DETERMINATION OF AN ENVIRONMENTAL BACKGROUND SPECTRUM AND DATA

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 2

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 (235 U) • Routine counting regimes: • Whole Body: 10 min, 30 min, 60 min (special counts) 3

Introduction and Background • Canberra Whole Body and Lung Counter system • 4 x BEGE Lung detectors and 1 x Scanning Na. I Whole Body detector • Shielded container 4

Introduction and Background 5

Introduction and Background Peak Centroid (Channel Number/Energy) 6

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 (40 K) • Problem Statement: • Artificial and environmental peaks close together: • 131 I - 364 ke. V and 214 Pb – 352 ke. V due to poor energy resolution • Differentiate between Environmental and Artificial peaks • Technical justification to exclude certain peaks from occupational exposure assessment 7

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 8

Mehod 9

Results Peak Centroid [ke. V] Possible Nuclides Source 188 Ra-226 (186 ke. V) U-238 Decay Series (U-235!) 243 Pb-212 (238 ke. V); Pb-214 (242 ke. V) U-238/Th-232 Decay Series 299 Pb-214 (295 ke. V) U-238 Decay Series 356 Pb-214 (351 ke. V) U-238 Decay Series (I-131!) 507 Annihilation (511 ke. V) Pair production in shielding material from high energy gamma/cosmic rays 609 Bi-214 (609 ke. V) U-238 Decay Series 938 Bi-214 (934 ke. V); Ac-228 (911 ke. V & 969 ke. V) U-238/Th-232 Decay Series 1136 Bi-214 (1120 ke. V & 1155 ke. V) U-238 Decay Series 1454 K-40 (1460) Primordial 10

Results Count Time : 7200 s Centroid Avg (STDev) [ke. V] CV 188 2. 2% 243 1. 0% 299 1. 0% 356 (3. 1) 0. 9% 507 0. 7% 609 0. 7% 938 0. 9% 1136 1. 5% 1454 0. 9 Net Peak Area Avg (STDev) [Counts] 1737 5839 1730 4905 (817) 2006 2919 1306 429 1208 CV 56. 1% 21. 5% 44. 6% 16. 7% 20. 0% 13. 7% 23. 5% 26. 2% 17. 1% CV high for Net Peal Area - ? ? ? Lookup Table with Upper and Lower limit (2 x STDev) 11

Results: Lookup Table E. g. Spectrum from Rad Worker: Peak at 360 ke. V with NPA = 6800 counts 12

Discussion • Environmental spectrum taken once a week for 2 hours • Terrestrial radiation from 238 U and 232 Th 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: 222 Rn 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 13

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. 131 I few μSv) 14

• THANK YOU • Question? Internal Dosimetry is more and art than science! Dosimetry is not an exact science! We are not chasing u. Sv! 15

Questions? www. necsa. co. za