  Far-infrared (far-IR) astrophysics missions featuring actively cooled
telescopes will offer orders of magnitude observing speed improvement at
wavelengths where galaxies and forming planetary systems emit most of their
light. The PRobe far-Infrared Mission for Astrophysics (PRIMA), which is
currently under study, emphasizes low and moderate resolution spectroscopy
throughout the far-IR. Full utilization of PRIMA's cold telescope requires
far-IR detector arrays with per-pixel noise equivalent powers (NEPs) at or
below 1 x 10-19 W/rtHz. We are developing low-volume Aluminum kinetic
inductance detector (KID) arrays to reach these sensitivities. We will present
on the development of our long-wavelength (210 um) array approach, with a focus
on multitone measurements of our 1,008-pixel arrays. We measure an NEP below 1
x 10-19 W/rtHz for 73 percent of our pixels.
