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KMID : 0364920200450020076
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Journal of Radiation Protection and Research
2020 Volume.45 No. 2 p.76 ~ p.80
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An Improved Movable 3 photomultiplier (3PM)-γ Coincidence Counter Using Logical Sum of Double Coincidences in β-Channel for Activity Standardization
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Hwang Han-Yull
Lee Jong-Man
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Abstract
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Background: To improve the measurement accuracy of liquid-scintillation counting for activity standardization, it is necessary to significantly reduce the background caused by thermal noise or after-pulses. We have therefore improved a movable 3 photomultiplier (3PM)-γ coincidence-counting method using the logical sum of three double coincidences for β events.
Materials and Methods: We designed a new data-acquisition system in which β events are obtained by counting the logical sum of three double coincidences. The change in β-detection efficiency can be derived by moving three photomultiplier tubes sequentially from the liquid-scintillation vial. The validity of the method was investigated by activity measurement of 134Cs calibrated at the Korea Research Institute of Standards and Science (KRISS) with 4π(PC)β-γ(NaI(Tl)) coincidence counting using a proportional counter (PC) for the β detector.
Results and Discussion: Measurements were taken over 14 counting intervals for each liquid-scintillation sample by displacing three photomultiplier tubes up to 45 mm from the sample. The dead time in each β- and γ-counting channel was adjusted to be a non-extending type of 20 μs. The background ranged about 1.2?3.3 s?1, such that the contributions of thermal noise or after-pulses were negligible. As the β-detection unit was moved away from the sample, the β-detection efficiencies varied between 0.54 and 0.81. The result obtained by the method at the reference date was 396.3±1.7 kBq/g. This is consistent with the KRISS-certified value of 396.0±2.0 kBq/g within the uncertainty range.
Conclusion: The movable 3PM-γ method developed in the present work not only succeeded in reducing background counts to negligible levels but enabled β-detection efficiency to be varied by a geometrical method to apply the efficiency extrapolation method. Compared with our earlier work shown in the study of Hwang et al. [2], the measurement accuracy has much improved. Consequently, the method developed in this study is an improved method suitable for activity standardization of β-γ emitters.
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KEYWORD
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Liquid Scintillation Counting, Movable 3PM-γ Coincidence Counter, Logical Sum of Double Coincidences, 4π(LS)β-γ(NaI(Tl)) Coincidence Counting, β-γ Emitter 134Cs
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