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

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

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

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

6 Radionuclides and Radiations Alpha particles (  ) helium Alpha particles (  ) helium Beta particles (      e- and e + Beta particles (      e- and e + Electronic Capture Electronic Capture Gamma rays (  ): Photons Gamma rays (  ): Photons – 137 Cs, 210 Pb, 241 Am,… EeEe EgEg 

7 Gamma-Matter Interaction: Interaction processus  Photoelectric effect Compton Pair production e-e- 2m 0 C 2 = 1,02 MeV

8 Gamma-Matter Interaction Attenuation of  Attenuation of  –I  (x) = I (0) e -  x I(0) I  (x) x  Attenuation coefficient cm -1 or cm 2 /g

9 Gamma attenuation Lead Aluminium

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

11 Hyperpur Germanium Detectors Semiconductor diodes having a p-i-n structure Semiconductor diodes having a p-i-n structure Intrinsic (I) region is sensitive to ionizing radiation, particularly x rays and  rays Intrinsic (I) region is sensitive to ionizing radiation, particularly x rays and  rays Under reverse bias, an electric field extends across the intrinsic or depleted region. 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. 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.

12 Hyperpur Germanium Detectors Relative efficiency, energy resolution, energy range, peak/compton ratio

13 Hyperpur Germanium Detectors

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

15 Hyperpur Germanium Detectors

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

17 Energy Calibration

18 Efficiency calibration Full – energy –peak efficiency:  (E) Full – energy –peak efficiency:  (E)  (E) = N(E) /R  (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.I  R = A.I  –A :Source Activity –I  : Gamma (  ) ray emission probability

19 Efficiency calibration  (E) depends on:  (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

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

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

22 Standard Sources Standard Sources

23 Efficiency Curves HPGe, coaxial – P Type: Rel. Eff.30%

24 Efficiency Curves P-type and N- type detectors

25 Efficiency fitting

26 Spectral Evaluation

27 a1, a2, b1 b2 N = N t - N b

28 Spectral Evaluation

29 Calcul of activity General Case If tc << T1/2 General Case If tc << T1/2

30 Calcul of activity : Areal activity

31 Uncertainties

32 Uncertainties IAEA TECDOC 1401

33 Backround

34 Detection Limit

35 Detection Limit

36 Correction factors Factor corrections (Fc) 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

37 Coïncidence –summing corrections (eg. two Radionuclides ) –N 1 = A I 1   ( 1 –  12 ) –C 1 = 1/(1-  12 ) –N 2 = AI 2  2 [1 – (I 1 /I 2 )  12 ] –C 2 = 1 / [1—(I 1 /I 2 )  12 ] –N 3 = AI 3         ] –C 3 = 1/[1+I 1   /(I 3  3 )  E 2 (I 2 )

38 Dead time and pile-up correction MCA : Real and live time MCA : Real and live time Pile - up correction rejector Pile - up correction rejector Pulser method: Pulser method: –N 0 = N f t/N p N et N 0 : measured and true number of counts in the peak respectively N et N 0 : measured and true number of counts in the peak respectively N p: number of counts in the pulser peak N p: number of counts in the pulser peak F: frequency of the pulser F: frequency of the pulser T: Counting time T: Counting time

39 Attenuation correction Attnuation law: I  (x) = I (0) e -  d  Attnuation law: I  (x) = I (0) e -  d  –  : masse attenuation coefficient – d : tickness of the sample –  : density of the ample Self-attenuation Facteur: Self-attenuation Facteur: –F (  d  )= [1-exp(-  d  )]/  d  Correction Facteur Correction Facteur –C a = F (  d  ) sample / F(  d  ) standard –(E > 100 keV): C a depends exclusively on the sample density –(E <100 keV): C a depends also on the chemical composition

40 Attenuation correction Boshkova and Minev, ARI 54 (2001)

41 Attenuation correction Can be determined Can be determined –Using analytical methods –Using the Monte-Carlo Computation techniques –Experimentally

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

43 Comparative measurements 137 Cs, 210 Pb, 226 Ra, 241 Am 137 Cs, 210 Pb, 226 Ra, 241 Am

Cs

Pb

Am

Ra

48 Comparison

Cs & 210 Pb Self-absorption Sediment Samples (100 m) HPGe 45% -N Type -

50 Conclusion Gamma spectrometry: Direct technique, without radiochemical separation, but requires some precautions: 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 –Factor effects –All sources of uncertainty –Quality Control Programme