L. Vermeeren

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.

Laser-spectroscopy measurements of 72–96Kr spins, moments and charge radii

Abstract The spins, moments and radii of krypton isotopes have been investigated by collinear fast-beam laser spectroscopy in combination with ultra-sensitive collisional ionization detection. The sequence of isotopes under study ranges from the neutron-deficient N = Z = 36 isotope 72 Kr to the neutron-rich 96 Kr ( N = 60). The mean-square charge radii in the neighbourhood of the N = 50 neutron-shell closure exhibit a pronounced shell effect which has recently been explained in the framework of relativistic mean-field theory. The results for the neutron-deficient nuclei are related to the shape coexistence of strongly prolate and near-spherical states which is known from nuclear spectroscopy. Here, an inversion of the odd-even staggering is observed below the neutron number N = 45. The neutron-rich transitional nuclei are influenced by the N = 56 subshell closure. In contrast to the N = 60 isotones 97 Rb, 98 Sr and 100 Zr, the new isotope 96 Kr is not strongly deformed.

Moments and mean square charge radii of short-lived argon isotopes

Abstract We report on the measurement of optical isotope shifts for 32–40 Ar and for 46 Ar from which the changes in mean square nuclear charge radii across the N = 20 neutron shell closure are deduced. The investigations were carried out by collinear laser spectroscopy in fast beams of neutral argon atoms. The ultra-sensitive detection combines optical pumping, state-selective collisional ionization and counting of β-radioactivity. By reaching far into the sd shell, the results add new information to the systematics of radii in the calcium region ( Z ≈ 20). Contrary to all major neutron shell closures with N ⩾ 28, the N = 20 shell closure causes no significant slope change in the development of the radii. Information from the hyperfine structure of the odd- A isotopes includes the magnetic moments of 33 Ar ( I = 1 2 ) and 39 Ar ( I = 7 2 ), and the quadrupole moments of 35 Ar, 37 Ar ( I = 3 2 ) and 39 Ar. The electromagnetic moments are compared to shell-model predictions for the sd and fp shells. Even far from stability a very good agreement between experiment and theory is found for these quantities. The mean square charge radii are discussed in the framework of spherical SGII Skyrme-type Hartree-Fock calculations.

The SPIG, improvement of the efficiency and beam quality of an ion-guide based on-line isotope separator

Abstract A SPIG (Sextupole Ion Beam Guide) has been constructed and tested to couple an ion-guide based ion source to an on-line isotope separator. In comparison to a skimmer, the SPIG offers both an increased beam quality and the possibility to operate the ion-guide at higher buffer gas pressures.

The Leuven isotope separator on-line laser ion source

An element-selective laser ion source has been used to produce beams of exotic radioactive nuclei and to study their decay properties. The operational principle of the ion source 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 installed at the Leuven Isotope Separator On-Line (LISOL), which is coupled on-line to the cyclotron accelerator at Louvain-la-Neuve. 54,55Ni and 54,55Co isotopes were produced in light-ion-induced fusion reactions. Exotic nickel, cobalt and copper nuclei were produced in proton-induced fission of 238U. The b decay of the 68–74Ni, 67–70Co, 70–75Cu and 110–114Rh isotopes has been studied by means of β–γ and γ–γ spectroscopy. Recently, the laser ion source has been used to produce neutron-deficient rhodium and ruthenium isotopes (91–95Rh, 98Rh, 90,91Ru) near the N=Z line in heavy ion-induced fusion reactions.

Nuclear ground state properties of 99Sr by collinear laser spectroscopy with non-optical detection

Collinear fast-beam laser spectroscopy, with improved sensitivity for ions with hyperfine split transitions, is performed to measure the hyperfine structure and the isotope shift of the well deformed short-lived 99Sr. The new method consists in ground state depopulation by a two-step optical pumping sequence prior to state selective neutralization and fast-atom counting. A definitive nuclear spin value I = 32, the change in mean square charge radius δ〈r2〉98,99 and the nuclear moments are derived. These results are compared to nuclear spectroscopy information and are interpreted in the frame of the particle plus deformed core model.

Two-step laser ionization schemes for in-gas laser ionization and spectroscopy of radioactive isotopesa)

The in-gas laser ionization and spectroscopy technique has been developed at the Leuven isotope separator on-line facility for the production and in-source laser spectroscopy studies of short-lived radioactive isotopes. In this article, results from a study to identify efficient optical schemes for the two-step resonance laser ionization of 18 elements are presented.

A LASER ION-SOURCE FOR ONLINE MASS SEPARATION

A laser ion source based on resonance photo ionization in a gas cell is proposed. The gas cell, filled with helium, consists of a target chamber in which the recoil products are stopped and neutralized, and an ionization chamber where the atoms of interest are selectively ionized by the laser light. The extraction of the ions from the ionization chamber through the exit hole and skimmer is similar to the ion-guide system. The conditions to obtain an optimal system are given. The results of a two-step one-laser resonance photo ionization of nickel, and the first results of laser ionization in a helium buffer gas cell are presented.

A detector setup to study the decay properties of exotic nuclei

Abstract General features of a detector setup for decay studies at on-line mass-separators are discussed. The detection system was used for measurements of β-decay properties of exotic nuclei with A ≈70. Results of GEANT Monte-Carlo simulations of the detector setup generally confirm the measurements. Both measurements and simulations show that even in case of low multiplicity events the Ge detector efficiency and peak to total ratio of γ-lines are reduced by large factor due to summing. Steps to improve the detection system are discussed.

Measurement of nuclear moments and radii by collinear laser spectroscopy and by β-NMR spectroscopy

Experiments using laser spectroscopy and nuclear magnetic resonance techniques have been performed at ISOLDE on the unstable isotopes of several light elements. The results include nuclear ground state spins, magnetic dipole and electric quadrupole moments, and the behaviour of mean square nuclear charge radii within isotopic strings. These give important information about the nuclear structure at shell closures and close to the drip lines.