1. 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

2. 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

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

4. Intercomparisons NRPB / HPA Annual Exercise
Since 1997
40 passive detectors
10 x transits – subtracted from exposures
10 x ‘low’ radon exposure ~ 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

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

6. 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

7. 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)

8. 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

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

10. Ageing / Fading Effects Hardcastle & Miles (1996)
Combined ageing & fading correction factor = M M

11. HPA Ageing & Fading Correction Factors

12. 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

13. 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 ?

14. Passive detectors in NRPB / HPA Intercomparisons