L. Johannsen

Observation of forward neutrons from the break-up of the 11Li neutron halo

Abstract The cross-sections and forward angular distributions of fast neutrons have been studied for the reaction (11Li, 9Li) at 29 MeV/u on targets of Be, Ni and Au. The cross-sections are large and arise essentially from peripheral reactions. The angular distributions of the neutrons are very narrow and indicate the existence of a neutron halo with radius of about 12 fm.

The 11Li nucleus as a three-body system

Abstract It is shown that a variational three-body calculation with two neutrons interacting with a core via weak, short-range interactions leads to a correlated two-neutron bound state. The most probable separation between the neutrons is 3.3 fm and their most probable centre of mass distance from the 9 Li core is 3.1 fm. The resulting 11 Li system is consistent with the experimental information about radius, energy, momentum distribution of the 9 Li core and also with the cross section for Coulomb dissociation.

Study of charged particles emitted in the β-decay of 6,8He

The beta-delayed charged particle spectra from He-6 and He-8 were measured with a telescope consisting of a gas counter and a Si surface barrier detector. Beta-delayed deuterons and tritons are emitted in the two cases with branching ratios of (7.6 +/- 0.6) x 10(-6), E(d) > 350 keV and (8.0 +/- 0.5) x 10(-3), respectively. Both spectra present anomalies that must be due to the special structure of these nuclei (an alpha core surrounded by several neutrons).

First observation of beta-delayed deuteron emission

Abstract Beta-delayed deuteron emission has been observed in the decay of 6 He with a branching ratio of (2.8±0.5)×10 −6 . Simple theoretical estimates predict an intensity that is significantly higher.

Super-Allowed Beta-Decay of Nuclei at the Drip-Line

We have investigated the continuous particle spectra following the beta decay of the neutron-rich nuclei that terminate the mass 6, 8, 9 and 11 isobaric chains. Strong beta transitions feeding the very top of the spectrum are found systematically. The favoured states are within a few MeV of the energy of the initial state. This phenomenon seems to be linked to the occurrence of a di-neutron structure or neutron halo in the drip-line nuclei.

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.

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.

Neutron Momentum Distributions from Core Break-up Reactions of Halo Nuclei

Neutron angular distributions from violent break-up reactions of Li-11 and Be-11 have been measured at 28 MeV/u and 280 MeV/u and at 41 MeV/u and 460 MeV/u, respectively. The derived neutron momentum distributions show a narrow component in transverse momentum that is within uncertainties independent of beam energy and target charge. This component is suggested to be simply related to the momentum distribution of the loosely bound halo neutron(s) in the projectiles.

Study of neutron rich neon isotopes

The half-lives andPn-values of the neutron rich isotopes26–29Ne have been determined. The results are compared to shell-model calculations and good agreement is found except for29Ne, where the half-life exceeds the predictions by more than an order of magnitude. This unexpectedly long half-life can be explained as due to a fp intruder configuration.

Candidates for electron-neutrino mass measurements. the case of 81Kr

The nuclear data of the known candidates for neutrino mass determination by resonant electron-capture beta decay are reviewed. New results are presented for the isotope 81Kr, where the transition to the 276 keV state has been found to have QEC=4.7±0.5 keV and a branching ratio of (3.0±0.2)×10−3