Thin solid films of poly(di-n-hexylsilane) were irradiated with a variety of high-energy ion beams, electron beams, and Co-60 gamma-rays of which linear energy transfer (LET) ranges from 0.2 to 1620 eV/nm. The beams caused nonhomogeneous reactions of crosslinking and main chain scission in the films. The molecular weight of the polymer was traced to give the efficiency of crosslinking reactions: G(x) based on the Charlesby-Pinner relationship. The value of G(x) increases from 0.042 to 0.91 with increasing values of LET. We adopt a reaction model in a single ion track to the crosslinking reactions, and the expanding chemical track along an ion trajectory is responsible for the increasing crosslinking G values. The theoretical aspects of the energy distribution in the penumbra area give a good interpretation of the chemical track radii obtained in this study.