  Nowadays, intense exotic beams are needed in order to study nuclei with very
short half-life. To increase the release efficiency of the fission products,
all the target characteristics involved must be improved (e.g. chemical
composition, dimensions, physicochemical properties such as grain size,
porosity, density etc). In this article, we study the impact of the target
thickness. Released fractions measured from graphite and uranium carbide
pellets are presented as well as Monte-Carlo simulations of the Brownian
motion.
