J. A. Brown

Production of A = 6,7 nuclides in the α+α reaction and cosmic ray nucleosynthesis

Cross sections for production of 6He, 6Li, 7Li, and 7Be in the alpha+alpha reaction were measured at bombarding energies of 159.3, 279.6, and 619.8 MeV, and are found to decrease rapidly with increasing energy. These cross sections are essential for the calculation of the rate of nucleosynthesis of the lithium isotopes in the cosmic rays and thereby play a key role in our understanding of the synthesis of Li, Be, and B. The results for 6Li differ significantly from the tabulated values commonly used in cosmic-ray production calculations and lead to lower production of 6Li.

REACTION CROSS SECTIONS IN SI OF LIGHT PROTON-HALO CANDIDATES 12N AND 17NE

Abstract Total reaction cross sections, σ R , on Si were measured near 40 A MeV for the proton-halo candidate 12 N and the two-proton-halo candidate 17 Ne, and were compared with σ R for other light proton-rich nuclei. The A -dependence shows enhanced σ R s for 12 N and 17 Ne, relative to their neighbors, but the effect is smaller than for 8 B which has been argued to have a proton halo. In general, nuclei with loosely bound last protons ( S p ⩽ 1.5 MeV) have significantly larger σ R s than their neighbors. Cross sections for charge-removal from 12 N and 17 Ne also were obtained.

Isovector giant resonances in 6He, 12B, 90Y, 120In, and 208Tl observed in the (7Li,7Be) charge-exchange reaction

Abstract The (7Li,7Be) and (7Li,7Beγ) reactions have been studied at bombarding energies of 50 A MeV on targets of 6Li, 12C, 90Zr, 120Sn, 208Pb, and polystyrene. The detection of Doppler-shifted γ-rays from excited 7Be permits the identification of the spinflip and non-spinflip characteristics. Transitions to over 20 discrete states and resonances have been observed. Angular distributions for 6Li and 12C are well described by microscopic one-step distorted-waves calculations with shell-model transition amplitudes. The transitions to the ground states of 6He and 12C exhibit significant Gamow-Teller strength, but ∼7% and ∼17%, respectively, of the calculated cross sections at θ ≈ 0° arise from the tensor interaction. A broad resonance at ∼5.6 MeV in the halo nucleus 6He with Jπ = (2+,1−,0+) seems to correspond to a structure predicted at this excitation energy containing soft modes of excitation with Jπ = 0+and/or 2+. A resonance in 12B at ∼4.5 MeV is described by a superposition of known 2− and 4− states, and another one at ∼7.5 MeV is dominated by a transition to a 2− state with weaker contributions from two 1− states (spin dipole and giant dipole). Four to six resonances each, mostly spin-dipole resonances, were identified in the final nuclei 90Y, 120In, and 208Tl. The non-resonant quasi-free charge-exchange reaction pbound(7Li,7Be)nfree was observed for all targets up to effective excitation energies of 50 MeV. The mechanism is related to that on free protons. The effective number of participating protons was determined.

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.

Elastic and inelastic scattering of protons on 38S in inverse kinematics

Abstract Elastic and inelastic scattering of protons on unstable 38 S nuclei has been measured in inverse kinematics by using a 38 S secondary beam at 39 MeV/nucleon. The excitation energy spectrum is measured up to 5 MeV. The first 2 + state is seen at 1.2 MeV excitation energy, and strength at around 3–5 MeV is also observed. Angular distributions for inelastic and elastic scattering are shown and compared to optical potential and folding model calculations. The preliminary β 2 value extracted for the first 2 + state is 0.38±0.05.

Proton scattering on

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 Mn/Mp 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.

Coulomb excitation of neutron rich light nuclei

We have produced beams of light neutron rich nuclei (Z<20) in the A1200 fragment separator at the NSCL by fragmenting high intensity 48Ca beams (E=70 and 80 MeV/nucl.) in a 379u2009mg/cm2 beryllium target. The exotic beams then impinged on a secondary gold target where they were excited in the Coulomb field of the target. The nuclei de-excite from a bound state by emission of a discrete photon, thus establishing the existence and energy of this state. Using a high efficiency position sensitive photon spectrometer we were able to detect photons from eight even-even nuclei far from the valley of stability (44,46Ar, 40,42,44S, 34,36,38Si) and to correct for the considerable Doppler shifts and Doppler broadening of the photo peaks.

Inelastic proton scattering on Neutron-Rich sulphur isotopes

The proton scattering on unstable38S nuclei has been measured in inverse kinematics by using a38S secondary beam at 39 MeV/nucleon. The first 2+ state is seen at 1.2 MeV excitation energy and its angular distribution is extracted. Optical and folding model calculations are compared with the elastic and the 21+ angular distributions and the β2 value is determined to be 0.35±0.04. This value is compared with the corresponding result from a Coulomb excitation measurement and theMn/Mp value is extracted.