n-Heptane freezes to a polycrystalline glass at 77 K and after $\gamma $ radiolysis is shown to emit four glow peaks. It is demonstrated that trapping is a property of the heptane matrix whilst emission intensity and temperatures of maximum glow depend on the nature and concentration of the luminescent additive. Dose-dependence measurements of glow peak intensities shows that the three lowest temperature peaks can be explained in terms of electron trapping in a fixed number of traps, while the highest temperature peak is produced by traps which are subject to a radiation-induced blocking mechanism. A possible blocking mechanism is proposed and it is shown that the traps are probably different types of cavity within the solid. The results are compared with earlier results for squalane, an amorphous hydrocarbon.