  The quantum-mechanical theory of the decay of unstable states is revisited.
We show that the decay is non-exponential both in the short-time and long-time
limits using a more physical definition of the decay rate than the one usually
used. We report results of numerical studies based on Winter's model that may
elucidate qualitative features of exponential and non-exponential decay more
generally. The main exponential stage is related to the formation of a
radiating state that maintains the shape of its wave function with
exponentially diminishing normalization. We discuss situations where the
radioactive decay displays several exponents. The transient stages between
different regimes are typically accompanied by interference of various
contributions and resulting oscillations in the decay curve. The decay curve
can be fully oscillatory in a two-flavor generalization of Winter's model with
some values of the parameters. We consider the implications of that result for
models of the oscillations reported by GSI.
