Coulomb excitation of 28,29,30,31Na

The neutron-rich radioactive isotopes {sup 28,29,30,31}Na have been produced by nuclear fragmentation of {sup 48}Ca and {sup 40}Ar 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 {sup 197}Au target. Experimental results suggest a large degree of collectivity in {sup 31}Na. Intrinsic ground state and transition quadrupole moments in {sup 28,29,30,31}Na are compared.

Mass dependence of the effective isovector charge in the sd-shell

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*=2206u200akeV) and the ground state (0gs+) 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.