A performance comparison of two kinds of liquid scintillation counters (LSCs) from PerkinElmer, Inc.

Transcription

1 Institute of Nuclear and New Energy Technology(INET), Tsinghua University A performance comparison of two kinds of liquid scintillation counters (LSCs) from PerkinElmer, Inc. Xiao-gui FENG 1), Guo-hua JIANG 2), Jian-hua HUANG 2), Jian-yu DU 3), Qian-ge HE 1), Jian-chen WANG 1), Jing CHEN 1) 1) Institute of Nuclear and New Energy Technology, Collaborative Innovation Center of Advanced Nuclear Energy Technology, Tsinghua University, Beijing , PR China 2) Analytical and Testing Center, Beijing Normal University, Beijing , PR China 3) PerkinElmer, Beijing , PR China

2 Outline Introduction Experimental methods Results and discussion Background count rate and counting efficiency α/β discrimination The instrument stability Conclusions 2

3 Key features of Quantulus 1220 Asymmetric lead shield PMT: photomultiplier tubes Liquid scintillation guard ,2: Guard PMT 3,4: Sample PMT 5: Sample Schematic diagram of lead shield and guard 3

4 Key features of Quantulus GCT 6220 BGO (Bismuth-Germanium Oxide) Guard GCT (Guard Compensation Technology) GCT is a mathematical method which determines the number of environmental background events that were not blocked or detected by the BGO guard, and then uses this information to reduce (in real time) sample background counts caused by the environment. Quantulus GCT 6220 Quantulus

5 Three LSCs involved in this study LSC ID Brand Model No. Serial No. Manufacture date Installation site LSC1 (18 ) LSC2 (*) LSC3 (18 ) Quantulus GCT 6220 A SGLO November 2015 Quantulus June 2005 Quantulus DG June 2011 Tsinghua University Tsinghua University The Institute of Crustal Dynamics *The cooling unit in LSC2 was shut down because it was broken. 5

6 LSC1 and LSC2 Quantulus 1220 (LSC2) Quantulus GCT 6220 (LSC1) 6

7 Samples and experiment plan Samples Unquenched standards in 20mL glass vials: Blank, 3 H, 14 C Home-made samples in 20mL plastic vials (cocktail:10ml OptiPhase HiSafe 3) 3 parts 3 blank samples: Blank1, Blank2, Blank3 2 samples of β-γ emitters: 60 Co, 137 Cs 1 pair of α or β emitters: 241 Am, 90 Sr/ 90 Y Part 1: background count rate and counting efficiency Blank, 3 H, 14 C, Blank1, Blank2, Blank3, 60 Co, 137 Cs Part 2: α/β discrimination 241 Am, 90 Sr/ 90 Y Part 3: instrument stability Blank, 3 H, 14 C 7

8 Counting conditions Assays of LSC1 (GCT=Off, -14.0, -12.4, -3.2, -2.6, -2.4, 0.1) CPM assay Alpha/Beta assay Protocols of LSC2 Protocol 1: H-3 or low energy beta counting Protocol 2: C-14 or high energy beta counting Protocol 3: Alpha/Beta counting (Guard active) Protocol 4: Alpha/Beta counting (Guard inactive) Protocol of LSC3 Optimize_GCT_Strength_Factors assay Protocol 1: H-3 or low energy beta counting 8

9 ROI (region of interest) ROI is always selected as the whole spectrum (unless otherwise specified) kev for LSC1 channels for LSC2 and LSC3 9

10 Samples in glass vials Blank 3 H 14 C Condition CR SD CR CR SD CR ε CR SD CR ε (%) (%) Off GCT for LSC1 Protocol for LSC CR: count rate; SD CR : the standard deviation of count rate; ε: counting efficiency 10

11 Samples in plastic vials Blank1 Blank2 Blank3 Condition CR SD CR CR SD CR CR SD CR Off GCT for LSC1 Protocol for LSC CR: count rate; SD CR : the standard deviation of count rate 11

12 Samples of 60 Co and 137 Cs 60 Co: β/γ emitter with cascade β-γ radiation Guard active Guard inactive Condition CR GCT for LSC1 Protocol for LSC2 60 Co 137 Cs ε R CR ε R (%) (%) Off % % Cs: β/γ emitter With no cascade β-γ radiation γ counting efficiency of guard: LSC1 CR: count rate; ε R : relative counting efficiency LSC2 12

13 Effect of quench on α/β discrimination α: 241 Am; β: 90 Sr/ 90 Y Composition of SimS: HNO 3 1.0mol/L Na 18.3 g/l Fe 6.0 g/l Al 5.7 g/l Ni 2.9 g/l Nd 1.5g/L LSC2 is better LSC1 is better Fig. 1 The effect of quench on the optimum PSA Fig. 2 The effect of quench on the misclassification at the optimum PSA Cross-over plot method: Optimum PSA, Misclassification PSA: pulse shape analysis SimS: Simulated Solution, the quenching agent represents the main composition of Chinese high level liquid waste. 13

14 Effect of α/β discrimination on background count rate at the optimum PSA Blank1 Blank2 Blank3 Condition CR SD CR CR SD CR CR SD CR MCA: multi-channel analyzer β-mca α-mca Off GCT for LSC LSC2: Protocol LSC1: all GCT LSC2: Protocol * The optimum PSA is 146 for LSC1 and 107 for LSC2. CR: count rate; SD CR : the standard deviation of count rate 14

15 Effect of α/β discrimination on counting efficiency near the optimum PSA 241 Am 90 Sr/ 90 Y Condition CR ε R CR ε R (%) (%) Off β-mca GCT for LSC Bremsstrahlung from 90 Sr/ 90 Y MCA: multi-channel analyzer α-mca LSC2: Protocol LSC1: all GCT b LSC2: Protocol b Aging of PMT in LSC2 The PSA is 150 for LSC1 and 110 for LSC2 CR: count rate; ε R : relative counting efficiency 15

16 Results of stability test for the three LSCs χ 2 NN 1 RRRRRR CCCC RRRRRR pppppppp Best Worst Reduced chi-square 2 Condition Item Blank 3 H 14 C CR LSC1: GCT=Off RSD CR (%) RSD pred (%) ε (%) CR LSC1: GCT=0.1 RSD CR (%) RSD pred (%) ε (%) CR LSC2: Protocol 1 RSD CR (%) RSD pred (%) ε (%) CR LSC3: Protocol 1 RSD CR (%) RSD pred (%) ε (%) χ 2 NN 1 Counting terminator RSD pred 0.125% 1 CR: count rate; RSD CR : observed relative standard deviation of CR; RSD pred : predicted relative standard deviation of CR; ε: counting efficiency 16

17 Conclusions If proper GCT is applied, the background count rate in β-mca for Quantulus GCT 6220 can be lower than that for Quantulus 1220, but the background count rate in α-mca for Quantulus GCT 6220 is always higher. For the counting efficiency, which LSC is higher depends not only on the LSC setting (including the aging of PMTs), but also on the decay properties of the radionuclide to-be-determined. The results of α/β discrimination vary with the quench level of the samples: Quantulus GCT 6220 is better for high quenched samples, while Quantulus 1220 is better for low quenched samples. It seems that Quantulus GCT 6220 exhibits better stability than Quantulus

18 References 1. PerkinElmer (2002) Wallac 1220 Quantulus ultra low level liquid scintillation spectrometer (instrument manual, ) 2. PerkinElmer (2015) QuantaSmart for the Tri-Carb liquid scintillation analyzer (Models 4810TR, 4910TR, 5110TR, and Quantulus TM GCT 6220) (reference manual, manual reorder number Rev. A) 3. Zhao HP, Feng XG (2011) The influences of anticoincidence shield on liquid scintillation count rates for various kinds of radionuclides. J Nucl Radiochem 33(6): (in Chinese) 4. Pujol L, Sanchez-Cabeza J-A (1997) Role of quenching on alpha-beta separation in liquid scintillation counting for several high capacity cocktails. Analyst 122: Pates JM, Cook GT, MacKenzie AB, Passo CJ (1998) Implications of beta energy and quench level for alpha-beta liquid scintillation spectrometry calibration. Analyst 123: Feng XG, He QG (2009) Simultaneous determination of 237 Np, Pu and 241 Am in HNO 3 solution by combining extraction, liquid scintillation counting, and α spectrometry. Nucl Instrum Methods Phys Res A 609: Feng XG, He QG, Wang JC, Chen J (2016) The effect of incidental radiations on the determination of α or β particles by liquid scintillation counting for low quenched samples. J Radioanal Nucl Chem 308: Knoll GF (1999) radiation detection and measurement (3rd Edition). John Wiley & Sons, Inc., New York/Chichester/Weinheim/Brisbane/Toronto/Singapore 18

19 Decay scheme of 137 Cs and 60 Co E ce = kev, ΣI ce = 9.37% 60 Co 137 Cs E β (kev) I β (%) E β (kev) I β (%) γ 84.99% ce 9.37% γ keV 100% 137m Ba (2.552m) γ keV 100% 137 Ba 60 Ni Decay scheme of 137 Cs Decay scheme of 60 Co 19

20 Misclassification (MR) MMMM α = R G + R 100% ROI PK Am:α-MCA MMMM β = B in ROIPK Y + B 100% Am:β-MCA Sr/Y:α-MCA Sr/Y:β-MCA SimS Volume =1.2mL,PSA=50 20

21 ROI PK for LSC1 and LSC2 Run No. SimS volume (ml) ROI PK for LSC1 (kev) ROI PK for LSC2 (channel) ROI PK : the peak area of α emitter 21

22 Cross-over Plot SimS Volume = 0mL SimS Volume = 1.2mL 22

23 Optimum PSA vs. regular quench index Quantulus 1220 Quantulus GCT

24 RSD CR and RSD pred RSD CR : the observed relative standard deviation of CR (30 cycles) RSD pred : the predicted relative standard deviation from Poisson distribution which is used to describe the nuclear counting system: (1) for Blank, RSD pred = 1/(CR t) % (where counting time t = 120 min); (2) for 3 H and 14 C, RSD pred is from counting terminator. The terminator for LSC1 is 2 Sigma % = 0.25%, implying RSD pred = 0.125%. The terminator for LSC2 and LSC3 is COUNTS = , therefore RSD pred =1/ % = 0.125%. 24

25 Optimizing_GCT_Strength_Factors_Procedure-Rev_B.pdf 25

26 Optimizing_GCT_Strength_Factors_Procedure-Rev_B.pdf 26