True Coincidence Summing Correction in Gamma Spectroscopy

More documents

Recommendations

Info

2 True Coincidence Summing 2.1 Introduction Gamma spectroscopy using HPGe detectors is commonly used for its high energy resolution. It can be used for the activity determination of gamma-emitting radioactive sources provided the full-peak efficiency calibration curve is known. In that case, the efficiency value for each gamma ray energy is obtained by interpolation. The measured peak efficiency curves should be corrected for cascade summing due to simultaneous detection of two or more gamma-rays from the same decay event inside the detector crystal. When the gamma rays arrive within the width of the amplifier output pulse they will not be recognized as separate events. The resulting output pulse will be equivalent to the height of the first pulse received plus the height of the second pulse. This situation is demonstrated in Figure-2.1, in which the calculated preamplifier and 17
amplifier output pulses shapes are plotted when the pulses arrive at the preamplifier input. The diagram shows that for every coincidence, the height of the combined output pulse measured at MCA, is the sum of the two pulses heights. Figure 2.1 Registered pulse shapes resulting in coincidence summing [6] The combined output pulse (coincidence) is undesirable because it causes counts to be lost from both full-energy peaks in the spectrum. If both gamma-rays arriving at the detector within the resolving time of the amplifier were fully absorbed and contribute to their respective full-energy peaks, the coincidence will result in the loss of one count from each peak and the appearance of a count somewhere else in the spectrum. This will create many difficulties in achieving a valid efficiency calibration for close geometry measurements. This problem is not a new one but it has been largely ignored in practice. The problems caused by true coincidence summing (TCS) are illustrated in the calibration curves for a 45% HPGe detector in Figure 2.2. These were derived using Eu-152 near point sources of moderate activity such that the count rate in each case was about 7700 cps. The lower curve is measured with the source at 115 mm from the detector and is smooth and consistent and is apparently satisfactory. The upper curve, measured with the source on the detector cap, is not at all satisfactory. The points do not lie on an orderly line and it would be difficult to draw an acceptable curve through them. The reason for this dramatic difference is due to TCS. 18
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: p c =1-e -2Rτ (1.6) R is the mean
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
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
show all

True Coincidence Summing Correction in Gamma Spectroscopy

Create successful ePaper yourself

Delete template?

Save as template?