Experimental study of the influence of the CdZnTe substrate thickness on the response of IR detectors under irradiation has been performed. Two detectors, with different substrate thickness 800 μm and 50 μm, were submitted to low flux 62 MeV proton irradiation.Images acquired under irradiation were analyzed. Whereas the detector with 800 μm substrate thickness has shown background signal increase under irradiation, the detector with the substrate partially removed has not shown any background signal elevation. The elevation of the background signal under irradiation in the detector with intact substrate is attributed to the large extension of one transient event as reveal by the derivation of one transient event. The results of the irradiation campaign are further compared to simulations. This comparison shows very good agreement and highlights the role of emission of low energy photons inside the substrate which are in turn detected by the light sensitive layer.