Design and development of a tantalum foil target for the production of high intensity radioactive beams

The design and development of a high power target and ion source for the production of Radioactive Beams at intensities approaching two orders of magnitude greater than currently possible is presented. This was a key aim of the RIST experiment, designed to utilise the proton synchrotron of the ISIS facility at Rutherford Appleton laboratory, Chilton, Oxfordshire, where an 800 MeV proton beam is available at currents of up to 200 μA. A number of different target designs were considered and analysed, and high temperature power dissipation tests were conducted. This culminated in the manufacture of a diffusion bonded structure comprising 6000 separate tantalum foil discs and spacer washers. The target was installed in the RIST facility, and thermal tests using electron beam heating demonstrated that the target was capable of dissipating 24 kW by thermal radiation, at the desired temperature of 2000 deg C. This is equivalent to running with the 800 MeV ISIS proton beam at a current of 100 μA. A smaller diameter target of otherwise similar geometry was successfully tested online at the ISOLDE facility, CERN, producing Radioactive Beam yields and release times at least as good as a normal ISOLDE target. A Monte Carlo program was written to investigate the parameters responsible for the delay time of radioactive atoms from their production within the target foils to extraction from the ion source. Data on the critical parameters, principally the diffusion coefficients and surface sticking times, is scarce for the elements of interest at the high temperatures required. By calculating the path length and number of surface interactions within the target geometry, it was possible in many cases to fit the calculated delay curves to the experimental results gained at ISOLDE, thus providing estimates of the diffusion constants and surface sticking times. This gave insight into which were the dominant mechanisms, and made it possible to predict the delay characteristics of the full size RIST target. (author)