Abstract <p>The paper theoretically and experimentally studies the kinetics of decay of radiation on the laser transition of Yb³⁺ ion in the matrix of ceramic samples made of yttrium oxide. nanopowder doped with Yb and Zr. The calculation results showed the presence of a source of pumping of the upper level of Yb³⁺, presumably in the form of defects with a nearby lower energy level, in the ceramic samples, which explains the longer “tail” of the oscillograms than follows from a purely exponential decay. During pumping, the levels of these defects are saturated with energy, and after the pumping ceases, they give it to ytterbium ion, transferring it to the upper level of Yb³⁺. The energy exchange is quite weak and, most likely, have an insignificant effect on the energy of laser radiation of the samples. We also study the influence of the well-known “radiation trapping” effect on photoluminescence decay time in samples of different thickness. The saturation effect of the increase in photoluminescence decay time has been experimentally detected depending on the thickness. For a sample of 5% Yb:Y₂O₃ with addition of ZrO₂, this thickness was 1180 microns. To explain this effect, a mathematical model was developed based on the numerical solution of Biberman–Holstein integro-differential equation. The agreement between the calculated and experimental data confirms the correctness of our view of the physical processes in ceramic samples.</p>