Influence of Decay in the Target on the Measurement of Release Times and Release Efficiency

We present a framework for the experimental determination of release times and release efficiencies of ISOL-targets devoted to production of neutron-rich nuclei. Since a wide (Z,A) distribution of products is generated in fission, decay in the target of nuclei more exotic than the one investigated is an extra production channel. This feeding, not in phase with the accelerator beam, modifies the release curves. A model which explicitly takes into account the occurence of β decay during diffusion in the grains or effusion between them has been designed. We also discuss the fact that in measurement of efficiency versus lifetime, an effective cross-section must be used which incorporates feedings by the mother nucleus and occasionally of isomers of the nucleus investigated. Release efficiency and release curves must be analysed in a consistent framework to obtain meaningful data.