Sodium particles in heavily irradiated synthetic doped NaCl and natural rocksalt have been studied by electron spin resonance (ESR), differential scanning colorimetry (DSC) and dielectric loss measurements. For the first time we have studied quantum size effects of Na colloids in irradiated NaCl at different temperatures between 140 K and 30 K. A weak temperature dependence of the electronic susceptibility of these particles is observed, that deviates from the simple metallic behaviour and may be due to interactions in the system. The magnetic properties of sodium particles in heavily damaged NaCl (3–15 mole % metallic Na) measured by ESR below 160 K might be associated with antiferromagnetic interactions between the extremely small colloids, which suggests a transition to a spin density or charge density wave ground state; this is in agreement with the observation of an anomalous peak in the AC conductivity at about 150 K.