Systematic comparison of ISOLDE-SC yields with calculated in-target production rates

Within the scope of the EURISOL project [1], the design of a large-scale European secondary-beam facility based on the ISOL (Isotope Separation On Line) method is actually being worked out. One of the tasks defined in the project is devoted to the predictions of the nuclide yields of the future facility. The efficiency with which specific nuclides are extracted can vary over many orders of magnitude, thus strongly influencing the final yields. The information on the behaviour of the extraction efficiency with isotopic half-lives can help identifying the issues that need most attention in the process of target and ion-source development. The overall extraction efficiencies have been obtained earlier for some nuclides by measuring the extracted yields of stable [2] or radioactive [3] tracers implanted into a target. This method requires a dedicated experimental effort for every nuclide and every target and ion-source system. We have used the ABRABLA code, benchmarked against recent GSI experimental cross-sections, to calculate the nuclide production rates in ISOLDE targets by proton-induced spallation at 600 MeV. We have compared these results to the published ISOLDE yields obtained with the proton beam from the CERN Synchrocyclotron (SC) [4]. In this way, we have systematically deduced the overall extraction efficiencies for a wide variety of isotopic chains from different target and ion-source systems, and studied their dependence on the isotope halflife. This dependence is found to follow a simple pattern in many different cases. For the shortest-lived isotopes, the extraction efficiency scales with a power function of the isotope half-life. For longer half-lives, the decay losses become negligible, and the extraction efficiency approaches the value valid for the stable isotopes. This behavior can be parameterized using the following function with three parameters that summarize the essential properties of the system in question: