Integrating Passive Detection
Passive detectors require no electrical power and integrate radon exposure over weeks to months. They are the standard method for determining annual average concentration—the metric used in health guidance and building regulations worldwide.
Alpha-Track Detectors (CR-39, LR-115)
Small plastic chips enclosed in filtered chambers. Alpha particles from radon and its decay products strike the plastic, creating microscopic damage tracks. After exposure (typically 90 days to 1 year), the detector is returned to a laboratory where chemical etching enlarges the tracks for counting.
Performance Characteristics
- •Detection limit: ~30 Bq/m³ for 1-month exposure (WHO); research-grade systems reach 7 Bq/m³
- •Uncertainty: Typically 10–15% for quality laboratory analysis
- •Duration: Minimum 2 months (ISO 11665-8); 3+ months recommended for regulatory compliance
- •Robustness: Insensitive to humidity, temperature, and background radiation
Electret Ion Chambers (E-PERM)
A charged Teflon disc (electret) inside a chamber serves as both electric field source and sensor. Ionization from radon decay gradually discharges the electret. Voltage drop over known time period equals integrated radon exposure.
Key Considerations
- •Short-term (2–15 days) and long-term (3–12 months) configurations available
- •Responds to all ionizing radiation—requires gamma background correction
- •Accuracy degrades at low concentrations due to background contribution (~25 Bq/m³ at 50 Bq/m³ measured)
- •Excellent precision when procedures followed correctly
Activated Charcoal Detectors
Radon adsorbs onto activated carbon during 2–7 day exposure. After sealing and 3-hour in-growth wait, gamma emissions from progeny are counted (for canisters) or liquid scintillation counting is used (for small vials).
Limitations
- •Screening tool only—not suitable for compliance decisions
- •Results weighted toward final days of exposure due to continuous adsorption/desorption
- •Sensitive to humidity, temperature, and exposure duration
- •Fast return to laboratory required (radon 3.8-day half-life)
- •Detection limit: ~20 Bq/m³ for 2–7 days (WHO)
Regulatory Use
European standards (ISO 11665-8), Health Canada, UK UKHSA, and many national protocols require long-term passive measurements (≥2–3 months) for the annual average determination. Short-term tests can vary from annual mean by factor of 2 or more due to temporal variability.
Selection Criteria
Compliance measurement: Alpha-track detectors, 90+ days
Initial screening: Charcoal detectors, 2–7 days
Seasonal monitoring: Electrets or alpha-track, quarterly deployment
Laboratory selection: Verify accreditation (ISO 17025, NRPP, C-NRPP)
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