Making possible beams of rare radioactive isotopes
For the experimental study of exotic, namely extremely short-lived radioactive isotopes, not only their production has to be optimised, but also their release from the target material. An essential requisite for the efficient delivery of short-lived nuclei in 'Isotope separation on-line' (ISOL) facilities is the fast release of radioactive ions from the target matrices and ion sources. Radionuclides, produced when primary beams of protons or heavy ions induce nuclear reactions in the target, need to reach the plasma ion source, be accelerated and mass-separated before they decay. Their release speed is influenced by diffusion within the matrix material, the effusion time of their random walk and the absorption/desorption cycles characterising each surface collision. The European project TARGISOL aimed to optimise ISOL target matrices in a systematic way by means of a Monte Carlo model. Simulating the release of different radioactive ions, researchers at the European Organisation for Nuclear Research predicted their release speed as a function of the units' geometry and the materials used. For ensuring an optimised release of certain key radionuclides, a database was created through an extensive literature research and new measurements performed at the ISOLDE facility at C.E.R.N. The Targisol database contains a collection of tables connected in a series of logical relationships that reflect the process and delay parameters influencing the delivery of intense beams of rare isotopes. Furthermore, an intelligent web interface ensured that project partners could access release parameters from remote sites at http://www.targisol.csic.es using any of the standard web-browsers. Approximately 2,400 entries were stored in the database during the TARGISOL project, which were used as input to the Monte Carlo simulation programme. On the basis of the simulation results, dedicated target matrices were designed to provide new and improved ISOL beams of the elements beryllium, oxygen, neon, magnesium and aluminium among others. These targets have been successfully been tested on-line not only at the ISOLDE facility, but at the SPIRAL facility of the Grand Accélérateur National d'Ions Lourds (GANIL) in France.
