True Coincidence Summing Correction in Gamma Spectroscopy

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that are emitted. Multichannel analyzers typically have 4k or 8k channels over an energy range of 0 to 2 MeV (the energy range depends on the amplifier gain setting). The output is a plot of pulse counts versus gamma energy. By analyzing the spectrum of gamma rays emitted, the user can determine the nuclidess which caused the gamma pulses. The present day MCA works as a multifunctional device, dealing with many aspects of data analysis, such as collection and sorting pulses, storing and sorting data, data display, data analysis and preparation of data for output. The output is in the form of a display of the number of counts shown on y-axis and the corresponding channel number displayed on x-axis. 1.6 Counting statistics The contents (N) of the observed γ lines are numbers resulting from a counting experiment, which are partially superimposed on a high background. This produces an uncertainty of the result which can seriously degrade the precision with which the net peak counts is measured. Counting statistics is applied to estimate this uncertainty which is expressed by the standard deviation of the result. For any counting experiment the result (N) of which is governed by a Poisson distribution, the standard deviation is σ = N (1.1) The above equation expresses the fact that a repetition of a counting experiment would in about 2/3 of the cases give a result in the range of N ± σ. For the analysis of a peak without background this is sufficient. But to subtract a background, the Poisson distribution should be approximated by a Gaussian distribution. This can be done without large errors for numbers of counts (N) greater than about 10. In this case, N net =N tot - N BG (1.2) σ = 2 2 σ + σ = Ntot BG N N (1.3) BG tot + 11
Figure 1.7 Peak and background areas for background subtraction [5] In many cases the background has a slope. Therefore the counts in the background areas have to be determined at both sides of the peak and averaged. By such calculation, the value and the statistical error of the activity concentration can be determined. 1.7 Calibrations The modern digital gamma ray spectrum displays number of pulses measured within small consecutive pulse height ranges. The detector is calibrated to interpret the spectrum in terms of energy rather than channel number or voltage and amount of radionuclides rather than number of pulses. A complicated spectrum analysis may need providing information about peak width variation and shape with energy or channel number. Therefore two main tasks have to be performed . • Energy calibration - the relationship between channel and energy • Efficiency calibration - the relationship between number of counts and disintegration rate. 1.7.1 Energy Calibration The energy calibration specifies the relationship between peak position in the spectrum (channel number) and the corresponding gamma-ray energy. This is accomplished by measuring the spectrum of a source emitting gamma-rays of 12
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: The gamma photons produce electron-
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
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0.900 0.850 0.800 0.750 0.700 0.650
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The sediment sample obtained from B
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Appendix A Detector 3: Marinelli be
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Detector 3: Pellets (1g/cm 3 ) 5 mm
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Nuclide Energy (keV) Detector 4: Ma
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Nuclide Energy (keV) Detector 4: Pe
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Detector 5: Marinelli beaker 0.5 L
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Detector 5: Pellets (1g/cm 3 ) Nucl
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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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