  We have mapped the [C I] emission at 492 GHz toward the supernova remnant Cas
A. We detect [C I] emission from the periphery of the diffuse Photon Dominated
Region (PDR) covering the disk of Cas A, as traced by the carbon recombination
lines, as well as from the denser PDRs associated with the molecular clouds
towards the south-east. [C I] emission is detected from both the Perseus and
Orion arm molecular clouds, with the -47 km s^-1 Perseus arm feature being
strong enough to be detected at all positions. We estimate the C/CO relative
abundance to be 0.2 at the position of the identified CO clouds and >1 for most
of the cloud. Here we show that the distribution of [C I] emitting regions
compared to the C+ region and molecular cloud is consistent with a scenario
involving PDRs. Using physical models for PDRs we constrain the physical
properties of the [C I] line-forming regions. We estimate the densities of the
[C I] emitting regions to be between 10^2 and 10^3 cm^-3. Based on rather high
volume filling factors (~50%) we conclude that [C I] emission mainly arises
from diffuse neutral gas in the Perseus arm.
