  The frequencies of transitions between hyperfine levels of ground-state atoms
can be measured with exquisite precision using magnetic-resonance techniques.
This makes hyperfine transitions ideal probes of QED effects originating from
the interaction of atoms with the quantized electromagnetic field. One of the
most remarkable effects predicted by QED is the Casimir-Polder shift
experienced by the energy levels of atoms placed near one or more dielectric
objects. Here we compute the Casimir-Polder shift and the width of hyperfine
transitions between ground-state Zeeman sub-levels of an hydrogen atom placed
in a micron-sized metallic cavity, over a range of temperatures extending from
cryogenic temperatures to room temperature. Results are presented also for
deuterium and tritium. We predict shifts of the hyperfine transitions
frequencies of a few tens of Hz that might be measurable with present-day
magnetic resonance apparatus.
