Superconducting tunnel junction X-rays detectors Ti/Nb/Al/AlO_x/Al/Nb/NbN with the Ti/Nb/Al/ killed electrode were studied under irradiation by X-rays photons of different energies produced by the fluorescence method. The nonlinearity of the detector response and the shape of the detector line were analyzed on the basis of the diffusion model taking into account the quasiparticle self-recombination and edge losses.