DETERMINATION OF AN ENVIRONMENTAL BACKGROUND SPECTRUM AND DATA TO BE USED FOR ROUTINE ANALYSES OF WHOLE BODY COUNTS F BEESLAAR

CONTENT Introduction and Background Method Results Discussion and Conclusion

INTRODUCTION AND BACKGROUND Occupational Monitoring of Necsa workers for incorporated nuclides In-Vivo technique: Whole Body Counting and Lung Counting Whole Body Counting: Monitor internal exposures to long/short lived fission and activation products medical isotope production industrial source production Lung Counting: Low energy gamma emitters (235U) Routine counting regimes: Whole Body: 10 min, 30 min, 60 min (special counts)

Introduction and Background Canberra Whole Body and Lung Counter system 4 x BEGE Lung detectors and 1 x Scanning NaI Whole Body detector Shielded container

Introduction and Background

Introduction and Background Peak Centroid (Channel Number/Energy)

Introduction and Background Energy spectrum contains spectral photo peaks from artificial nuclides incorporated during occupational exposure and photo peaks due to radioactivity in counting environment Natural radiation Cosmic -, Terrestrial – and Internal Radiation (40K) Problem Statement: Artificial and environmental peaks close together: 131I - 364 keV and 214Pb – 352 keV due to poor energy resolution Differentiate between Environmental and Artificial peaks Technical justification to exclude certain peaks from occupational exposure assessment

Method Over several months acquired spectrums for different counting times 10 min, 30 min and 60 min, 120 min (improved detection) Blank spectrum: i.e. no human or phantom Peak Centroid determined and Net Peak Area calculation done by software (GENIE 2000) Allocate peaks to expected Environmental Peaks Basic Statistical Analyses: Peak Energy (centroid) and Net Peak Area (counts) – Variations? Avg, SD, CV and determined Upper and Lower Limit

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Results Peak Centroid [keV] Possible Nuclides Source 188 Ra-226 (186 keV) U-238 Decay Series (U-235!) 243 Pb-212 (238 keV); Pb-214 (242 keV) U-238/Th-232 Decay Series 299 Pb-214 (295 keV) 356 Pb-214 (351 keV) (I-131!) 507 Annihilation (511 keV) Pair production in shielding material from high energy gamma/cosmic rays 609 Bi-214 (609 keV) 938 Bi-214 (934 keV); Ac-228 (911 keV & 969 keV) 1136 Bi-214 (1120 keV & 1155 keV) 1454 K-40 (1460) Primordial

Results Count Time : 7200 s CV high for Net Peal Area - ??? Lookup Table with Upper and Lower limit (2 x STDev) Centroid Avg (STDev) [keV] CV Net Peak Area Avg (STDev) [Counts] 188 2.2% 1737 56.1% 243 1.0% 5839 21.5% 299 1730 44.6% 356 (3.1) 0.9% 4905 (817) 16.7% 507 0.7% 2006 20.0% 609 2919 13.7% 938 1306 23.5% 1136 1.5% 429 26.2% 1454 0.9 1208 17.1%

Results: Lookup Table E.g. Spectrum from Rad Worker: Peak at 360 keV with NPA = 6800 counts

Discussion Environmental spectrum taken once a week for 2 hours Terrestrial radiation from 238U and 232Th decay chains However there are changes from day to day and seasonal. Even from morning to afternoon. Variability in Net Peak Area due to Net Peak Area: 222Rn daughters - seasonal, time of day, weather, Energy Centroid: room temperature Daily QC check – energy correcting (adjusting gain) Software report a peak if above Environmental Peak

Discussion Lookup Table used for routine evaluation of staff counts Judgement by technical expert (Spectroscopy and RP) Knowledge of exposure environment Deliberation captured in report Missed dose in case of wrong allocation to environmental peak as false negative (e.g. 131I few μSv)

THANK YOU Question? Dosimetry is not an exact science! Internal Dosimetry is more and art than science! We are not chasing uSv!

Questions?