A Passive Indoor Radon Witness Cheap Enough to Matter: Design and Fully Computed Sensitivity Budget for a 90-Day CR-39 Diffusion-Chamber Detector at Under US$5 in Parts

Design study (SIMULATED, NOT MEASURED) of a passive no-electronics CR-39 diffusion-chamber radon witness with US$4.75 in parts for a 90-day deployment. Computed practical minimum detectable concentration 8.56 Bq/m3 and 18.9% total uncertainty at 100 Bq/m3: it resolves the WHO 100 Bq/m3 reference level and the US EPA 148 Bq/m3 action level as screening decisions with error rates of a few percent, but cannot certify borderline concentrations. All numbers reproduce from the archived compute.py.