  The PRobe far-Infrared Mission for Astrophysics (PRIMA) is working to develop
kinetic inductance detectors (KIDs) that can meet the sensitivity targets of a
far-infrared spectrometer on a cryogenically cooled space telescope. An
important ingredient for achieving high sensitivity is increasing the
fractional-frequency responsivity. Here we present a study of the responsivity
of aluminum KIDs fabricated at the Jet Propulsion Laboratory. Specifically, we
model the KID's temporal response to pair-breaking excitations in the framework
of the Mattis-Bardeen theory, incorporating quasiparticle recombination
dynamics and the pair-breaking efficiency. Using a near-IR laser, we measure
time-resolved photon pulses and fit them to our model, extracting the
time-resolved quasiparticle density and the quasiparticle recombination
lifetime. Comparing the fit to the known energy of the laser provides a
measurement of the pair-breaking efficiency. In addition to photon-sourced
excitations, it is important to understand the KID's response to phonon-sourced
excitations from cosmic rays. We measure the rate of secondary cosmic rays
detected by our devices, and predict the dead time due to cosmic rays for an
array in L2 orbit. This work provides confidence in KIDs' robustness to cosmic
ray events in the space environment.
