True Coincidence Summing Correction in Gamma Spectroscopy

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Abstract In environmental or low-level γ-ray spectrometry, the determination of activities or application of efficiency curves is hampered by the phenomenon of true coincidence summing. It is intended to bring the sample close to the detector and the probability of the detector to register two photons emitted by the same decaying nucleus as one is no longer negligible. All this becomes even more complex when the sample’s specific activity is so low that it is essential to measure a larger volume of sample material. In that case, the detector efficiency can no longer be considered constant over the source volume and the voluminous true-coincidence summing effect comes into play. Completely neglecting true coincidence summing effects when identifying isotopes may result in negative consequences, such as biased result for radionuclide activities and the associated uncertainties, and missed or falsely identified radionuclides. A simple procedure for correction for cascade summing can be achieved by obtaining the peak-to-total ratio for the HPGe detector.These ratios are measured using single line emitters placed at an arbitrary distance from the detector. This procedure saves money and time since only a set of uncallibrated point sources are needed to obtain the P/T ratios for a detector. Results obtained are improved to within 5% of the true value. ii
List of Figures: Figure 1.1 - Comparative pulse height spectra recorded using NaI and Ge(Li) ………………………………4 Figure 1.2 - The basic construction of germanium detector …………………………………………..……..….5 Figure 1.3 - Configuration of detector generally available together with the used energy range …..…….....5 Figure 1.4 - A typical germanium detector, cryostat and liquid nitrogen reservoir ……………………………6 Figure 1.5 - (a) - A Display of the FWHM of a peak ……………………………………………………….……7 Figure 1.5 - (b) - Energy spectrum of Co-60 …………………………………………………………………….7 Figure 1.6 - A simple schematic electronic system for gamma spectroscopy …………………………………9 Figure 1.7 - Peak and background areas for background subtraction ………………………………………..12 Figure 1.8 - Energy Calibration ……………………………………………………………………………………13 Figure 1.9 - Full peak energy calibration curve for a Marinelli Beaker …………………….…...…...………...15 Figure 2.1 - Calculated pulse shapes resulting coincidence summing ………………………………………..18 Figure 2.2 - Efficiency curves on top of and at 115 mm from the detector end cap using Eu-152 ………...19 Figure 2.3 - Simplified decay scheme of Eu-152 ………………………………………………………………...20 Figure 2.4 - Geometrical arrangement used to obtain the data ………………………………………………..21 Figure 2.5 - Sum peaks above the 1408 keV peak of Eu-152 ………………………………………………….22 Figure 2.6 - Ratio of count rates in the full-energy peaks for a point source positioned at a short distance d and a large distance d 0 ……………………………………………………………………………..24 Figure 2.7 - Summarized Cs-134 results from the 1989 NPL environmental radioactivity intercomparison exercise ……………………………………………………………………………..25 Figure 2.8 - The efficiency curve constructed using a ‘best fit’ computer program ………………………28 Figure 2.9 - Relative peak areas for sources of different geometry and density ……………………………...29 Figure 2.10 - Simple illustrative decay scheme liable to true coincidence summing …………………………..30 Figure 3.1 - The decay scheme and a spectrum of Ba-133 showing the summing-in summing out peaks ..36 Figure 3.2 - Diagram of Multiple Genie 2000 Network ……………………………………….…………………..42 Figure 3.3 - A Typical Energy Show Display ………………………………………………………………………43 Figure 3.4 - Show Efficiency Calibration ……………………….…………………………………………………..43 Figure 3.5 - Geometry Composer ..................................................................................................................45 Figure 3.6 - Comparison of Cascade Corrected to Empirical Efficiency …………………………………..……46 Figure 3.7 - LabSOCS Calibration Software is launched, used for data entry, and then generates the Efficiency Calibration Curve …………………………………………………………………………..46 Figure 4.1 - One of the detectors in the laboratory ………………………………………………………………..49 iii
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: Acknowledgements My recognition to
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 and 44: • The larger Eu-152 factors for t
Page 45 and 46: As discussed in chapter 2, the emis
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
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Detector 5: Pellets (1g/cm 3 ) Nucl
Page 87 and 88:
Detector 6: Marinelli beaker Nuclid
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Detector 6: Pellets (1g/cm 3 ) 5 mm
Page 91 and 92:
References [1] Radiation Detection
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True Coincidence Summing Correction in Gamma Spectroscopy

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