  A detailed analytical model has been developed to simulate isotope-release
curves from thin-foils ISOL targets. It involves the separate modelling of
diffusion and effusion inside the target. The former has been modelled using
both first and second Fick's law. The latter, effusion from the surface of the
target material to the end of the ionizer, was simulated with the Monte Carlo
code MolFlow+. The calculated delay-time distribution for this process was then
fitted using a double-exponential function. The release curve obtained from the
convolution of diffusion and effusion shows good agreement with experimental
data from two different target geometries used at ISOLDE. Moreover, the
experimental yields are well reproduced when combining the release fraction
with calculated in-target production.
