[en] Comparisons between diameters of uranium and thorium neutron induced fission tracks in glass made possible to test indirectly the hypothesis that observation efficiency of spontaneous ²³⁸U and induced ²³⁵U fission tracks may be considered the same. The sources of fission fragments were uranium and thorium thin films irradiated with neutrons with and without Cd-cover. After irradiation, the glasses were etched for 50 s by HF 24% at 15⁰C followed by another etching for 30 s in same conditions. Diameters were measured after each chemical etching. Only tracks presenting a ratio between the longer and shorter diameters lesser than 1.1 have been analyzed. The results indicates the hypothesis is valid