With the aim to determine an optimal indium (In) concentration in Cd0.9Zn0.1Te:In nuclear radiation detectors for a good material quality, low-temperature photoluminescence (LTPL) measurements have been performed at 5 K in the energy range of 1.2–1.7 eV. The LTPL spectrum of initial sample contained a neutral acceptor bound exciton (A0X) peak at 1.635 eV, donor–acceptor pair (DAP) peak at 1.606 eV with its 1LO phonon replica, and deep-level defects (D) peak at 1.455 eV. In doping leads to the appearance of a new relatively intensive peak at 1.652 eV that is caused by a shallow neutral donor bound exciton (D0X) due to In substituting cadmium vacancies (VCd). At In concentrations of about 8 × 1016–1017 cm−3 the value of ratio is maximal, which indicates a low density of detrimental carrier traps and, therefore, high-material quality. We believe that at such concentrations the majority of as-grown Cd vacancies will be substituted by In atoms and, therefore, noted concentrations are optimal, which was confirmed by the correlation with mobility–lifetime product.