Positronium formation in solids

The spur processes described for the liquid state should apply to solids. As compared to the scheme of reactions (I)—(IX), solvation is to be excluded. Localization in the sense that the particle would preserve some non negligible mobility as such, is also to be discarded in most cases. 

Whereas Ps can be formed anywhere in liquids, considered as a very soft state of matter, this is no more true in most solids. Thus, besides the chemical concepts of the spin- model, structural considerations must be taken into account, since Ps, as a physical particle, cannot form and survive in the absence of sufficient free space in the matrix (except if delocalized). 

Most generally, spur aspects appear whenever a chemically very active compound (essentially, electron scavengers) is present in the solid matrix, either as a normal constituent or as an additive. This has been demonstrated recently in a variety of irradiated polymers, where radiolysis of the matrix induces the production of either free radicals, inhibiting Ps formation by electron scavenging, or of trapped electrons, enhancing the Ps formation probability, depending on the temperature [41]. 

The free spaces where Ps can form and o-Ps can have a reasonably long lifetime may be extrinsic defects, as just illustrated, or intrinsic defects, such as created when heating a pure solid compound. More generally, they may correspond to the natural voids present in any solid matrix (e.g., free volume in polymers, treated elsewhere in this book). Ps can be formed not only in molecular solids, including frozen liquids, but also in a number of ionic solids, even when the open spaces are rather small. For example, Ps is formed in such a highly packed lattice as KC1 [44, 45] where the largest space available corresponds to the tetrahedral sites circumscribed by 4 Cf anions, with a radius of only 0.0845 nm, resulting in an o-Ps lifetime of about 0.65 ns. 

---