Passive Radon Witness (CR-39) — Design Blueprint: 90-Day Diffusion-Chamber Detector

DESIGN BLUEPRINT — SIMULATED, NOT MEASURED — NOT BUILT. Build-ready design blueprint derived from 'A Passive Indoor Radon Witness Cheap Enough to Matter' (DOI: 10.5281/zenodo.24108450). Specifies a passive CR-39 diffusion-chamber radon witness: 20 x 10 x 0.9 mm chip (2.0 cm2) in a 250 mL cup of 35 mm internal height with a glass-fibre diffusion barrier, deployed 90 days, etched in 6.25 mol/L NaOH at 70 C for 6 h, and read over the whole chip by smartphone-microscope counting. Includes bill of materials with source-stated unit costs (total US$4.75), dimensions and tolerances, assembly, deployment, batch calibration and readout procedures, operating envelope with printed failure regimes (borderline readings compatible with roughly 62-138 Bq/m3 at 100 Bq/m3 cannot be certified; 7-day deployment collapses), maintenance and consumables, failure modes, and explicit non-claims. Design calibration factor 0.0665 tracks cm-2 per (Bq m-3 d); practical MDC 8.56 Bq/m3; total uncertainty 18.9% at 100 Bq/m3.