R. W. Ibbotson

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.

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.

Proton scattering on radioactive proton sd-shell nuclei and comparison to Coulomb excitation

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 scat...

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.

Single-particle structure of radioactive beams from one-nucleon knockout reactions

Studies of the single-particle structure of radioactive beams produced in fragmentation reactions are described. The experiments are based on observing the individual final states of the projectile residues produced in one-nucleon knockout reactions. The measured partial cross sections to the various final states of the projectile residue and the shape of the corresponding longitudinal momentum distributions reflect the single particle properties, spectroscopic factors and the angular momentum l of the removed nucleon. Applications to 26,27P and 15C are discussed.

In-beam Coulomb-excitation studies of odd-A ν(fp)- and π(sd)-shell nuclei

A group of odd-mass neutron-rich nuclei with neutron excesses of 9–11 in the 13⩽Z⩽17 region has been produced and studied by in-beam intermediate-energy Coulomb excitation using an array of NaI(Tl) detectors. The excitation energies of the observed states and B(E2) values connecting these states to the ground-states have been measured. For the 41S and 43S cases the measurements have been compared to particle-rotor and particle-vibrator calculations. The measurements for 41S are consistent with an interpretation of the low-energy behavior of this nucleus as rotations of a deformed core, whereas for 43S no distinction can be made between the deformed (rotational) and spherical (vibrational) calculations.