By bombarding a target with stable ion beams it is possible to produce nuclear radioactive species. The EXCYT facility at the Istituto Nazionale di Fisica Nucleare - Laboratori Nazionali del Sud (INFN-LNS), in Catania, is based on a K-800 superconducting cyclotron injecting high intensity beam (up to 500 W) in a Target Ion Source (TIS) assembly, and on a 15 MV Tandem for post-accelerating the Radioactive Ion Beams (RIB). Radioactive species are produced in a solid porous target where, by diffusion, they will reach the target surface. After effusion through the target container, radioactive atoms are ionized, extracted at low energy, and changed in charge (if needed). Two stages of isobaric mass separators will clean the resulting radioactive beam from contaminants, thus the RIB is injected into the Tandem for the post-acceleration. TIS is operating at high temperature (up to 2700 K) under vacuum (10-4 mbar). The production of 8Li radioactive beam was performed by injecting a 13C4+ primary beam of 45 MeV/u on a porous graphite target. In this work we present a study to optimize the target geometry in order to increase the production of the RIB. Our first RIB was 8Li which mean life time is 1.212 s. The generation density of 8Li inside the solid target is calculated by EPAX II code. A simple diffusion model which takes into account either the nuclei generation and their decay, was computer simulated to estimate the efficacious diffusion coefficient of 8Li in porous graphite which resulted to be in the range of 10-6 - 10-5 cm2/s. Since the porous nature of the target, many diffusion mechanisms are active, our assumption, supported by experimental evidence, is that at such high temperature multi-grain diffusion prevails. Diffusion length inside the target of 8Li, before its decay, resulted to be sensible smaller than the target size, therefore we modified its geometry enhancing the RIB production by a factor of 3-4.