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Overview of Analytical Procedures in Sediment Core Dating Techniques

This document provides a comprehensive overview of analytical procedures relevant to sediment core dating, focusing on radionuclides and their interactions through gamma spectrometry. Key topics include the principles behind hyper-pure germanium detectors, detection calibration, and activity calculation, along with a detailed examination of both natural and man-made radionuclides, such as 137Cs, 210Pb, and 241Am. The course emphasizes practical aspects like efficiency calibration, correction factors, and uncertainties in measurement, making it essential for researchers involved in environmental and geological studies.

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Overview of Analytical Procedures in Sediment Core Dating Techniques

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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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