Measuring Tritium in Water and Urine with Liquid Scintillation Counting Kun-Ho Chung, Mun-Ja Kang Korea Atomic Energy Research Institute Nuclear Environment Safety Research Center Page 1

Nuclear Decay Process-Radioactivity Radionuclides ? Nuclear Decay Process-Radioactivity 1. alpha (α) – emission of helium nucleus( 4He) composed of two protons and two neutrons with a momogenic energy between 2-8 MeV 2. Beta (β) – electrons with either a positive (positron) or negative (negatron) charge and emitted with a continuum of energy (0-2000 KeV) 3. Gamma (γ) – a high energy, short wavelength electromagnetic radiation, originating in the nucleus of the atom Page 2

Radioactivity Decay Law Radionuclides ? Radioactivity Decay Law N = N0e-λt N0 = number of atoms or activity (e.g. Bq) at staring time t0 N = number of atoms or activity after time interval t  = decay constant or 0.693 / t1/2 t1/2 = half-life of isotope H-3 half-life has been accepted to be 12.32 y = 4500 + - 8 d, TU = Tritium Unit = 0.119 Bq/kg (1 TU = 1 3H atoms per 10exp(18) H atoms) Page 3

1 Curie (1 Ci) = 2.22 x 1012 dpm 1 Bequerel (1 Bq) = 1 dps Unit of radioactivity 1 Curie (1 Ci) = 2.22 x 1012 dpm 1 Bequerel (1 Bq) = 1 dps 1 Ci = 2.22 x 1012 dpm = 3.7 x 1010 dps = 3.7 x 1010 Bq = 37 GBq 1 mCi = 2.22 x 109 dpm = 3.7 x 107 dps = 3.7 x 107 Bq = 37 MBq 1 μCi = 2.22 x 106 dpm = 3.7 x 104 dps = 3.7 x 104 Bq = 37 KBq H-3 half-life has been accepted to be 12.32 y = 4500 + - 8 d, TU = Tritium Unit = 0.119 Bq/kg (1 TU = 1 3H atoms per 10exp(18) H atoms) Page 4

Principals of LSC : Energy transfer process Chemical Quench Color b hn Page 5

Principals of LSC : Energy transfer process Page 6

Principals of LSC : Energy transfer process b-particle Solvent molecule Excited Scintillator Emitted Photon P S b / Illustration of the Collision Process Page 7

Principals of LSC : Detector system of LSC (Quantulus 1220) PMT Liquid scintillator LEAD(passive shield) A/D CONVERTER COINC. UNIT SUMMING AMPL . MCA1 HALF1 TRIGGER MCA SPLIT INHIBIT ANALOG MCA2 HALF2 Page 8

Radionuclides ? Emax Number of beta particles Beta particle energy Page 9

Both cosmogenic and man-made Tritium 3H, Emax = 18.6 keV, half-life 12.32 y Both cosmogenic and man-made Low energy – non hazardous, biological half-life 10 days 3H appears as part of water molecule as HTO Drinking water limit 100 Bq/L Measurable at this limit with any LSA in a reasonable time Concentrations much smaller in environmental samples Page 10

Weapons testing from 1954 to 1963 Tritium Anthropogenic tritium is from atmospheric weapons testing and nuclear fuel cycle Weapons testing from 1954 to 1963 Natural levels are now back to the levels of pre-atmospheric bomb tests The present day activity in precipitation is approximately 2 Bq/L Cosmic rays 14N(n,3H)12C 16O(n,3H)14O 2H(n,) 3H Nuclear Tests 235U(n,f)3H 14N(n,3H)12C 16O(n,3H)14O 2H(n,) 3H NPP 6Li(n,)3H 10B(n,2)3H 2H(n,) 3H Page 11

Minimum Detectable Activity Detection Limit Minimum Detectable Activity MDA = (4.65 sqrt(B*t)+ 2.71)/(E*V*t) E = counting efficiency V = sample volume B = background count rate t = counting time An American National Standard on Performance Criteria for Bioassay, HPS N13.30-1996, Health Physics Society, McLean 1996, 112 p. Page 12

Detection Limit vs. Counting Time 10 mL sample, 25 % counting efficiency, background 1 CPM Page 13

Measurement of Tritium by LSC Direct addition - mix with cocktail and measure (sample 10 mL) less labor intensive distillation or purification by Eichrom 3H column is needed to remove impurities from a low activity sample Electrolytic enrichment (starting volume 100-300 mL) enrichment system required, no commercially made systems readily available time consuming Benzene synthesis (C6H6 contains 3 times as much 3H as H2O) synthesis apparatus required, no commercially made synthesizers readily available labor intensive, time consuming carcinogenic end product Page 14

typical 3H eff typical bkg detection limit How do methods compare ? typical 3H eff typical bkg detection limit Direct counting 25 % 1.0 CPM 2.5 Bq/L Benzene synthesis 60 % 1.2 CPM 0.37 Bq/L Enrichment (20 x) 25 % 1.0 CPM 0.13 Bq/L Direct counting and enrichment calculations are made for 10 mL water and 500 min counting time 20 mL benzene is equivalent to 30 mL water with 100 % yield. Numbers are typical for Quantulus Page 15

Distilled & Electrolytically Enriched Samples Sample quench is constant for all samples, and no quench correction needed Sample activity = Ao * (S-B)/(Std-B), where Std, S, and B are reference, sample and blank count rates, respectively, Ao is reference standard activity and counting efficiency E = (Std-B)/Ao Spectrum analysis software does the calculation based on raw spectra When sample S and blank B count rates are low, the random noise signal & chemiluminescence is constant and low (typically 0.15 CPM in Quantulus) and subtracted in the above calculation Page 16

Constant quench & EASY View software of Quantulus Page 17

Internal standard method, ISO Standard 9698:1989(E) Samples measured first as they are Very small volume of known activity standard material added and recounted Efficiency verified for each sample and activity calculated Advantage: Based on raw data, no quench curves needed Works on any counter (performance is an issue) Disadvantages: Destroys samples, recounting not possible Page 18

Counting with quench curves / Tri-Carb Page 19

Measurement of Tritium by LSC Cocktails for aqueous 3H samples Ultima Gold LLT, high capacity, acceptance of mineral acids Ultima Gold XR, high capacity, acceptance of mineral acids OptiPhase HiSafe 3, multipurpose cocktail, lower water capacity than Ultima Gold’s Cocktails are based on di-isopropyl- naphthalene solvent, which has very low vapor pressure and high flash point (148°C) Page 20

Non-radioactive plastic vials for lowest activities Ordinary polyethylene vials good with safe cocktails super PE vial , 20 mL, 6008117 (1200-420 PE vial, 20 mL) Teflon coated polyethylene vials for classic cocktails low diffusion PE vial, 6000477, 20 mL (1200-422 low diffusion PE vial, 20 mL) Copper-teflon vials 15 and 20 mL, 1220-502, 1220-503 (expensive, to be washed and reused, only for Quantulus) Page 21

Standard reference material Reference materials 3H counting Conventional size standards for low level counting (20 mL glass vial) 6018917 3H standard (set of ten) ~3 x 104 DPM/vial Internal Standards 6004052 3H water, ~2.5 x 106 DPM/g, 10 mL Internal standard kits 3H W 1210-121 3H for aqueous solvents, 40 capsules ‘Dead’ water needed for background correction Page 22

Direct counting of 3H in urine samples: LSC source for 3H add H-3 standard of 200 ml in vial: 3003 Bq/g add Ultima Gold-LLT cocktail of 15~20 mL add “dead” water: 5~0 mL total volume in vial: 20 mL counted by liquid scintillation counter Page 23

Preparation of spiked urine samples with 3H standard add H-3 standard of 200 ml in vial: 3003 Bq/g or 300 Bq/g add Ultima Gold-LLT cocktail of 19 mL add urine sample: 1 mL total volume in vial: 20 mL counted by liquid scintillation counter Page 24

LSC quenching spectrum and curve for 3H Page 25

What is the SQP(E)? SQP(E)= 839.93 1% 99% Page 26

Activity (Bq/L) = (Csamp- CBK) /60(100/E)(1/V) Calculation Activity (Bq/L) = (Csamp- CBK) /60(100/E)(1/V) MDA (Bq/L) = {2.71 + 4.65sqrt(CBK t)}/(60t)  (100/E)(1/V) Csamp and CBK: count rate (cpm) of sample and background E: % efficiency of H-3 in LSC V: sample quantity (L) t: counting time (min) Page 27

Measuring 3H in Urine: urine sample spiked with 3H standard Page 28

Measuring 3H in Urine: urine sample spiked with 3H standard SQP(E) LSC efficiency (%) Spiked activity (Bq/L) Measured Rel. deviation (%) Spiked urine sample 1 845.75 47.38 61,000 58,900 (1674.3 cpm) 3.4 sample 2 845.98 47.41 620,000 617,400 (17562 cpm) 0.4 Page 29

MDA of the method for 3H in urine samples Sample quantity: 1 mL LSC efficiency: 47% (50~300 windows) Count time: 30min Back ground count rates: 5.83 cpm (50~300 windows) MDA : 75.89 Bq/L Page 30

TR-LSC Application Note P10763: Literature TR-LSC Application Note P10763: Time Resolved Liquid Scintillation Counting: (TR-LSC) for Environmental 3H Analysis by Charles J. Passo, Jr. and Dr. Gordon Cook PerkinElmer Life Sciences 2002 LSC Handbook of Environmental Liquid Scintillation Spectrometry, Packard 12/1994, PMC0387 (out of print) Quantulus bibliography February 2006 46 references on LS measurements of 3H Page 31

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