The Balloon Experiment for Galactic INfrared Science (BEGINS) is a concept for a sub-orbital observatory that will operate from \(\lambda\) = 25 to 250 \(\upmu\) m to characterize dust in the vicinity of high-mass stars. The mission’s sensitivity requirements will be met by utilizing arrays of 1840 lens-coupled, lumped-element kinetic inductance detectors (KIDs) operating at 300 mK. Each KID will consist of a titanium nitride (TiN) parallel strip absorbing inductive section and parallel plate capacitor deposited on a Silicon (Si) substrate. The parallel plate capacitor geometry allows for reduction of the pixel spacing. At the BEGINS focal plane, the detectors require optical NEPs from \(2\times 10^{-16}\) to \(6\times 10^{-17}\) W/ \(\sqrt{\text {Hz}}\) from 25 to 250 \(\upmu\) m for optical loads ranging from 4 to 10 pW. We present the design, optical performance and quasiparticle lifetime measurements of a prototype BEGINS KID array at 25 \(\upmu\) m when coupled to Fresnel zone plate lenses. For our optical set up and the absorption efficiency of the KIDs, the electrical NEP requirement at 25 \(\upmu\) m is \(7.6\times 10^{-17}\) W/ \(\sqrt{\text {Hz}}\) for an absorbed optical power of 0.36 pW. We find that over an average of five resonators the the detectors are photon noise limited down to about 200 fW, with a limiting NEP of about \(7.4\times 10^{-17}\) W/ \(\sqrt{\text {Hz}}\) . Future arrays will be coupled to microlens arrays and have higher optical efficiencies.
