  We have measured the spontaneous neutral particle emission from copper
cluster anions (Cu$_n^-$, $n=3-6$) stored at cryogenic temperatures in one of
the electrostatic ion storage rings of the DESIREE (Double ElectroStatic Ion
Ring ExpEriment) facility at Stockholm University. The measured rate of
emission from the stored Cu$_3^-$ ions follows a single power law decay for
about 1 ms but then decreases much more rapidly with time. The latter behavior
may be due to a decrease in the density of available final states in Cu$_3$ as
the excitation energies of the decaying ions approach the electron detachment
threshold. The emissions from Cu$_4^-$, Cu$_5^-$ and Cu$_6^-$ are
well-described by sums of two power laws that are quenched by radiative cooling
of the stored ions with characteristic times between a few and hundreds of
milliseconds. We relate these two-component behaviors to populations of stored
ions with higher and lower angular momenta. In a separate experiment, we
studied the laser-induced decay of Cu$_6^-$ ions that were excited by 1.13 eV
or 1.45 eV photons after 46 milliseconds of storage.
