1 – 5 September 2008 ICNTS-24

Sources of error in etched track measurements – or –

why aren’t my results as good as yours whenwe’re using the same detectors?

Fero Ibrahimi

Accuracy & PrecisionPassive radon detectors

Assessing Accuracy

Reference value / Calibration - Radon Chamber / BoxChamber / 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

IntercomparisonsNRPB / HPA Annual Exercise

IntercomparisonsNRPB / 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%

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

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Best Mean Absolute % Difference

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

Holder Design Detector Material Best Mean % SD

Non-conducting NRPB PADC 4.6

Non-conducting ANPA LR115 / PADC 4.7

Non-conducting RAD E77 PADC 4.9

Conducting NRPB/SSI PADC 2.5

Conducting TASL PADC 3.7

Conducting Karlsruhe FN Polycarbonate 4.3

Conducting Radosys 2000 PADC 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 detectorsDiffusion cups / casings – Rn-220 (Tn)Etched track material / polymer:

• chemicals – monomer, initiator, plasticiser• cure cycle variation

Etching chemicals

Laboratory Personnel

Track overlapCalibration curve corrections

Counting whole etched tracks Counting foreground pixels (px)

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Ageing / Fading EffectsHardcastle & Miles (1996)

Combined ageing & fading correction factor = 0.0007M2 + 0.0142M + 0.9528

HPA Ageing & Fading Correction Factors

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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

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

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Closed DetectorsOpen Detectors