  The supernova remnant Cassiopeia A (Cas A) is one of the few remnants in
which it is possible to observe unshocked ejecta. A deep 1.64 micron image of
Cas A shows a patch of diffuse emission from unshocked ejecta, as well as
brighter emission from Fast-Moving Knots and Quasi-Stationary Flocculi.
Emission at 1.64 micron is usually interpreted as [Fe II] emission, and spectra
of the bright knots confirm this by showing the expected emission in other [Fe
II] lines. We performed NIR spectroscopy on the diffuse emission region and
found that the unshocked ejecta emission does not show those lines, but rather
the [Si I] 1.607 micron line. This means that the 1.64 micron line from the
unshocked ejecta may be the [Si I] 1.645 line from the same upper level, rather
than [Fe II]. We find that the [Si I] line is formed by recombination, and we
use the [Si I] to [Si II] ratio to infer a temperature about 100 K, close to
the value assumed for analysis of low frequency radio absorption and that
inferred from emission by cool dust. Our results constrain estimates of Cas A's
total mass of unshocked ejecta that are extremely sensitive to temperature
assumptions, but they do not resolve the ambiguity due to clumping.
