  We report experimental measurements of the rates of blackbody
radiation-induced transitions between high-lying (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.
