Future space-based far-infrared astrophysical observatories will require exquisitely 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 \(6.5\times 10^{-19}~\text {W/Hz}^{1/2}\) . 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.
