  Ultrafast time-resolved photoinduced reflectivity is measured for the
itinerant antiferromagnet UNiGa$_{5}$ ($T_{N} \approx$85 K) from room
temperature to 10 K. The relaxation time $\tau$ shows a sharp increase at
$T_{N}$ consistent with the opening of a spin gap. In addition, the temperature
dependence of $\tau$ below $T_{N}$ is consistent with the opening of a spin gap
leading to a quasiparticle recombination bottleneck as revealed by the
Rothwarf-Taylor model. This contrasts with canonical heavy fermions such as
CeCoIn$_{5}$ where the recombination bottleneck arises from the hybridization
gap.
