  Luminescence and optical absorption due to radiation damage centers in
silicon has been studied exhaustively for decades, but is receiving new
interest for applications as emitters for integrated silicon photonic
technologies. While a variety of other optical transitions have been found to
be much sharper in enriched $^{28}$Si than in natural Si, due to the
elimination of inhomogeneous isotopic broadening, this has not yet been
investigated for radiation damage centers. We report results for the well-known
G, W and C damage centers in highly enriched $^{28}$Si, with optical linewidth
improvements in some cases of over two orders of magnitude, revealing
previously hidden fine structure in the G center emission and absorption. These
results have direct implications for the linewidths to be expected from single
center emission, even in natural Si, and for models for the G center structure.
The advantages of $^{28}$Si can be readily extended to the study of other
radiation damage centers in Si.
