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Lecture 5 Overview on the Analytical Procedures ( g )

IAEA Regional Training Course Sediment Core Dating Techniques - RAF/7/008 Project CNESTEN, Rabat, 05-09 July 2010. IAEA. CNESTEN. Lecture 5 Overview on the Analytical Procedures ( g ). Moncef Benmansour CNESTEN, Rabat Morocco. Contents. Radionuclides and radiations

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Lecture 5 Overview on the Analytical Procedures ( g )

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  1. IAEA Regional Training Course Sediment Core Dating Techniques - RAF/7/008 Project CNESTEN, Rabat, 05-09 July 2010 IAEA CNESTEN Lecture 5Overview on the Analytical Procedures (g) Moncef Benmansour CNESTEN, Rabat Morocco

  2. Contents • Radionuclides and radiations • Basis of Gamma spectrometry • Hyper Germanium detectors • Detection calibration • Activity calculation, uncertainty, detection limit • Correction factors • Comparative measurements: 137Cs, 210Pb, 226Ra, 241Am

  3. Radionuclides and Radiations Natural Radionuclides Cosmogenic Radionuclides 14C, 3H, 22Na, 7Be…. Primordial Radionuclides (Singly) 40K, 87Rb, 50V, 144Nd... Primordial Radionuclides ( Natural series) 238U, 235U, 232Th series

  4. Radionuclides and Radiations • Man made Radionuclides • Fissions Products • 137Cs, 90Sr, 89Sr, 131I, 99Tc • Activation Products • 239Pu, 240Pu, 241Pu, 241Am, 242Cm, • 60Co, 65Zn, 54Mn, 55Fe… • - Nuclear Weapons testing • Chernobyl Accident • Discharges from reprocessing

  5. Radionuclides and Radiations • Alpha particles (a) helium • Beta particles (b-, b+) e- and e+ • Electronic Capture • Gamma rays (g): Photons • 137Cs, 210Pb, 241Am,… Ee g g Eg

  6. Gamma-Matter Interaction: Interaction processus Photoelectric effect Compton Pair production g e- 2m0C2 = 1,02 MeV

  7. Gamma-Matter Interaction • Attenuation of g • Ig(x) = I (0) e-mx x I(0) Ig(x) m : Attenuation coefficient cm-1 or cm2/g

  8. Gamma attenuation Lead Aluminium

  9. Gamma spectrometry: General Basis • Interaction of g photons with the detector • Production of electric pulses : Amplitude proportionnel to photon energy emitted by the source • Whole information contained in a gamma spectrum ( gamma energy, & activity)

  10. Hyperpur Germanium Detectors • Semiconductor diodes having a p-i-n structure • Intrinsic (I) region is sensitive to ionizing radiation, particularly x rays and g rays • Under reverse bias, an electric field extends across the intrinsic or depleted region. • When photons interact with the material charge carriers (holes and electrons) are produced and are swept by the electric field to the p and n electrodes.

  11. Hyperpur Germanium Detectors • Relative efficiency, energy resolution, energy range, peak/compton ratio

  12. Hyperpur Germanium Detectors

  13. Electronic parmeters • Power Supply: H.V • Amplifier • Gain : Coarse and Fine • Shaping time • Zero pole • Parameters of MCA

  14. Hyperpur Germanium Detectors

  15. Energy Calibration • Energy – Canal Relation • Two sources ( 137Cs, 60Co) • Multi-gamma sources

  16. Energy Calibration

  17. Efficiency calibration • Full – energy –peak efficiency: e (E) • e (E) = N(E) /R • N (E): count rate in the peak corresponding to the Energy E • R: rate at which photons of Energy E are emitted from the source • R = A.Ig • A :Source Activity • Ig: : Gamma ( g ) ray emission probability

  18. Efficiency calibration • e (E) depends on: • Source dimension and source –detector distance • Dimensions of the detector housing and of the sensitive and insensitive zones of the detector • Elementary composition and density of all materials traversed by the photons • Photon attenuation coefficients of these materials • Energy-and angle-dependent cross sections of the detector material for the various photon interactions • Information on the electron and positron transport in the detectors

  19. Efficiency calibration • Efficiency calculation • Monte Carlo codes, but many constraints • Uncertainties in the shape and size of the effective or sensitive crystal volume • Uncertainties on the photons and electron interaction probaility and angular distributions • Efficiency measurements • Calibration sources: easier and more accurate than calculation • e (E) VS Energy (keV)

  20. Standard sources • Liquid multi-gamma sources with certified activities purchased from an international provider • Different Marked matrixes prepared by the supplier in different geometries • Reference Materials: (e.g.. IAEA)

  21. Standard Sources

  22. Efficiency CurvesHPGe, coaxial – P Type: Rel. Eff.30%

  23. Efficiency Curves P-type and N- type detectors

  24. Efficiency fitting

  25. Spectral Evaluation

  26. N = Nt - Nb a1, a2, b1 b2

  27. Spectral Evaluation

  28. Calcul of activity General Case If tc << T1/2

  29. Calcul of activity: Areal activity

  30. Uncertainties

  31. Uncertainties IAEA TECDOC 1401

  32. Backround

  33. Detection Limit

  34. Detection Limit

  35. Correction factors • Factor corrections (Fc) • Coïncidence –summing corrections ( two or more photons within the resolving time of the spectrometer). • Dead –time and pil-up corrections • Attenuation correction: self-absorption attenuation

  36. Coïncidence –summing corrections (eg. two Radionuclides ) • N1 = A I1e1 ( 1 – e12) • C1 = 1/(1- e12) • N2 = AI2 e2 [1 – (I1/I2) e12] • C2 = 1 / [1—(I1 /I2) e12] • N3 = AI3 e3 + AI1 e1 e2 ] • C3 = 1/[1+I1e1 /(I3e3) E2 (I2)

  37. Dead time and pile-up correction • MCA : Real and live time • Pile - up correction rejector • Pulser method: • N0 = N f t/Np • Net N0: measured and true number of counts in the peak respectively • Np: number of counts in the pulser peak • F: frequency of the pulser • T: Counting time

  38. Attenuation correction • Attnuation law: Ig(x) = I (0) e-mdr • m: masse attenuation coefficient • d : tickness of the sample • r : density of the ample • Self-attenuation Facteur: • F (mdr)= [1-exp(-mdr)]/ mdr • Correction Facteur • Ca = F (mdr)sample /F(mdr)standard • (E > 100 keV): Ca depends exclusively on the sample density • (E <100 keV): Ca depends also on the chemical composition

  39. Attenuation correction Boshkova and Minev , ARI 54 (2001) 777-783

  40. Attenuation correction • Can be determined • Using analytical methods • Using the Monte-Carlo Computation techniques • Experimentally

  41. Attenuation correction: Experimentally • Point Source on the top of containers: • with unknown sample, standard, and air Cutshall et al., NIM PR A 206 (1983) 309-312

  42. Comparative measurements 137Cs, 210Pb, 226Ra, 241Am

  43. 137Cs

  44. 210Pb

  45. 241Am

  46. 226Ra

  47. Comparison

  48. 137Cs & 210PbSelf-absorption Sediment Samples (100 m) HPGe 45% -N Type -

  49. Conclusion • Gamma spectrometry: Direct technique, without radiochemical separation, but requires some precautions: • Selection of suitable HPGe detectors • Selection of suitable standards • Sample preparation and geometry of counting • Efficiency curve • Background • F actor effects • All sources of uncertainty • Quality Control Programme

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