  Low temperature Kinetic Inductance Detectors (KIDs) are attractive candidates
for producing quantumsensitive, arrayable sensors for astrophysical and other
precision measurement applications. The readout uses a low frequency probe
signal with quanta of energy well-below the threshold for pair-breaking in the
superconductor. We have calculated the detailed non-equilibrium quasiparticle
and phonon energy spectra generated by the probe signal of the KID when
operating well-below its superconducting transition temperature Tc within the
framework of the coupled kinetic equations described by Chang and Scalapino.[1]
At the lowest bath temperature studied Tb/Tc = 0.1 the quasiparticle
distributions can be driven far from equilibrium. In addition to the low
frequency probe signal we have incorporated a high frequency (~ 1 THz) source
signal well-above the pair-breaking threshold of the superconductor.
Calculations of source signal detection efficiency are discussed
