  We report muon spin relaxation rate measurements on films composed of
aluminum grains having a size of a few nm, with a large energy level splitting
of the order of 100 K. The films range from weakly metallic to insulating. In
the insulating case the low temperature relaxation rate is consistent with the
presence of single electron spins in grains having an odd number of electrons.
The relaxation rate temperature dependence follows an activation law having an
energy scale in agreement with the average level splitting. In weakly metallic
films the relaxation rate is smaller and decreases faster with temperature.
Overall our observations are in line with the presence of a Kubo spin in Al
nano-size grains due to quantum size effects.
