Page 1 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 2

Radionuclides ?

Nuclear Decay Process-Radioactivity

1. alpha (α) – emission of helium nucleus( 4 He) 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 3

Radionuclides ?

Radioactivity Decay Law N = N

0

e

-λt N 0 = number of atoms or activity (e.g. Bq) at staring time t 0 N = number of atoms or activity after time interval t



= decay constant or 0.693 / t 1/2 t 1/2 = half-life of isotope

Page 4

Unit of radioactivity

1 Curie (1 Ci) = 2.22 x 10

12

dpm 1 Bequerel (1 Bq) = 1 dps 1 Ci = 2.22 x 10

12

dpm = 3.7 x 10

10

dps = 3.7 x 10

10

Bq

= 37 GBq 1 mCi = 2.22 x 10 9 dpm = 3.7 x 10 7 dps = 3.7 x 10 7 Bq = 37 MBq 1 μCi = 2.22 x 10 6 dpm = 3.7 x 10 4 dps = 3.7 x 10 4 Bq = 37 KBq

Page 5

Principals of LSC : Energy transfer process

b

h

n

Chemical Quench Color Quench

Page 6

Principals of LSC : Energy transfer process

Page 7

Principals of LSC : Energy transfer process

S / b -particle Solvent molecule Excited Scintillator Emitted Photon / S

P

S / S / S S b / /

P P

S

P

S /

Illustration of the Collision Process P

/

P P

Principals of LSC : Detector system of LSC (Quantulus 1220)

LEAD(passive shield)

Liquid scintillator PMT

Page 8 COINC. UNIT

INHIBIT MCA SPLIT

COINC. UNIT SUMMING AMPL

.

ANALOG

A/D CONVERTER MCA1 HALF1 MCA2 HALF2

TRIGGER

Radionuclides ?

Page 9

Beta particle energy Emax

Tritium



3 H, E max

•

= 18.6 keV, half-life 12.32 y Both cosmogenic and man-made

•

Low energy – non hazardous, biological half-life 10 days



3 H 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

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

14 N(n, 3 H) 12 C 16 O(n, 3 H) 14 O 2 H(n,  ) 3 H

Nuclear Tests

235 U(n,f) 3 H 14 N(n, 3 H) 12 C 16 O(n, 3 H) 14 O 2 H(n,  ) 3 H

NPP 6 Li(n,



) 3 H 10 B(n,2



) 3 H 2 H(n,



) 3 H Page 11

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

MDC Bq/liter vs. counting time 12.0

10.0

8.0

6.0

4.0

2.0

0.0

0 500 1000

min

1500 2000

10 mL sample, 25 % counting efficiency, background 1 CPM

2500

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 3 H 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 (C 6 H 6

•

contains 3 times as much synthesizers readily available 3 H as H synthesis apparatus required, no commercially made 2 O)

• •

labor intensive, time consuming carcinogenic end product Page 14

How do methods compare ?

typical 3 H eff typical bkg detection limit



Direct counting 25 %



Benzene synthesis 60 %



Enrichment (20 x) 25 % 1.0 CPM 1.2 CPM 2.5 Bq/L 0.37 Bq/L 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

Page 16



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

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 3 H 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 3 H counting

•

Conventional size standards for low level counting (20 mL glass vial) 6018917

–

3 H standard (set of ten) ~3 x 104 DPM/vial

•

Internal Standards 6004052

–

3 H water, ~2.5 x 106 DPM/g, 10 mL

•

Internal standard kits 3 H W 1210-121

–

3 H for aqueous solvents, 40 capsules

•

‘Dead’ water needed for background correction Page 22

Direct counting of 3 H in urine samples: LSC source for 3 H

 add H-3 standard of 200 m l 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 3 H standard

 add H-3 standard of 200 m l 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 3 H

Sample Spectrum 150 100 50 0 0 100 200 300 400 500 600 700 800

H-3 Efficiency curve 2007.11.14

900 1,000 60.0

55.0

50.0

45.0

y = -0.0009614x

R 2 + 1.7896459x - 778.5347571

2 = 0.9588690

40.0

830 840 850 860 870

SQPE

880 890 900 910 ...S ...S ...S ...S 관련 \ 방사능분석 \ 관련 \ 방사능분석 \ 관련 \ 방사능분석 \ 관련 \ 방사능분석 \

Page 25

What is the SQP(E)?

SQP(E)= 839.93

Sample Spectrum 70 60 50 40 30 20 10 0 0 100 200 300 400 500 600

99%

700 800

1%

900 1,000 ...S 관련 \ 방사능분석 \

Page 26

Calculation



Activity (Bq/L) = (C samp - C BK ) /60



(100/E)



(1/V)



MDA (Bq/L) = {2.71 + 4.65



sqrt(C BK



t)}/(60



t)



(100/E)



(1/V)



C samp and C BK : 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 3 H in Urine: urine sample spiked with 3 H standard

Sample Spectrum 150 100 50 0 0 100 200 300 400 500 600 700 800 900 1,000 ...S 관련 \ 방사능분석 \ ...S 관련 \ 방사능분석 \ ...S 관련 \ 방사능분석 \

Page 28

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

samples Spiked urine sample 1 Spiked urine sample 2 SQP(E) 845.75

845.98

LSC efficiency (%) Spiked activity (Bq/L) Measured activity (Bq/L) Rel. deviation (%) 47.38

47.41

61,000 620,000 58,900 (1674.3 cpm) 617,400 (17562 cpm) 3.4

0.4

Page 29

MDA of the method for 3 H 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

Literature

Page 31 TR-LSC Application Note P10763: Time Resolved Liquid Scintillation Counting: (TR-LSC) for Environmental 3 H 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 3 H

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