Sources of error in etched track measurements – or – why aren’t my results as good as yours when we’re using the same detectors? Fero Ibrahimi 1 – 5 September 2008 ICNTS-24

Accuracy & Precision Passive radon detectors Assessing Accuracy Reference value / Calibration - Radon Chamber / Box Chamber / Box Instrumentation – itself calibrated / intercomparison ! Assessing Precision Standard error of the mean (SEM) (s / n) Standard deviation (SD) Assessing Both Internal control - Blind test / dummy customer External control – Certification Assurance / Proficiency Test / Validation Scheme External control - Intercomparison Exercises Radon-222 Sources: Primary National Standards Organisation – e.g. NPL, UK or PTB, Germany Standard error of the mean (SEM) (s / n) - minimum 5 replicates ? Standard deviation (SD) – more conservative than SEM - HPA 6 / 110

Intercomparisons NRPB / HPA Annual Exercise 10 x transits – overseas parcels – custom delays

Intercomparisons NRPB / HPA Annual Exercise Since 1997 40 passive detectors 10 x transits – subtracted from exposures 10 x ‘low’ radon exposure ~ 0.1 - 0.2 MBq m-3 h 10 x ‘UK action level’ exposure ~ 0.2 – 1.0 MBq m-3 h 10 x ‘high’ exposure ~ 1.0 – 2.0 MBq m-3 h 3 different radon exposures & equilibrium factors (F) For each exposure set (Net) Absolute % Difference % Standard Deviation For all 3 exposure sets Mean % Difference Mean % Standard Deviation Sum Rank results Grade ‘A’ < 10% 10 x transits – overseas parcels – custom delays

How accurate can I hope to be? NRPB / HPA intercomparisons

How precise can I hope to be? NRPB / HPA intercomparisons Holder Design Detector Material Best Mean % SD Non-conducting NRPB PADC 4.6 ANPA LR115 / PADC 4.7 RAD E77 4.9 Conducting NRPB/SSI 2.5 TASL 3.7 Karlsruhe FN Polycarbonate 4.3 Radosys 2000 5.2

Sources of Measurement Uncertainty Radon calibration reference value Radon-222 source  3.1% at 2 sigma (95%) Confidence Level - PTB HPA Radon Chamber - minimum  5.3% Laboratory Etching equipment Counting system - Track Recognition, Focus, Scratches Track overlap – calibration curve correction Ageing / Fading effects Seasonal / temperature corrections (Miles, 2001)

Other sources of error Passive detectors Laboratory Diffusion cups / casings – Rn-220 (Tn) Etched track material / polymer: chemicals – monomer, initiator, plasticiser cure cycle variation Etching chemicals Laboratory Personnel Diffusion cups / casings – material variation Etched track material: material variation: chemicals – monomer, initiator, plasticiser; cure variation Personnel – training, QA testing

Track overlap Calibration curve corrections Counting whole etched tracks Counting foreground pixels (px)

Ageing / Fading Effects Hardcastle & Miles (1996) Combined ageing & fading correction factor = 0.0007M2 + 0.0142M + 0.9528

HPA Ageing & Fading Correction Factors

What Quality Assurance checks are you doing? Why aren’t your results as good as somebody else’s when you’re using the same detectors? What Quality Assurance checks are you doing? Personnel – adequate + continued training / support Radon Chamber / Box instrumentation calibration / intercomparison Detector calibration - material sensitivity & background – HPA each sheet Etch System – HPA every time Count System– HPA every time Track overlap – calibration linearity Ageing & Fading effects Seasonal / temperature effects on annual average concentration

Any Questions / Comments ? Why aren’t your results as good as somebody else’s when you’re using the same detectors? continued How often should you assess your measurement system? Minimum: internal blind test – every 6 months ? Better: internal blind test – every batch of etch track material Even better: external proficiency / intercomparison test – 3 diff exposures Best: all of the above! Any Questions / Comments ?

Passive detectors in NRPB / HPA Intercomparisons