Abstract The radial size of the damaged region induced by the proton beams in the CR-39 has been determined by small angle neutron scattering (SANS). The radii of the track core have been estimated by SANS technique using Guinier-plots and have been found to be about 0.7 ± 0.2 , 0.9 ± 0.4 and 1.5 ± 0.4 nm at the fluences of 10 11 , 10 12 and 5 × 10 12 cm - 2 , respectively. These results are compared with those calculated using Waligorski radial dose distribution and those obtained by Yamauchi et al. using atomic force microscopy (AFM) and UV spectrophotometer. Finally, a correlation between the present results and those obtained in the previous work permits to propose a chemical structure of the proton latent track in CR-39.