Review of ion-source developments for radioactive ion-beam facilities

The ion-sources dedicated to the production of radioactive ion beams (RIB) shall be highly efficient, selective and fast. This efficiency is mandatory since only limited amounts of radionuclides are produced. Chemical selectivity is needed to confine other elements near to the production site and to suppress isobaric contaminants. Eventually, the ion-source shall only decay the radioisotopes by a fraction of their half-life to reduce decay losses. The world wide spread RIB facilities came up with a large variety of solutions to meet part or all of these requirements such as: ion traps, surface, plasma, sputtering, electron cyclotron resonance and laser ion- sources. In this review, the latest developments are presented and their applications to charge states breeder systems proposed for post-acceleration are discussed. (59 refs)

Exotic ion-beams targets and sources

Exotic beams of short-lived radioisotopes are produced in nuclear reactions such as thermal neutron induced fission, target or projectile fragmentation and fusion reactions. For a given radioactive ion beams (RIB), different production mode are in competition. For each of them the cross section, the intensity of the projectile beam and the target thickness define an upper production rate. The final yield relies on the optimisation of the ion-source, which shall be fast and highly efficient in view of the limited production cross section and on minimum diffusion time out of the target matrix or fragment catcher to reduce decay losses. Eventually, either chemical or isobaric selectivity is needed to confine unwanted elements near to the production site. These constraints are discussed for pulsed or dc driven RIB facilities and the solutions to some of the technical challenges will be illustrated by examples of actually scarcely produced near drip line elements

Transverse emittance investigation of the ISOLDE target ion sources

In order to produce target-ion sources allowing for a high isotopic resolution in the separator, CERN/ISOLDE (Isotope Separator On Line) has purchased a commercial emittance metre, capable of measuring transverse phase-space emittances for ion-beam intensities down to approximately 1 nA. It was installed at the ISOLDE off-line separator where targets are tested with respect to material purity and the ion-source efficiencies are determined. Now, also the emittance and brightness are measured for different stable elements. An extensive programme has been launched aiming at a complete survey of the emittance dependence on the various ion-source parameters. Results from the measurements on the different ISOLDE ion-source types, with associated analysis, are presented

Radioactive ion beams produced by neutron-induced fission at ISOLDE

The production rates of neutron-rich fission products for the next-generation radioactive beam facility EURISOL are mainly limited by the maximum amount of power deposited by protons in the target. An alternative approach is to use neutron beams to induce fission in actinide targets. This has the advantage of reducing: the energy deposited by the proton beam in the target; contamination from neutron-deficient isobars that would be produced by spallation; and mechanical stress on the target. At ISOLDE CERN, tests have been made on standard ISOLDE actinide targets using fast neutron bunches produced by bombarding thick, high-Z metal converters with 1 and 1.4 GeV proton pulses. This paper reviews the first applications of converters used at ISOLDE. It highlights the different geometries and the techniques used to compare fission yields produced by the proton beam directly on the target with neutron-induced fission. Results from the six targets already tested, namely UC2/graphite and ThO2 targets with tungsten and tantalum converters, are presented. To gain further knowledge for the design of a dedicated target as required by the TARGISOL project, the results are compared to simulations, using the MARS code interfaced with MCNP libraries, of the neutron flux from the converters interacting with the actinide targets

Alkali suppression within laser ion-source cavities and time structure of the laser ionized ion-bunches

The chemical selectivity of the target and ion-source production system is an asset for Radioactive Ion-Beam (RIB) facilities equipped with mass separators. Ionization via laser induced multiple resonant steps Ionization has such selectivity. However, the selectivity of the ISOLDE Resonant Ionization Laser Ion-Source (RILIS), where ionization takes place within high temperature refractory metal cavities, suffers from unwanted surface ionization of low ionization potential alkalis. In order to reduce this type of isobaric contaminant, surface ionization within the target vessel was used. On-line measurements of the efficiency of this method is reported, suppression factors of alkalis up to an order of magnitude were measured as a function of their ionization potential. The time distribution of the ion bunches produced with the RILIS was measured for a variety of elements and high temperature cavity materials. While all ions are produced within a few nanoseconds, the ion bunch sometimes spreads over more than 100 ms. This demonstrates that ions are confined within high temperature metallic cavities. It is the internal electrical field of these cavities that causes the ions to drifts to the extraction region and defines the dwell time of the ions in the cavity. Beam optics calculations were carried out to simulate the pulse shape of a RILIS ion bunch and are compared to the actual measurements

The High Resolution Spectrometer at ISOLDE

ISOLDE's HRS (High Resolution isotope Separator) was recommissioned in mid 2000, after a period out of service. Since then, the separator has routinely run with a mass resolution of 3000-4500 and during 2001 it delivered 72% of ISOLDE's radioactive output. A concerted effort has been made to understand the ion optics and optimise performance. In this paper we present an overview of the HRS and investigate the factors which limit the attainable mass resolution: ion-source emittance; optical aberrations; beam instrumentation and magnet stability

H- Beam formation simulation in negative ion source for CERN's Linac4 accelerator

The caesiated surface negative ion source is the first element of CERN's LINAC4 a linear injector designed to accelerate negative hydrogen ions to 160 MeV. The IS03 ion source is operated at 35 mA beam intensity and reliably feeds CERN's accelerator chain, H- ions are generated via plasma volume and caesiated molybdenum plasma electrode surface mechanisms. Studying the beam extraction region of this H- ion source is essential for optimizing the H- production. The 3D Particle-in-cell Monte Carlo code ONIX (Orsay Negative Ion eXtraction), written to study H- beam formation processes in neutral injectors for fusion, has been adapted to single aperture accelerator H- sources. The code was modified to match the conditions of the beam formation and extraction regions of the Linac4 H- source. A set of parameters was chosen to characterize the plasma and to match the specific volume and surface production modes. Simulated results of the extraction regions are presented and benchmarked with experimental results obtained at the Linac4 test stand

Cavitation bubble behavior inside a liquid jet

The growth and collapse of laser-induced vapor cavities inside axisymmetric free-falling liquid water jets have been studied. Bubbles of different size are generated at various distances from the jet axis and the effects on the jet interface are recorded by means of ultrafast cinematography. The configuration is characterized by two dimensionless parameters: the bubble to jet diameter ratio delta and the eccentricity coefficient epsilon defined as the radius of bubble generation divided by the jet radius. For high delta and epsilon, microjets and droplets are ejected from the liquid jet at speeds exceeding 100 meter per second. The observed jet fragmentation shows similarities with experiments conducted on a liquid mercury jet hit by a pulsed proton beam, a candidate configuration for future accelerator based facilities