Olof Tengblad

Chemically selective laser ion-source for the CERN-ISOLDE on-line mass separator facility

Abstract Radioactive atoms produced in proton-induced nuclear reactions and released from thick targets have been ionized resonantly by laser radiation in a hot tube connected to the target container. Pulsed tuneable lasers with a repetition rate as high as 10 kHz have been applied for stepwise resonant excitation and photoionization in the last step. In this way the efficiency and selectivity of the target and ion source system which serves as an injector to the on-line isotope separators at CERN-ISOLDE could be improved. In a series of off-line and on-line studies the ionization of Sn ( E i = 7.3 eV), Tm ( E i = 6.2 eV), Yb ( E i = 6.2 eV) and Li ( E i = 5.4 eV) was investigated. An ionization efficiency of up to 15% was obtained for Yb. The ratio of the laser-ionized and surface-ionized ion currents was measured as a function of temperature for different ionization cavity materials (W, Ta, Nb and TaC). It was shown that this ratio, i.e. the selectivity, rises for Tm from 10 to 10000 with falling temperature and is strongly dependent on the material. Since the lasers are pulsed the ion beam becomes bunched with a pulse width of about 10–50 μs. This width is strongly dependent on the potential drop along the tube (caused by the electric current used for heating the tube) and on the alignment of the laser beams with respect to the tube axis. The selectivity could be further improved by a factor of 10 using gated detection of the bunched ion beam.

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

Accelerated radioactive beams from REX-ISOLDE

In 2001 the linear accelerator of the Radioactive beam EXperiment (REX-ISOLDE) delivered for the first time accelerated radioactive ion beams, at a beam energy of 2 MeV/u. REX-ISOLDE uses the method of charge-state breeding, in order to enhance the charge state of the ions before injection into the LINAC. Radioactive singly-charged ions from the on-line mass separator ISOLDE are first accumulated in a Penning trap, then charge bred to an A/q < 4.5 in an electron beam ion source (EBIS) and finally accelerated in a LINAC from 5 keV/u to energies between 0.8 and 2.2 MeV/u. Dedicated measurements with REXTRAP, the transfer line and the EBIS have been carried out in conjunction with the first commissioning of the accelerator. Thus the properties of the different elements could be determined for further optimization of the system. In two test beam times in 2001 stable and radioactive Na isotopes (Na-23-Na-26) have been accelerated and transmitted to a preliminary target station. There Ni-58- and Be-9- and H-2-targets have been used to study exited states via Coulomb excitation and neutron transfer reactions. One MINIBALL triple cluster detector was used together with a double sided silicon strip detector to detect scattered particles in coincidence with gamma-rays. The aim was to study the operation of the detector under realistic conditions with gamma-background from the beta-decay of the radioactive ions and from the cavities. Recently for efficient detection eight tripple Ge-detectors of MINIBALL and a double sided silicon strip detector have been installed. We will present the first results obtained in the commissioning experiments and will give an overview of realistic beam parameters for future experiments to be started in the spring 2002.

Positron-Neutrino Correlation in the 0 + → 0 + Decay of 32 Ar

4 pages, 1 table, 4 figures.-- PACS nrs.: 23.20.En, 13.30.Ce, 23.40.Bw, 24.80.+y.-- Erratum to this paper in: Phys. Rev. Lett. 83(15): 3101-3101 (1999).The positron-neutrino correlation in the 0[sup +][r arrow]0[sup +] [beta] decay of [sup 32]Ar was measured at ISOLDE by analyzing the effect of lepton recoil on the shape of the narrow proton group following the superallowed decay. Our result is consistent with the standard model prediction. For vanishing Fierz interference we find a=0.9989[plus minus]0.0052[plus minus]0.0039 , which yields improved constraints on scalar weak interactions. [copyright] [ital 1999] [ital The American Physical Society]

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).

8He-6He: a comparative study of nuclear fragmentation reactions

Dissociation of 227 MeV/u He-8 in a carbon target has been studied in kinematically complete experiments. The data include the relative energy spectrum, angular distributions in the neutron knock-out channel (He-6 + n) as well as diffractive dissociation and inelastic scattering into the (He-6 + 2n) channel. The data are compared with corresponding results from the well-known halo nucleus He-6. In both cases it is found that neutron knock-out is the: dominating reaction channel. The relative energy spectrum (He-6 + n) shows a structure, which is interpreted as being due to the I-pi = 3/2(-) resonance in the He-7 ground state with about equal contribution from its I-pi = 1/2(-) spin-orbit partner. The He-7 resonance shows a spin alignment similar to that observed in He-5, but with a smaller anisotropy indicating that the structure of the He-8 ground state is more complicated than that of He-6. The data in the (He-6 + 2n) channel were used to identify resonances in the excitation energy spectrum of He-8. If the spectrum is interpreted as two overlapping resonances, the spin-parity assignment for these is found to be 2(+) and 1(-), respectively.

Searching for the 5H resonance in the t+n+n system

19 pages, 7 figures, 2 tables, 2 appendices.-- PACS nrs.: 27.10.+h; 25.60.Gc.-- Printed version published Jul 28, 2003.

Beta-decay to the proton halo state in 17F

In the first experiment at the newly constructed ISOLDE Facility the first-forbidden beta-decay of Ne-17 into the first excited state of F-17 has been measured. It is a factor two faster than the corresponding mirror decay and thus gives one of the largest recorded asymmetries for beta decays feeding bound final states. Shell-model calculations can only reproduce the asymmetry if the halo structure of the F-17 state is taken into account.

The β2p decay mechanism of 31Ar

Abstract We have measured the beta-decay of 31 Ar with a high granularity setup sensitive to multiparticle decay branches. Two-proton emission is observed from the isobaric analog state in 31 Cl to the four lowest states in 29 P and furthermore from a large number of states fed in Gamow–Teller transitions. The mechanism of two-proton emission is studied via energy and angular correlations between the two protons. In all cases the mechanism is found to be sequential yielding information about states in 30 S up to 8 MeV excitation energy. Improved data on the β -delayed one-proton branches together with the two-proton data provide precise information about the beta-strength distribution up to 15 MeV excitation energy.

Invariant mass spectrum and alpha-n correlation function studied in the fragmentation of He-6 on a carbon target

Momentum distributions and invariant mass spectra from the breakup of He-6 ions with an energy of 240 MeV/u interacting with a carbon target have been studied. The data were used to extract information about the reaction mechanism which is influenced by the structure of He-6. It is found that the dominant reaction mechanism is a two-step process: knock out of one neutron followed by the decay of the He-5 resonance. The shape of the (alpha+n) two-body invariant mass spectrum is interpreted as mainly reflecting the 5He ground state which is a J(pi) = 3/2(-) resonance. However, no evidence for correlations between cu particles and neutrons is observed in the momentum widths of the distributions. It is demonstrated that a combined analysis of the two-body invariant mass spectrum and an appropriate correlation function may be used to determine the properties of the intermediate resonance