Superconducting Tunnel Junctions are promising devices in next generation nuclear and optical detectors. Unfortunately, a number of relaxation processes in the number of initially produced excitations degrades the performances of such devices. In order to investigate the role of charge loss mechanisms in Nb and/or at the Nb/Si interface we have fabricated three terminal in-plane double tunnel junctions, in which quasi-particles are injected into a Nb common strip and detected at different distances by tunnel junctions. The detection efficiency η decreases of about one order of magnitude from T= 4.2 K to T= 1.2 K for any injector-detector distance. The presence of charge trapping centers, activated at low temperatures, can qualitatively explain such a result. The estimation of the quasi-particle diffusion constant D= 28±5 cm 2/s at T= 1.2 K is in agreement with other similar values reported in literature.