  We present observations at optical wavelengths with the Cassini Spacecraft's
Imaging Science System of the Phoebe ring, a vast debris disk around Saturn
that seems to be collisionally generated by its irregular satellites. The
analysis reveals a radial profile from 80-260 Saturn radii ($R_S$) that changes
behavior interior to $\approx 110 R_S$, which we attribute to either the moon
Iapetus sweeping up small particles, or to orbital instabilities that cause the
ring to flare up vertically. Our study yields an integrated I/F at 0.635 $\mu$m
along Saturn's shadow in the Phoebe ring's midplane from 80-250 $R_S$ of
$2.7^{+0.9}_{-0.3} \times 10^{-9}$. We develop an analytical model for the
size-dependent secular dynamics of retrograde Phoebe ring grains, and compare
this model to the observations. This analysis implies that 1) the "Phoebe" ring
is partially sourced by debris from irregular satellites beyond Phoebe's orbit
and 2) the scattered light signal is dominated by small grains ($\lesssim
20\mu$m in size). If we assume that the Phoebe ring is generated through
steady-state micrometeoroid bombardment, this implies a power-law size
distribution with index $> 4$, which is unusually steep among solar system
rings. This suggests either a steep size distribution of ejecta when material
is initially released, or a subsequent process that preferentially breaks up
large grains.
