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Determination of 226 Ra in Environmental and Personal Monitoring Samples

Determination of 226 Ra in Environmental and Personal Monitoring Samples. Billy Lawrie Geoffrey Schofield Laboratories. Overview. Introduction Why measure 226 Ra? Properties of Ra Methods available Difficulties Experimental Results and Discussion Conclusions. Why Measure 226 Ra?.

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Determination of 226 Ra in Environmental and Personal Monitoring Samples

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  1. Determination of 226Ra in Environmental and Personal Monitoring Samples Billy Lawrie Geoffrey Schofield Laboratories

  2. Overview • Introduction • Why measure 226Ra? • Properties of Ra • Methods available • Difficulties • Experimental • Results and Discussion • Conclusions

  3. Why Measure 226Ra? There are 4 naturally occurring isotopes of Ra: • 232Th decay series 224Ra ( emitter, t½ 3.66 days) 228Ra (- emitter, t½ 5.75 years) • 235U decay series 223Ra ( emitter, t½ 11.4 days) • 238U decay series 226Ra ( emitter, t½ 1600 years)

  4. Why Measure 226Ra? (2) • 226Ra causes the most concern • long half life • radiological effects • Toxic • Widespread • Concentrates in bones • increases internal radiation dose of individuals

  5. Properties of Radium • Alkaline earth metal • Only one oxidation state (+2) • Not easily complexed • majority of compounds are simple ionic salts • Ca, Sr, Ba & Ra form a series of closely related elements • problems in chemically isolating Ra • Few complexes that pass into the organic phase • solvent extraction not suitable

  6. Methods of Determining Radium • Detection following BaSO4 co-precipitation • Radon emanation • Gamma ray spectrometry • Thermal ionisation mass spectrometry (TIMS) • Alpha spectrometry

  7. Barium Sulphate Co-precipitation • Ba(Ra)SO4 • Tedious • Slow • Analyst dependent

  8. Radon Emanation • Can only measure 226Ra • Indirect • Time consuming • Slow (20 days to achieve full equilibrium) • 222Rn is a gas • potential problems during sample handling • Low levels of detection require large sample volumes

  9. Gamma Ray Spectrometry • Limited to the analysis of 226Ra • Direct determination (186keV) • low -emission probability • 235U interference • Determination via 214Bi • time consuming equilibration between 226Ra and 214Bi • Distribution of 214Bi must be homogeneous throughout sample • Standard must have the same configuration & density as sample • neither of these two options are easy to achieve.

  10. TIMS • Shorter analytical time • Improvement in analytical precision • Reduction of the Ra sample size • Requires Ra load to be extremely pure • in particular it has to be free from Ba

  11. Alpha Spectrometry • Can determine all -emitting Ra nuclides directly • No consideration of equilibria loss • High resolution surface barrier detectors • determination virtually specific • very few spectral interferences • Can be performed in a timely manner • 2 days source prep. + counting time • Electrodeposition gives robust source

  12. Difficulties • Adsorption • Dissolution • Tracer • Electrodeposition • Interferences • chemical • spectral

  13. Adsorption • Ra adsorbs onto suspended particles, colloids & container walls • Precautions to avoid loss of radium • sampling • analysis • Water samples acidified • Collected in acid washed plastic containers • Sample vigorously shaken before sub-sampling • re-suspend any adsorbed radium

  14. Dissolution • Total dissolution required • Must be free from suspended particles • eliminates loss of radium • Microwave digestion • Fusion

  15. Suitable Tracer • Tracer v’s parallel standards • 224Ra • occurs naturally • short t½ (3.66 days) • 225Ra • not naturally occurring • also has a short t½ (14.8 days)

  16. Suitable Tracer (2) • 133Ba • acceptably long t½ (10.66 years) • non-isotopic • susceptible to error regardless of chemical similarity1 • 226Ra parallel standard • parallel sample spiked with 226Ra • best option for a batch laboratory 1. Sill, C.W. Nucl. Chem. Waste Manage. 1987, 7, 239

  17. Chemical Interferences • Major interferent is Ba • A little makes a significant difference • 10g can result in a 50% reduction in recovery of Ra1 • Can be minimised by: • limiting sample size to ~ 0.1g • washing column with 1.5M HCl1 • adding (COONH4)2 to the ED solution2 1. Alvarado et al J. Radioanl. Nucl. Chem. 1995, 1, 163 2. Orlandini et al Radiochim. Acta 1991, 55, 205

  18. Spectral Interferences • 226Ra produces a doublet • 4.602MeV (5.55%) & 4.785MeV (94.45%) • 226Ra daughters 222Rn, 218Po & 214Po are present • Main possible interferent is 234U (4.773MeV) • 234U in equilibrium with it’s parent 238U (4.194MeV) • if no 238U then 234U won’t be contributing to 226Ra peak • Shouldn’t pose a significant problem • U should be removed at the separation stage

  19. Electrodeposition • One of the most awkward species to ED • Partial deposition observed during deposition of 228Th1 • Method for deposition at pH 8-9 developed by Roman2 • Orlandini et al showed advantage of using Pt and (COO NH4)2 • Add g amounts of Pt3 • ‘Pt black’ film produced • Ra quantitatively plated • robust source • (COO NH4)2 increases efficiency in presence of 5-10g Ba3 1. Sill, C.W. et al Anal. Chem. 1974, 46, 1725 2. Roman, D. Int. J. Appl. Radiat. Isot., 1984, 35, 990 3. Orlandini, K.A. et al Radiochim Acta 1991, 55, 205

  20. Experimental - Overview • Microwave digestion • double digestion procedure • HCl, HNO3 & HF followed by EDTA & H3BO3 • Separation (BioRad AG-50W-X8 resin)1 • Ba elimination (EIChroM Sr.spec resin)2 • Electrodeposition [400g Pt, 0.17M (COO NH4)2, 0.14M HCl] • Counting (-spectrometry) 1. Alvarado et al J. Radioanal. Nucl. Chem., 1995, 1, 163 2. Chabaux et al Chem. Geol., 1994, 114, 191

  21. Experimental dissolve evaporate sample (1) 1M HCl (2) 1.5M HCl (3) 6M HCl BioRad AG-50W-X8 (3) (1) & (2) waste sample (1) evaporate 3M HNO3 EIChroM Sr.spec (2) 3M HNO3 -spec (1) (2) ED sol. waste evaporate electrodeposit Pt

  22. Intercomparison Results

  23. Recoveries from Marine Sediment Reference Sample IAEA-135

  24. Results from Environmental & Personal Monitoring Samples

  25. Results Summary • Intercomparison results compare favourably with actual levels • Marine sediment reference material IAEA-135 • mean result obtained from intercomparison exercise • 66 labs participated • 64 used -ray spectrometry; 2 used Rn emanation • 11 outliers • recommended value 23.9Bqkg-1 • confidence interval 20.6 - 25.0Bqkg-1 @ 95% confidence level • range of accepted results 13.6 - 36.0Bqkg-1 • from replicate parallel analysis obtained 23.6  3.6Bqkg-1 • Good recoveries achievable on a variety of problematic matrices

  26. Conclusions • Robust method • Capable of analysing wide variety of matrices • Good LOD’s achievable • 20mBqL-1 for water samples (100ml) • 20mBqg-1 for solid samples (0.1g) • Future work will focus on 228Ra • -spectroscopy • electrodeposition disc used for 226Ra

  27. Acknowledgements • Thanks are expressed to: • Jim Desmond • Debbie Spence • Scott Anderson • Clare Edmondson

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