We report experimental measurements of the rates of blackbody-radiation-induced transitions between
highlying (n > 60) S, P, and D Rydberg levels of rubidium atoms in a magneto-optical trap using a hybrid field
ionization and state-selective depumping technique. Our results reveal significant deviations of the measured
transition rates from theory for well-defined ranges of the principal quantum number. We assume that the
most likely cause for those deviations is a modified blackbody spectrum inside the glass cell in which the
magneto-optical trap is formed, and we test this assumption by installing electrodes to create an additional
microwave cavity around the cell. From the results, we conclude that it should be possible to use such external cavities to control and suppress the blackbody-radiation-induced transitions."