We report on the effects of cosmic ray interactions with the kinetic inductance detector (KID)-based focal plane array for the terahertz intensity mapper (TIM). TIM is a NASA-funded balloon-borne experiment designed to probe the peak of the star formation in the Universe. It employs two spectroscopic bands, each equipped with a focal plane of four \(\sim \,\) 900-pixel, KID-based array chips. Measurements of an 864-pixel TIM array show 791 resonators in a 0.5 GHz bandwidth. We discuss challenges with resonator calibration caused by this high multiplexing density. We robustly identify the physical positions of 788 (99.6 %) detectors using a custom LED-based identification scheme. Using this information, we show that cosmic ray events occur at a rate of 2.1 \(\,\mathrm {events/min/cm^2}\) in our array. 66 % of the events affect a single pixel, and other 33 % affect \(<\,\) 5 KIDs per event spread over a 0.66 \(\,\mathrm {cm^2}\) region (2 pixel pitches in radius). We observe a total cosmic ray dead fraction of 0.0011 % and predict that the maximum possible in-flight dead fraction is \(\sim \,\) 0.124 %, which demonstrates our design will be robust against these high-energy events.
