We have measured the temperature dependence and magnitude of the superfluid density rho(s)(T) via the magnetic field penetration depth lambda(T) in PuCoGa5 (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. Penetration depths lambda(0)=265(5) and >= 498(10) nm are derived for the fresh and aged samples, respectively. The temperature dependence of the superfluid density rho(s)(T)/rho(s)(0)=lambda(0)(2)/lambda(T)(2) is well described in both materials by a model using d-wave gap symmetry. Within the context of a strong-coupling dirty d-wave model a zero-temperature gap value Delta(0)=3.0k(B)T(c0) is obtained in the fresh sample for a scattering rate Gamma=0.005 pi k(B)T(c0), which is consistent with Abrikosov-Gor'kov (AG) pair-breaking theory. This Delta(0) should be compared to the weak-coupling value for a clean d-wave gap Delta(0)=2.14k(B)T(c0). In the aged sample the same model yields Delta(0)=2.4k(B)T(c0) for Gamma=0.010 pi k(B)T(c0). This value of Gamma is much less than required by the AG pair-breaking formalism. Furthermore, the aged rho(s)(0) is reduced by at least 70% compared to the fresh sample, which is also incompatible with Delta T-c/T-c0 similar to 20%, according to AG theory. We conclude that the data in aged PuCoGa5 support the postulate that the scattering from radiation-induced defects is not in the limit of the AG theory of an order parameter which is spatially averaged over impurity sites, but rather in the limit of short-coherence-length superconductivity. We show that a model calculation consistent with this assumption fits rho(s)(T) in the aged sample rather well.
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