  We have measured the temperature dependence and magnitude of the superfluid
density $\rho_{\rm s}(T)$ via the magnetic field penetration depth $\lambda(T)$
in PuCoGa$_5$ (nominal critical temperature $T_{c0} = 18.5$ K) using the muon
spin rotation technique in order to investigate the symmetry of the order
parameter, and to study the effects of aging on the superconducting properties
of a radioactive material. The same single crystals were measured after 25 days
($T_c = 18.25$ K) and 400 days ($T_c = 15.0$ K) of aging at room temperature.
The temperature dependence of the superfluid density is well described in both
materials by a model using d-wave gap symmetry. The magnitude of the muon spin
relaxation rate $\sigma$ in the aged sample, $\sigma\propto
1/\lambda^2\propto\rho_s/m^*$, where $m^*$ is the effective mass, is reduced by
about 70% compared to fresh sample. This indicates that the scattering from
self-irradiation induced defects is not in the limit of the conventional
Abrikosov-Gor'kov pair-breaking theory, but rather in the limit of short
coherence length (about 2 nm in PuCoGa$_5$) superconductivity.
