The use of the CR-39 track detector in a mixed field of radiation (γ-rays + α-particles) has been carried out. The response function V(R) and V(REL) 300 eV has been evaluated at different γ dose values. A delaying effect of track appearance in plastic detectors was observed and found to be γ dose dependent. Results of the present work shows that γ dose values can be extracted from the measurements of α track diameter and dissolved thickness layer. An interesting semi-empirical formula has been developed and used for such purpose.