Ionising radiation leaves radicals and pairs of radicals in irradiated
solids. The distance between observed radicals in a pair is between 0
.3 and 1.0 nm. Pairs are identifiable by their electron spin resonance
spectra and a method is described to calculate the distance between s
pins. Observations of pairs in dense polyethylene and thymine (at 77K)
are described as a function of dose, for irradiations with gamma rays
and with 32 MeV protons.