Piet Van Duppen

Beams of short lived nuclei produced by selective laser ionization in a gas cell

Abstract An on-line laser ion source has been developed for the production of elemental and isobaric pure beams of radioactive ions. It is based on selective resonant laser ionization of nuclear reaction products thermalized and neutralized in a noble gas at high pressure. The ion source has been tested in a wide range of recoil energies going from 1.3 MeV to ∼ 90 MeV. Efficient schemes of two step laser ionization through autoionizing states have been found for nickel, cobalt and rhodium. Residence times of the reaction products in a gas cell have been measured for helium and argon as buffer gas. Elementally pure beams of 54,55Ni and 54Co, produced in a light-ion induced fusion-evaporation reaction, and of 113Rh, produced in proton-induced fission of 238U, were obtained. An efficiency of the ion source of 6.6% for fusion reactions and of 0.22% for fission reactions has been obtained. A selectivity of the ion source of 300 for fusion and 50 for fission reactions has been achieved.

A gas cell for thermalizing, storing and transporting radioactive ions and atoms. Part II: On-line studies with a laser ion source

Abstract The application of a gas cell filled with noble gas (helium or argon) for thermalizing, storing and transporting trace radioactive ions and atoms has been studied in on-line conditions. Radioactive ions produced in nuclear reactions and stable energetic ions have been resonantly re-ionized by laser light via a two-step resonant process after thermalization and neutralization in high-pressure noble gas. The influence of the ion–electron density created by the projectile beam on the recombination of exotic ions has been investigated in different experimental conditions, including DC and RF electrical fields in the gas cell. Results for the laser ion source efficiency and selectivity for heavy-ion induced fusion reaction products are given. For the Rh isotopes the efficiency reaches up to 12%.

Intensity limitations of a gas cell for stopping, storing and guiding of radioactive ions

Abstract The possibility to use a gas cell filled with noble gas (He or Ar) for thermalizing, storing and transporting radioactive ions is explored by studying experimentally ion–electron recombination of stable Ni, resonantly ionized by laser light. Combined with a literature study on ionization chambers, especially developed for high-intensity applications, conclusions are drawn on the maximum intensity of the incoming ion beam. A practical limit is encountered when the space-charge induced voltage fully counteract the applied voltage on the electrodes collecting the electrons.

LASER ION SOURCES FOR ON-LINE ISOTOPE SEPARATORS

Abstract The application of resonant photoionisation using tuneable pulsed laser systems for the production of short-lived radioactive isotopes is discussed. The different ways the laser ionisation is implemented at existing on-line isotope separators is reviewed and the properties, limitations and expectations of the laser ion sources are highlighted.

Atom-at-a-time laser resonance ionization spectroscopy of nobelium

Optical spectroscopy of a primordial isotope has traditionally formed the basis for understanding the atomic structure of an element. Such studies have been conducted for most elements and theoretical modelling can be performed to high precision, taking into account relativistic effects that scale approximately as the square of the atomic number. However, for the transfermium elements (those with atomic numbers greater than 100), the atomic structure is experimentally unknown. These radioactive elements are produced in nuclear fusion reactions at rates of only a few atoms per second at most and must be studied immediately following their production, which has so far precluded their optical spectroscopy. Here we report laser resonance ionization spectroscopy of nobelium (No; atomic number 102) in single-atom-at-a-time quantities, in which we identify the ground-state transition 1S0 1P1. By combining this result with data from an observed Rydberg series, we obtain an upper limit for the ionization potential of nobelium. These accurate results from direct laser excitations of outer-shell electrons cannot be achieved using state-of-the-art relativistic many-body calculations that include quantum electrodynamic effects, owing to large uncertainties in the modelled transition energies of the complex systems under consideration. Our work opens the door to high-precision measurements of various atomic and nuclear properties of elements heavier than nobelium, and motivates future theoretical work.

Status of the laser ion source at the Leuven isotope separator online

An online laser ion source has been used at the Leuven isotope separator online for the production of pure beams of exotic nuclei. The operational principle of the ion source is based on the element-selective multistep laser resonance ionization of nuclear reaction products thermalized and neutralized in a high-pressure noble gas. A number of improvements has been carried out to obtain stable and reproducible operation. The ion source has been optimized for the production of beams of exotic nuclei, created in proton-induced fission reactions. The efficiency of the ion source has been improved by incorporating the sextupole ion guide to separate laser-produced ions from the gas jet and to transport them to the acceleration stage of the mass separator. A gas purification system has been installed to purify the noble gas down to ppb level. High selectivity and efficiency of the ion source allowed to collect nuclear spectroscopic information for the neutron deficient 54Ni and neutron-rich 68–74Ni isotopes.

Evidence for a beta-decaying 1/2- isomer in 71Ni

We report on the investigation of the population mechanism for the 454-keV level in 71 Cu. This level was identified for the first time in a recent Coulomb excitation measurement with a radioactive beam of 71 Cu. The selective nature of the Coulomb-excitation process as well as nuclear-structure considerations constrain the possible spin values for the newly observed state to I π = 1/2 ― . A reexamination of the data set obtained in a β-decay study at the Leuven isotope separator on-line mass separator (LISOL) revealed that this state is also populated in the decay of 71 Ni, most probably by direct feeding from a newly identified 1/2 ― β-decaying isomer having a T 1/2 = 2.3(3) s. In this paper, we investigate the proposed scenario by reanalyzing the β-γ and γ-γ coincidences obtained in the β-decay study at LISOL.

β-delayed deuteron emission from Li11: decay of the halo

The deuteron-emission channel in the beta decay of the halo nucleus (11)Li was measured at the Isotope Separator and Accelerator facility at TRIUMF by implanting postaccelerated (11)Li ions into a segmented silicon detector. The events of interest were identified by correlating the decays of (11)Li with those of the daughter nuclei. This method allowed the energy spectrum of the emitted deuterons to be extracted, free from contributions from other channels, and a precise value for the branching ratio B(d)=1.30(13)x10(-4) to be deduced for E(c.m.)>200 keV. The results provide the first unambiguous experimental evidence that the decay takes place essentially in the halo of (11)Li and that it proceeds mainly to the (9)Li+d continuum, opening up a new means to study the halo wave function of (11)Li.

Technological challenges of new radioactive beam facilities

Worldwide new second-generation radioactive beam facilities, based on the isotope separator on line as well as on the in-flight technique, are discussed. The aim of these projects is an improvement of the experimental figure of merit by at least three orders of magnitude. After a brief introduction of both production schemes, the experimental figure of merit is defined and a selection of technological challenges needed to reach this goal are discussed.

The SPEDE spectrometer: combined in-beam

The SPEDE spectrometer [1] aims to combine a silicon detector, for the detection of electrons, with the MINIBALL γ-ray detection array for in-beam studies employing radioactive ion beams at the HIE-ISOLDE facility at CERN. The setup will be primarily used for octupole collectivity [2] and shape coexistence studies [3, 4] in Coulomb excitation experiments. In the shape coexistence cases the transitions between states of the same spin and parity have enhanced E0 strength [5]. Additionally the 0→0 transitions, typically present in nuclei exhibiting shape coexistence [6], can only occur via E0 transitions, i.e. via internal conversion electron emission.