G. Schrieder

Invariant-mass spectroscopy of 10Li and 11Li

Break-up of secondary Li-11 ion beams (280 MeV/nucleon) on C and Pb targets into Li-9 and neutrons is studied experimentally. Cross sections and neutron multiplicity distributions are obtained, characterizing different reaction mechanisms. Invariant-mass spectroscopy for Li-11 and Li-10 is performed. The E1 strength distribution, deduced from electromagnetic excitation of Li-11 up to an excitation energy of 4 MeV comprises similar to 8% of the Thomas-Reiche-Kuhn energy-weighted sumrule strength. Two low-lying resonance-like structures are observed for Li-10 at decay energies of 0.21(5) and 0.62(10) MeV, the former one carrying 26(10)% of the strength and likely to be associated with an s-wave neutron decay. A strong di-neutron correlation in Li-11 can be discarded. Calculations in a quasi-particle RPA approach are compared with the experimental results for Li-10 and Li-11

Exclusive and restricted-inclusive reactions involving the 11Be one-neutron halo

Reactions of a 41 MeV/u beam of the radioactive halo nucleus Be-11 have been studied with a counter telescope coupled to an array of neutron detectors covering angles up to 97-degrees. The technique allows to determine single-neutron inclusive and exclusive angular distributions. The targets (Be, Ti and Au) were chosen to illustrate the relative roles played by nuclear and Coulomb mechanisms. The channels leading to Be-10, the dissociation channels, correspond to impact parameters larger than the sum of the radii of the target and the Be-10 core. It is shown that for the dissociation process it is possible to account almost quantitatively for the integral, single- and double-differential cross sections from models without free parameters including the Coulomb, Serber and Glauber (diffraction-dissociation) mechanisms. The neutron distributions from the nondissociative reaction channels show little individuality and it is convenient to group them together as the channel neutron plus anything different from Be-10. We refer to these as restricted-inclusive reactions. These seem to be a promising tool for obtaining accurate information on the halo wave function in momentum coordinates.

Longitudinal and transverse momentum distributions of 9Li fragments from break-up of 11Li

Transverse and longitudinal momentum distributions of Li-9 fragments from Li-11 break-up reactions in C, Al and Pb targets have been measured at 280 MeV/u. The two-neutron removal cross-section was measured to be sigma(-2n), = 0.26 +/- 0.02 b for the carbon target, sigma(-2n) = 0.47 +/- 0.08 b for the aluminum target and sigma(-2n), = 1.9 +/- 0.4 b for the lead target. No significant difference is observed between the narrow widths (FWHM approximate to 47 MeV/c) of the transverse and longitudinal momentum distributions of the Li-9 fragments. The physical implications of this are discussed.

Probing the 11Li halo structure through β-decay into the 11Be∗(18 MeV) state

An experimental study of beta-delayed charged particles (H, He, Be) from Li-11 shows a beta-feeding to an excited state at 18.15(15) MeV in Be-11 with a large Gamow-Teller strength, B-GT greater than or equal to 1.6. Branching ratios and reduced widths of 2-, 3- and 5- particle decay channels of this state are extracted, A strong suppression of the neutron branch from the 18 MeV state to the Be-10(g.s.) is observed, We suggest that the measured partial width may be sensitive to the (p(1/2))(2) component in the Li-11 halo, A rough estimate indicates that the (p(1/2))(2) component is not the dominant one in the halo wave-function.

Two-neutron removal reactions for very neutron-rich nuclei

Abstract The two-neutron removal reactions of beams of 11 Li, 14 Be and 8 He upon Be, Ni and Au targets were studied at 30 MeV/u. The cross sections and the neutron forward angular distributions were measured; they correlate strongly with the two-neutron separation energy of the projectile. Even though the coverage of the neutron detectors was limited, a rough neutron-neutron distribution could be extracted. A simplified interpretation of the data is presented.

Dissociation reactions of the 11Be one-neutron halo. The interplay between structure and reaction mechanism

The angular distributions of the forward neutrons in the exclusive (Be-10 + n) channel have been measured. They can be accounted for quantitatively and without free parameters in terms of Coulomb and diffraction dissociation. The results show that the transverse momentum distributions result from an interplay between the tail of the wave function (the halo) and the reaction mechanism.

Quasi-elastic nucleon transfer and single charge exchange in 48Ti + 42Ca collisions

Abstract Single nucleon transfer and single charge exchange have been studied for quasi-elastic collisions of 48 Ti and 42 Ca at E lab = 240, 300 and 385 MeV. Specific features below 12 MeV excitation energy are displayed in all of the measured particle energy spectra for each channel. These have been accounted for in the case of nucleon transfer by a one-step direct transfer mechanism in which all of the available hole and bound particle states in the donor and acceptor nucleus, respectively, have been included. This core excitation model was extended to single charge exchange (SCX) two-step transfers in which a proton and a neutron are exchanged. The absolute magnitude and distribution of strength in the SCX channel was obtained. The contributions from direct charge exchange via the isovector parts of the NN interaction has also been investigated in a microscopic model and is found to contribute of the order of 1–5% to the charge exchange cross section. The distribution of Fermi and Gamow-Teller strength is discussed.

New information on b-delayed neutron emission from 12,14Be

The β-delayed neutron branches from neutron-rich Be-isotopes have been measured with a setup sensitive down to thermal energies. For 14Be a new value of Pn = 101 ± 4% has been found for the total neutron-emission probability and an upper limit of 2.4% (95% confidence level) is imposed on the combination P2n + 3P3n of multi-neutron emission probabilities. Moreover, an improved value of Pn = 0.50 ± 0.03% has been obtained for 12Be. A time-correlation method, previously developed for a continuous radioactive beam to determine neutron branching ratios, has been revised and adapted to the pulsed secondary beam at ISOLDE, CERN.

6He and neutron momentum distributions from 8He in nuclear break-up reactions at 240 MeV/u

Neutron and He-6 momentum distributions from He-8 break-up reactions in a C target have been measured at 240 MeV/u. The two-neutron removal cross section was found to be sigma(-2n) = 0.27 +/- 0.03 b. The nature of the momentum distributions is interpreted in some simple reaction scenarios.

Isoscalar multipole strength decomposition in 40Ca with the (e,e′α0) reaction

Abstract A multipole decomposition of the angular correlation functions of the reaction 40Ca(e,e′, α)36Arg.s. in the excitation energy range Ex = 10 – 20 MeV is presented. The response in 40Ca is dominated by isoscalar E0 and E2 strength exhausting 12.8(2.6)% and 8.4(1.7)% of the respective energy-weighted sum rules. The deduced energy distributions are in qualitative agreement with results from α-induced coincidence experiments on 40Ca, but the transition strengths are about a factor of two smaller. Some possible origins of this discrepancy are discussed.