  We present results from SEPPCoN, an on-going Survey of the Ensemble Physical
Properties of Cometary Nuclei. In this report we discuss mid-infrared
measurements of the thermal emission from 89 nuclei of Jupiter-family comets
(JFCs). All data were obtained in 2006 and 2007 with the Spitzer Space
Telescope. For all 89 comets, we present new effective radii, and for 57 comets
we present beaming parameters. Thus our survey provides the largest compilation
of radiometrically-derived physical properties of nuclei to date. We conclude
the following. (a) The average beaming parameter of the JFC population is
1.03+/-0.11, consistent with unity, and indicating low thermal inertia. (b) The
known JFC population is not complete even at 3 km radius, and even for comets
with perihelia near ~2 AU. (c) We find that the JFC nuclear cumulative size
distribution (CSD) has a power-law slope of around -1.9. (d) This power-law is
close to that derived from visible-wavelength observations, suggesting that
there is no strong dependence of geometric albedo with radius. (e) The observed
CSD shows a hint of structure with an excess of comets with radii 3 to 6 km.
(f) Our CSD is consistent with an intrinsic distribution that lacks many
sub-kilometer objects.
