Results concerning the response to* Fe X-rays and 241Am a-particles of high quality Nb-based Superconducting Tunnel Junctions (STIs) on Si substrate are presented. The results in terms of the collected charge and the signal risetimes are discussed, considering the role of the substrate. We report the observation of a linear response of the device in the X-ray spectrum. The linearity has been extrapolated at higher energies, allowing the identification of the two obsverved peaks in the o-particles spectrum due to ionization energy loss and Y-ray absorption.