  Future space-based far-infrared astrophysical observatories will require
exquis-itely sensitive detectors consistent with the low optical backgrounds.
The PRobe far-Infrared Mission for Astrophysics (PRIMA) will deploy arrays of
thousands of superconducting kinetic inductance detectors (KIDs) sensitive to
radiation between 25 and 265 $\mu$m. Here, we present laboratory
characterization of prototype, 25 -- 80 $\mu$m wavelength, low-volume, aluminum
KIDs designed for the low-background environment expected with PRIMA. A compact
parallel plate capacitor is used to minimize the detector footprint and
suppress TLS noise. A novel resonant absorber is designed to enhance response
in the band of interest. We present noise and optical efficiency measurements
of these detectors taken with a low-background cryostat and a cryogenic
blackbody. A microlens-hybridized KID array is found to be photon noise limited
down to about 50 aW with a limiting detector NEP of about $6.5 \times
10^{-19}~\textrm{W/Hz}^{1/2}$. A fit to an NEP model shows that our optical
system is well characterized and understood down to 50 aW. We discuss future
plans for low-volume aluminum KID array development as well as the testbeds
used for these measurements.
