  We investigate the lifetime of indirect excitons in extremely high purity
diamond grown by the chemical vapor deposition method. A clear correlation is
found between the lifetime and small strain (magnitude $< $10$^{-4}$) assessed
using birefringence, thanks to the effective absence of impurity traps. A
surface recombination model, extended for nonradiative recombination at
dislocations due to exciton diffusion, is proposed to explain the
temperature-dependent lifetime. Based on the derived radiative lifetime, our
model enables the prediction of lifetimes at any temperature as well as the
highest achievable internal quantum efficiency of exciton luminescence in
diamond, which can generally be applicable to a wide range of materials with
high exciton binding energies.
