Radon Measurement Fundamentals and Physics
What you must understand
Radon-222 is a colorless, odorless, chemically inert noble gas produced in the uranium-238 decay chain. Its immediate parent is radium-226; its short-lived progeny (polonium-218, lead-214, bismuth-214, polonium-214) are the solid, charged particles that attach to aerosols and deliver most of the lung dose. Know the 3.82-day half-life - long enough to migrate from soil into buildings, short enough that concentrations respond to ventilation within hours. Units: 1 pCi/L = 37 Bq/m³. Working Level (WL) describes progeny energy; 1 WL = 100 pCi/L at 50% equilibrium, and the equilibrium ratio (progeny activity divided by radon activity) is typically 0.3–0.7 indoors. Decay is exponential: activity halves each half-life, so roughly five half-lives (~19 days) reduce a sealed sample's radon to about 3% of its initial value.
Traps
Questions conflate radon with its progeny (dose vs. gas measurement), confuse half-life with the 3.05-minute polonium-218, or assume 1 pCi/L equals 1 WL. Watch for unit conversions and for claims that a filter alone measures radon gas - filters collect progeny, not the gas.
Worked example
A 1 L chamber holds 4.0 pCi/L. After 7.64 days (two half-lives), activity = 4.0 × (1/2)² = 1.0 pCi/L, plus ingrowth from any radium in the chamber - negligible in a clean, radium-free device. Rule of thumb: each 3.82 days halves the count; each 10 °C is a minor factor compared with ventilation.