  Ensembles of identical atoms exhibit peculiar collective properties in their
interaction with radiation depending on geometry and environment where they are
embedded in. A remarkably clean and versatile platform to study collective
effects in resonant light scattering are M\"ossbauer nuclei placed in planar
x-ray waveguides. Here we conceive and demonstrate experimentally distinct
temporal emission characteristics in these systems, ranging from a tunable
accelerated exponential decay all the way to a pronounced oscillatory emission
pattern, depending on the waveguide geometry and mode of excitation. The
observed temporal and spatial emission characteristics of the collectively
excited nuclear state in the waveguide -- the nuclear exciton -- are well
reproduced by a unified theoretical model. Our findings pave the way for
applications ranging from fundamental studies of cooperative emission at hard
x-ray frequencies up to new methods of narrowband x-ray control via the
engineering of collective radiation patterns.
