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Alpha Spectrometry

1. Alpha Spectrometry. A useful tool for the detection and quantification of alpha-emitting radionuclides. A. Laissaoui. IAEA Regional Training Course on Sediment Core Dating Techniques RAF7/008 Project Rabat, 05 – 09 July 2010.

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Alpha Spectrometry

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  1. 1 Alpha Spectrometry A useful tool for the detection and quantification of alpha-emitting radionuclides A. Laissaoui IAEA Regional Training Course on Sediment Core Dating Techniques RAF7/008 Project Rabat, 05 – 09 July 2010 Centre National de l’Énergie des Sciences et des Techniques Nucléaires, Rabat - Morocco

  2. 2 Introduction Alpha emitting radionuclides produce alpha particles (4He) Alpha particles are heavy charged particles, they lose their energy easily in materials Unlike gamma spectrometry, alpha requires radiochemistry which consists of a series of chemical steps to: - Species of high purity Interferences - Thin counting sources Self absorption Centre National de l’Énergie des Sciences et des Techniques Nucléaires, Rabat - Morocco

  3. 3 Basic steps for radiochemical analisis by alpha spectrometry • Sample preparation. • Pre-concentration. • Chemical separation. • Source preparation. • Counting and analysis of spectrum. Centre National de l’Énergie des Sciences et des Techniques Nucléaires, Rabat - Morocco

  4. 4 Sample preparation Preliminary treatment is performed to generally prepare the sample for subsequent chemical processing Water samples: Acidification (pH < 1) and filtration Sediment samples: Drying, homogenizing, Ashing • Pre-concentration • Techniques: evaporation and co-precipitation • radionuclides in a small volume Centre National de l’Énergie des Sciences et des Techniques Nucléaires, Rabat - Morocco

  5. 5 Recovery measurement • For quantitative analysis: use of internal standard (tracer) is essential because losses are unavoidable. • Tracers are added at the start of the analysis. Requirements for Tracers: • are alpha-emitters • Pure • have certified activities • don’t interfere in spectrum-analysis • not present in the sample Centre National de l’Énergie des Sciences et des Techniques Nucléaires, Rabat - Morocco

  6. 6 Chemical separation: • Isolation of the element of interest • elimination of all organic and mineral substances and possible interferences of other alpha-emitting nuclides. • Two techniques are currently used: • 1. Liquid-liquid extraction • 2. Exchange resin Centre National de l’Énergie des Sciences et des Techniques Nucléaires, Rabat - Morocco

  7. 7 Source preparation • flat, thin and uniform counting sources • Techniques: • Electrodeposition on a flat and polished Stainless Steel disks for actinides and transuranics. • Evaporation • Microprecipition and filtration on a membrane filter • Self-deposition for Po on Ag or Pt discs Centre National de l’Énergie des Sciences et des Techniques Nucléaires, Rabat - Morocco

  8. 8 Counting and analysis of spectrum: • Detector: DJ, PIPS , SSB • Controled vacuum • Acquisition Software: Maestro , Genie 2000 • Calculation of activity (Bq/L or Bq/kg) • Net peak area • Backgound values • MDA • Chemical recovery Uncertainties • data of background values and calibration values (efficiency, resolution). Centre National de l’Énergie des Sciences et des Techniques Nucléaires, Rabat - Morocco

  9. U-238 U-232 U-234 5.32MeV 68% 5.26MeV 32% U-235 9 Excellent Radiochemistry Centre National de l’Énergie des Sciences et des Techniques Nucléaires, Rabat - Morocco

  10. 10 NOT TOO BAD Centre National de l’Énergie des Sciences et des Techniques Nucléaires, Rabat - Morocco

  11. 11 Centre National de l’Énergie des Sciences et des Techniques Nucléaires, Rabat - Morocco

  12. 12 210Pb Measurement in sediment 210Pb is a member of the 238U decay series 238U decays through several daughters to 226Ra 226Ra in rocks, soil and seawater decays to 222Rn some 222Rn escapes to the atmosphere and decays to 210Pb 210Pb in the atmosphere is delivered to marine systems via rainfall or dry fallout Adsorbe deposit UNSUPPORTED 210Pb Unsupported 210Pb in sediments must be distinguished from the supported fraction (226Ra) Centre National de l’Énergie des Sciences et des Techniques Nucléaires, Rabat - Morocco

  13. 210Pb (Bq/g) total 226Ra (210Pb) Depth in sediment (cm) 13 210Pb Measurement in sediment 210Pbxs Supported 210Pb is not measured directly with alpha spectrometers. It is generally estimated by the down-core asymptotic total 210Pb activity. 226Ra is measured by an independent analytical technique (alpha, gamma or LSC) and subtracted from the total lead in each stratigraphic level. Centre National de l’Énergie des Sciences et des Techniques Nucléaires, Rabat - Morocco

  14. T0 T1 T2 210Po 210Po 210Po 210Pb 210Pb 14 210Pb Measurement in sediment In sediment core dating, 210Pbex must be measured in each stratigraphic sample down to a depth where no measurable unsupported lead remains. In alpha spectrometry, total 210Pb activity is measured by alpha counting of the daughter radionuclide 210Po. Two cases: 1.- Old samples (almost 02 years): 210Pb is assumed to be in equilibrium with 210Po 2.- Freshly collected samples: no equilibrium in the topmost layers. Two elapsed measurments of 210Po Centre National de l’Énergie des Sciences et des Techniques Nucléaires, Rabat - Morocco

  15. 15 Radiochemistry of Po Weigh a required amount of the sample to be analyzed and spike it with a weighed aliquot of the tracer (208 or 209Po). Digest the sample using strong acids (HCl, HNO3, HF) Dissolve the sample in HCl 0.5N and filtrate Place a silver disc coated on one side in the bottom of the beaker keep on a hot plate at 70-80°C while being stirred occasionally After 6 hours, remove the disc and rinse it with H2O and ethanol The source is ready to be counted in an alpha spectrometer Centre National de l’Énergie des Sciences et des Techniques Nucléaires, Rabat - Morocco

  16. Po-209 Po-210 16 Polonium Spectrum If a second self-deposition is to be done, the remaining solution should be kept for 3 – 6 months. But before storing the sample, Po must be removed. A second deposition is carried out just after the first to remove Po, but not completely. Another procedure to remove the remaining Po is the use of anion exchange resin in Cl- form. In 10M HCl medium, Po is strongly adsorbed while Pb is not adsorbed onto the column.

  17. æ ö ( ) I B - I B ç ÷ - e e = ç ÷ A t t A è ø e t . R . . M = e b t A . q e æ ö M I B ç ÷ - t t ç ÷ t t è ø t b 17 Activities Calculation Activity of 210Pb at the sampling date: Activity of 210Po at the sampling date: Centre National de l’Énergie des Sciences et des Techniques Nucléaires, Rabat - Morocco

  18. 18 Transuranics (Pu and Am) At least four oxidation forms Pu(III), Pu(IV), Pu(V) and Pu(VI) exist in marine waters Pu radiochemistry is based mainly on purification by anion exchange resin and oxidation states adjustment 239Pu and 240Pu can not be discriminated by alpha spectrometry The most likely chemical form in the marine environment is Am(III) Radiochemistry of Am is quite complicated Centre National de l’Énergie des Sciences et des Techniques Nucléaires, Rabat - Morocco

  19. Sequential measurement of Pu and Am in sediments 19 Sample + tracers (242Pu & 243Am) Wet leaching using HNO3 and H2O2 (No need for total digestion) Put in HNO3 8M and add NaNO2 to convert all Pu into Pu(IV) [Pu(NO3)6]2- Pass the solution through resin Wash the resin with HNO3 8M Wash the column with HCl 10M Elute Pu with HCl 10M and NH4I Electrodeposition Am fraction Pt wire Chimney Power supply SS Disc Base Centre National de l’Énergie des Sciences et des Techniques Nucléaires, Rabat - Morocco

  20. 20 Americium purification Am co-precipitated with calcium oxalate (Calcium chloride and axalic acid) Extracted into Dibutyl-N,N-diethyl carbamyl phosphate (DDCP) in 12M HNO3 Separated from rare earths by anion exchange in mineral acids-methanol media Source counting preparation by electrodeposition in H2SO4/NH4OH media Centre National de l’Énergie des Sciences et des Techniques Nucléaires, Rabat - Morocco

  21. 21 Pu and Am Spectra 242Pu 239+240Pu 241Am 243Am 238Pu Acquisition time = 10 days Centre National de l’Énergie des Sciences et des Techniques Nucléaires, Rabat - Morocco

  22. 22 Conclusion - The high sensitivity of  detectors makes them suitable for counting low level activities in sediment cores. High resolution - All steps must be performed correctly. Errors can invalidate the results of big efforts, increasing the cost of analysis and causing disturbances in the normal course of operations Centre National de l’Énergie des Sciences et des Techniques Nucléaires, Rabat - Morocco

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