  Conditions for the appearance of a nuclear exciton in a crystal consisting of
excited and unexcited nuclei of a given type are determined. The total
probabilities for spontaneous -emission by a single excited nucleus or by an
arbitrary number of excited nuclei in the crystal are derived. It is shown that
the formation of a nuclear exciton is connected with an increase of the width
of the emitting level and with the concentration of the radiation in a narrow
solid angle.
