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Processes involving concentrations (or finite numbers) of particles moving randomly in space and reacting either upon encounter or via distance-dependent reaction rates are ubiquitous in condensed matter physics. Some of the many examples are recombination of ions or holes and electrons, excitation energy migration and quenching, trapping of particles by other species, in energy minima of random potential landscapes or in dangling ends of complex structures, coagulation, polymerization, and growth of dendrites or nuclei of a new phase.
Lindenberg et al. (Tue,) studied this question.