True Coincidence Summing Correction in Gamma Spectroscopy

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3 Implementation of Cascade Corrections in Genie 2000 3.1 Introduction A high efficiency detector and its optimal measurement geometry is the basic necessity for the following reason: • To attain highest possible analysis sensitivity for environmental samples • To reduce the count time to reach a desired precision for an observed activity or to reach lower minimum detectable activity MDAs. The optimal geometry is attained by positioning the source as close to the detector as possible and selecting the shape of the source most suitable to reach the highest possible efficiency (e.g., Marinelli beaker). With measurements conditions described above, true coincidences may cause systematic errors that can reach levels of more than ten percent for some radionuclides. 35
As discussed in chapter 2, the emissions resulting from the majority of decays from a mother nuclide to the ground state of its daughter contain several γ- or X-ray photons in cascades within a negligible time frame. This time frame is within the resolution time of the detector system where the detector accumulates all energy deposited in its crystal. Due to this reason, determination of energy of each individual photon emitted by each nuclear level transition is not possible. The detector system can only register the sum effect as the summed energy. The true coincidence may result in coincidence gaining (summing-in) or coincidence losing (summing-out). The coincidence summing-in effect (S g probability) increases the observed peak area of the analytical γ-line (N p,g ), whereas the summing-out effect (L g probability) decreases the peak area. 356 276 53 384 303 223 Ba-133 Decay 81 161 80 Summing in Summing out Summing out Counts 45000 40000 35000 30000 25000 20000 15000 10000 5000 0 81 keV line Summing out Sediment Spectrum 303 keV line Summing out 75 125 175 225 275 325 375 Energy (keV) 384 keV line Summing in Figure-3.1 The decay scheme and a spectrum of Ba-133 showing the summing-in summing out peaks 36
Page 1 and 2: Universität Bremen Faculty of Envi
Page 3 and 4: 2.2 The Origin of Summing 19 2.3 Su
Page 5 and 6: Acknowledgements My recognition to
Page 7 and 8: List of Figures: Figure 1.1 - Compa
Page 9 and 10: List of Tables: Table 1.1 - Typical
Page 11 and 12: the photopeak area is related to el
Page 13 and 14: Figure 1.1 Comparative pulse height
Page 15 and 16: 1.4.2 Packing of the Detector HPGe
Page 17 and 18: Table 1.1 Typical FWHM values as a
Page 19 and 20: The gamma photons produce electron-
Page 21 and 22: Figure 1.7 Peak and background area
Page 23 and 24: • Absolute full-peak energy effic
Page 25 and 26: p c =1-e -2Rτ (1.6) R is the mean
Page 27 and 28: amplifier output pulses shapes are
Page 29 and 30: Figure 2.3 Simplified decay scheme
Page 31 and 32: Counts 5000 4000 3000 2000 1000 Eu-
Page 33 and 34: Figure 2.6 Ratio of count rates in
Page 35 and 36: participate in summing. However, ca
Page 37 and 38: Figure 2.8 The efficiency curve con
Page 39 and 40: The above discussion leads to the c
Page 41 and 42: The ratio n 2 /n’ 2 would be used
Page 43: • The larger Eu-152 factors for t
Page 47 and 48: ∫ ∫ ε r) ε t ( r) dV ( r) dV
Page 49 and 50: For equation (3.8); N p,g = measure
Page 51 and 52: comprehensive spectrum analysis for
Page 53 and 54: Tentative Nuclide Identification An
Page 55 and 56: correction uses LabSOCS technology
Page 57 and 58: 4 Results, Validations and Discussi
Page 59 and 60: assumed to be constant for detector
Page 61 and 62: 4.2.3 Geometry Description The prop
Page 63 and 64: Table 4.4- Cascade correction facto
Page 65 and 66: Table 4.6- Cascade correction facto
Page 67 and 68: Figure 4.8 Comparison between the f
Page 69 and 70: 4.4 Validations and Discussion To v
Page 71 and 72: 0.900 0.850 0.800 0.750 0.700 0.650
Page 73 and 74: The sediment sample obtained from B
Page 75 and 76: Appendix A Detector 3: Marinelli be
Page 77 and 78: Detector 3: Pellets (1g/cm 3 ) 5 mm
Page 79 and 80: Nuclide Energy (keV) Detector 4: Ma
Page 81 and 82: Nuclide Energy (keV) Detector 4: Pe
Page 83 and 84: Detector 5: Marinelli beaker 0.5 L
Page 85 and 86: Detector 5: Pellets (1g/cm 3 ) Nucl
Page 87 and 88: Detector 6: Marinelli beaker Nuclid
Page 89 and 90: Detector 6: Pellets (1g/cm 3 ) 5 mm
Page 91 and 92: References [1] Radiation Detection

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