principles of liquid scintillation counting: theories and … · principles of liquid scintillation...

Paul Scherrer Institut • 5232 Villigen PSI lecture: principles of LSC, RISO, September 6, 2013

PAUL SCHERRER INSTITUT

Principles of liquid scintillation counting: theories and applications

Jost EikenbergDivision for Radiation Safety and Security

Paul Scherrer InstitutCH-5232 Villigen PSI / Switzerland



Development and implementation of radiochemical separation techniques for emission and immission measurements such as actinides in soil samples Ra isotopes in groundwater etc.Research projects with universities: dating

of Pleistocene samples with applications in volcanology, paleoearth quake dating , ice core samples (global warming)

R&D projects of the PSI radioanalytical laboratory



Table of Contents

1. Introduction to the LSC counting technique2. Quenching effects and correction procedures3. Alpha/Beta separation via pulse shape analysis4. Introduction to the triple to double coincidence

ratio measurement technique (TDCR)5. Example of a simultaneous alpha/beta analysis

using Radium isotopes in drinking water



Example: decay scheme of -emitting 228Ra

ZA

ZAX Y e decay energy

1

1

Z = 88

Z = 89228Ac (6.13 h)

1-

0.0

0.0

0.0067

228Ra (5.75 y)

Particle radiation, pure -emitting isotopes



Schematic view of radioactive transformation

1 (95.5 %)1

Z = 88

Z = 86222Rn (3.82 d)

2 (4.5 %)

0.0

0.0

0.186

226Ra (1600 y)

88226

86222

24 4 78Ra Rn He MeV .



principle of liquid scintillation spectrometry (LSC)

• Sample is in a cocktail with a organic scintillator and homogenously mixed

• Particle radiation ( or ) disturbs the electron configuration (orbital) of the scintillation liquid

• During recombination of the electron structure light (h*) is emitted which is measured in photomultiplier tubes



page 7

Liquid Scintillation Counting

Samples are dissolved or suspended in a "cocktail" containing an aromatic solvent and small amounts of other additives known as fluors Beta particles emitted from the sample transfer energy to the solvent molecules, which in turn transfer their energy to the fluors; the excited fluor molecules dissipate the energy by emitting light In this way, each beta emission results in a pulse of light



Principle of LS Coincidence/Anticoincidence Counting

page 8

log Scale

3H 14C 32P

The counter has two photomultiplier tubes connected in a coincidence circuit. The coincidence circuit assures that genuine light pulses, which reach both photomultiplier tubes, are counted, while other pulses (due to noise, for example), which would only affect one of the tubes, are ignored.



PMT 1

COINCI‐DENCE

PMT 2

SUM‐MATION ADCAMPLIFIER

DIGITIZEDSUMMED

COINCIDENCEPULSE

SPECTRALYZERSPECTRUMANALYZER

COINCIDENCE PULSE

Classical two photomultiplier LS electronic set-up



principle of LSC coincidene/anticoincidence counting

PMT 1 PMT 2PMT 2

PMT 2PMT 2

sample chamber

PMT 2 PMT 2

sample vial



LSC, Perkin Elmer Model TriCarb 3100 TR/SL LSC with BGO detector guard



HIDEX Model SL 300 with guard detector



LSC: relevant isotopes in the nuclear fuel cycle, radiopharmaceutical

application and environmental analysis• Pure - or ec-emitters Tritium (H-3) and

Radiocarbon (C-14) others, : Cl-36, Fe-55, Ni-63, Sr-90, Biological application: P-32, P-33, etc.

• Alpha-emitters, actinides : Th, U, Pu, Am, Cm• Alpha- and Beta-emitters from the U and Th decay

series: Ra-228, Ra-226, Pb-210, Po-210



-continuum-spectra: 3H, 14C

1 10 1000

200

400

600

800

1000

1200

LSC: spectrum of 14C und 3H

E max

=18

.6 k

eV

E max

= 1

56 k

eV

3H 14C

inte

nsity

(not

to s

cale

)

energy (keV)



Calculation of activity concentrations via classical and TDCR measurements

• Direct counting (standardization) and two window counting (e.g. 3H, 14C)

• Use of quench curves• Efficiency tracing methods (CIEMAT/NIST, needs

modeling) • Triple to double coincidence ratio measurements

(TDCR)



general relationship between activity and counting rate

N

tIA

)14(.)()3( 14, CcpmcmpcpmcpmHcpm BC

B

AnetAA

Example H-3, C-14, two window countingwindow A: H-3 + C-14, window B: C-14



Determination of the counting efficiency(a) by adding spike of the same isotope

AN

(b) Automatically via quench curve correctionaddedi

additionbeforeiadditionafterii A

NN

,

,,



Three window Sr-isotope counting

1 10 100 10000

20

40

60

80

100

120

composite spectrum

90Y

90Sr

85Sr

Region A:85Sr

Region C:89Sr + 90Y

Region B:90Sr + 90Y + 89Sr

-energy [keV] (LS-scale)

coun

ts/c

hann

el (n

ot to

sca

le)



• Quenching within a sample refers to any mechanism which reduces the amount of light being emitted from the vial. • Reducing the amount of light reaching the PMT’s results in a reduction of the pulse height.• Any factor, which reduces the efficiency of the energy transfer or causes the absorption of photons (light), results in quenching in the sample.

Quenching

page 19


PAUL SCHERRER INSTITUTQuenching

Sabine Mayer – 1 Introducion to radiological physics page 20

Chemical Quenching… occurs during the transfer of energy from the solvent to the scintillator.

Chemical agents (e.g. water and other solvents) added to the cocktail with the radioactive sample interfere with the transfer of kinetic energy between the solvent and the fluor(s).

The results are:Reduction and loss of lightReduced counting efficiency

Colour Quenching…is an attenuation of the photon of light.

The photons produced are absorbed or scattered by the colour in the solution, resulting in reduced light output available for measurement by the PMT’s.



14C spectrum of waste water: examples for not quenched filtrated

water and a quenched spectrum (direct measurement)

1 10 1000

200

400

600

800

1000

1200

E quen

ched

< E

max

E max

= 1

56 k

eV

14Cquenched

14C(not quenched)

rela

tive

inte

nsity

energy (keV)



Effects of Quenching

page 22

1) A shift in the pulse height spectrum of the particles to lower energy

2) A reduction in the measured CPM of the sample (loss of counting efficiency). This effect occurs especially with low energy particles.



Quench parameter calculation

k

ii

k

iii

XI

XIXkQPES

1

1

)(

)(

QPES or tSIE = quenching parameter (or center ofgravity or spectral index) of the external standardXi and I(Xi) refer to the channel numbers and the countrates of the external standard in each channel number.



3H, 14C quench curves: efficiency in dependency of the spectral index from the external standard

100 150 200 250 3000

20

40

60

14C 3H

% e

ffici

ency

tSIE



Alpha/Beta Separation

• Classical: use of spill-over curves and pulse shape analysis (PSA)

• TDCR (Hidex 300 SL): use of pulse length index (PLI) and energy differentiation



Alpha/Beta spectrum of Pu-isotopes



(iii) Alpha LSC countingalpha/beta-discrimination, PSA, Pulse

Shape Analysis)

20 40 60 80 100 120 140 1600

200

400

600

800

1000

1200

alpha-pulse

beta-pulserela

tive

inte

nsity

pulse decay time [ns] 100 110 120 130 140 1500

2

4

6

8

10

12

14

in in

spillo

ver [

%]

discriminator setting [ns]



Pulse Length Index (PLI) discrimination with HIDEX SL 300

27 38 52 72 99 135

185

252

343

466

634

861

1169

1588

2156

2927

3973

5393

7319

9934

1348

1

1829

6

0

4

8

12

16

20

24

28



Analytical separation of two interfering -peaks

0 200 400 600 800 10000

20

40

60

80

100

120

140

window B:B + Aspill

window A:A + Bspill

AspillBspill

BA

window Bwindow A239+240Pu 236Pu-Tracer

-energy [MeV]

coun

ts/c

hann

el (n

ot to

sca

le)



mathematical equations: 2-window counting

AA

V spill1 BB

V spill2

spillA BAN spillB ABN

21

21239 1

1VV

NVNVAAN BAspill

21

12236 1

1VV

NVNVBBN ABspill



Isotope Spiking

Pu

ss Y

NA )(239

)(239

Pu

tt Y

NA )(236

)(236

)(236)(236

)(239)(239 t

t

ss A

NN

A



The triple coincidence to double coincidence ratio (TDCR) counting technique

SAMPLE

PM R

PM S

PM T



The theory of TDCR

nx

exnnxP

!),(

1!

:0xnx

x

nenPZ ),0(

neZ 11

In counting statistics the number of events (x) that are detected from a mean number of (n) events follows a discrete probability or Poissondistribution provided that the events occur with a known average rate and independently of the time since the last event i.e.

From equation (9-53) it follows for the extreme case of no detection

Therefore, for case of no detection or zero detection (Z) it follows for 1 photomultiplier, for instance for “R”:

And, for the detection probability or detection efficiency ():

(9-56)

(9-53)

(9-54)

(9-55)



2111 nnn eee

32 111 nnn eee

31 neRST

nn eeRS 21

Equation (9-56) shows that the value for the detection efficiency varies between 0 (no interaction between the decaying isotopeand the scintillation cocktail) and 1 (i.e. when high energetic beta particles undergo strong interaction with the cocktail medium, i.e. produce a very high number of photons that are detected via LSC). From equation (9-56) it is obvious that for 2 photomultipliers (R+S) connected in coincidence, the total coincidence efficiency is:

And, consequently, for three photomultipliers (R, S, T) connected in a coincidence circuit:

In the following the possible quantity of coincidences is listed for the one case of triple coincidences tubes (R-S-T)and the other three cases of double coincidences, i.e. (R-S, R-T, S-T).

3-coincidences:

Equation (9-60) shows the coincidence probability for the R-S couple with zero detection from the T-tube.In analogy it can be written for the combination of the R-T-tubes (with zero detection from the S-tube):

(9-62)

2-coincidences:

(9-57)

(9-58)

(9-59)

(9-60)

nn eeRT 212-coincidences:



nn eeST 21

nnn eeeall 23 131

nnn eeeall 311 2

nn eeall 211 2

And, finally for the combination S-T (with zero detection from the R-tube):2-coincidences:

(9-62)The logical sum of these 4 possible combinations, i.e. the sum of all coincidences is:

(9-63)This expression can be rewritten by reducing both terms in equation (9-63) with (1-e-n)2 i.e.,

(9-64)

(9-65)



32 1213 nn eeall

32

3

12131

nn

n

eee

alltriple

cidencesdoublecoincidencestriplecoinTDCR

1..11: TDCReien n

0...11:0 23 TDCReieen nn

Multiplying these terms and converting the result in binominal expressions yields finally:

Therefore the ratio of triple coincidences to all coincidences, referred in the literature as TDCR, is in reality:

In the following, two extreme cases have to be regarded, i.e. the number of photoelectrons produced in the multiplier (n) tends to a) an unlimited high number and, b) tends to zero value, i.e.

(9-68)

(9-69)

(9-66)

(9-67)

For a high number of events the measured ratio TDCR is a function of the counting probability detection, i.e. the efficiency.With common pure -emitting radionuclides and less quenched cocktail medium (e.g. 14C in organic compounds such as benzenemixed with a high aromatic scintillation liquid) the TDCR value approximates roughly the counting efficiency.For the Hidex 300 SL LS-spectrometer the efficiency is given as linear function of the TDCR-value or,

1 mwithTDCRm (9-73)



0.0000

0.1000

0.2000

0.3000

0.4000

0.5000

0.6000

0.7000

0.0000 0.1000 0.2000 0.3000 0.4000 0.5000 0.6000 0.7000

TDCR

Eff

The first approximation linear relationship between the counting efficiency and the TDCR



TDCR vs. Efficiency using high purity radionuclide standard solutions

0.4 0.5 0.6 0.7 0.8 0.9 1.00.4

0.5

0.6

0.7

0.8

0.9

1.0

TDCR = efficiency

226Ra

210Pb

228Ra

TDC

R

efficiency



TDCR blank correction

2211 MCMCMC mm

00 TDCRrTDCRrTDCRr nnbb

For linear systems the following mass relationship hold for mixtures (m) of two omopnents, M1 and M2 with the concentrations C1 and C2

This relationship can be transferred directly to TDCR measurements and the true or net-TDCR (TDCRn) has to be calculated from the brut-TDCR („mixture“) (TDCRb) and the TDCR-value of a blank measurement (TDCR0) .

with

0rrr bn 000 TDCRrTDCRrrTDCRr nbbb

0

00

rrTDCRrTDCRrTDCR

b

bbn

nnTDCR

n

b

n

n rrrA

0

bbb

b

TDCRrTDCRrrrA

0

20

follows:

for pure beta emittes holds:

with:

follows for the activity calculation



Terrestrial radioisotopes:

238U-series 235U-series

232Th-series



238U 234U

230Th

234Pa

234Th

226Ra

222Rn

218Po 214Po 210Po

210Bi

210Pb 206Pb

235U

231Pa

231Th 227Th

227Ac

223Ra

219Rn

232Th 228Th

228Ac

224Ra228Ra

220Rn

215Po

211Pb 207Pb

211Bi 212Bi

216Po 212Po

212Pb 208Pb

207Tl 208Tl

214Bi

214Pb

4.47x109y 2.45x105y

24.1d 7.54x104y

1.17min

4.196MeV

4.776MeV

4.688MeV

4.784MeV

5.490MeV

6.003MeV

1600y

3.825d

3.11min

19.9min

26.8min 22.3y

7.04x108y

6.038MeV

4.395MeV

5.013MeV

3.28x104y

18.7d

21.8y

11.4d

4.010MeV

1.41x1010y 1.91y

5.75y

5.423MeV6.13h

3.66d

5.686MeV

55.6s

6.288MeV

1.6x10-4s

5.01d

7.687MeV

5.304MeV

stable

138.4d

1.06d

5.716MeV

6.819MeV

3.96s

1.8x10-3s

2.14min

36.1min stable

7.386MeV

6.623MeV

4.77min

66.3%

0.15s

6.779MeV 1.01h

33.7%

6.051MeV10.6h

3.05min

stable

8.784MeV

3x10-7s

238U-Series 235U-Series 232Th-Series

UPaThAcRaFrRnAtPoBiPbTl



Th-Ra relationship in the 232Th and 238U decay series

230 7 54 10 226 1 60 10 222 3 82 2184 3

Th Ra Rn Po daughtersa a d : . : . : .

232 1 40 10 228 5 76 10 228 613 22810 0

Th Ra Ac Th daugthersa a h : . : . : .

228 19110 224 3 66 220 55 6 2160

Th Ra Rn Po daugthersa d s : . : . : .



Continental water: relevant isotopes

• Alpha-emitter: U-238, U-234• Beta-emitter: Ra-228 (with fast ingrowing Ac-228)• Alpha-emitter: Ra-226 (with Rn-222 prognies)• Beta-emitter: Pb-210 (with ingrowing Bi-210)• Alpha-emitter: Po-210



relevant isotopes in continental water and their measurement methods at PSI

radionuclide analytical technique

234U, (235U), 238U U/TEVA separation, electro-deposition, -spectrometry

226Ra, 228Ra, 210Pb filtration (RadDisc), OptiPhase Hisafe3 cocktail, LSC

210Po spontaneous deposition on silver disc, -spectrometry



Relationship parent / daughterparent isotope

half-life parent

ingrowing daughter

half-life daughter

210Pb () 22.3 years 210Bi () 6.02 days

226Ra () 1602 years 222Rn () 3.82 days

228Ra () 5.76 years 228Ac () 6.13 hours



Method implementation: low level determination of 210Pb, 226Ra+228Ra in drinking water

• Filtration of the sample (2 liter) through 3 Empore RadDisc (Mn-oxide impregnated) membrane filters

• Elution of Pb with Diammonium Hydrogen Citrate• Elution of Ra with alkaline Na-EDTA solution• Measuring via LSC with optimized -

discrimination



-spectrum of Radium-226: PERALS-LSC



-spectrum of 226Ra with ingrowing daughters 2 h and 8 h after separation using HIDEX 300 SL LSC

Alpha

0

20

40

60

80

100

120

140

10 100 1000 10000



-spectrum of 226Ra with ingrowing daughters obtained 6 days after separation

Alpha

0

20

40

60

80

100

120

140

160

10 100 1000 10000



-spectrum of 228Ra with ingrowing 228Ac 1 h and 8 h after separation using HIDEX 300 SL LSC

Beta

0

5

10

15

20

25

30

35

40

45

50

1 10 100 1000



long lived mother – short lived daughter relationship

A t A e e A ey xy

y x

xt yty

yt( ) ( ) ( ) ( )

0 0

A t A e ey xy

y x

xt yt( ) ( ) ( )

0

x yy

y x

1

)1()0()( txy

yeAtA



LS-spectrum interference correction

228228

AcB

A

AcR

)()( 228228,

228, AcrRrRar B

nAcAmn

Acorn

ARa

cornRa

RarA

228

228,

228

)(

BAc

Bn

AcAcrA228

228

228)(



Comparison of measured 228Ra and 228Ac activities with calculated decay/ingrowth curves

0 10 20 30 40 50 60 70 800.0

0.5

1.0

1.5

2.0

2.5

228Ac: measured data

228Ra decay curve

228Ra: measured data

228Ac ingrowth curve

rela

tive

activ

ity

time after separation [h]



Comparison of measured 226Ra and the progeny isotopes 222Rn, 218Po and 214Po with calculated decay/ingrowth curves

0 5 10 15 20 25 300.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

222Rn+218Po+214Po: measured data

226Ra decay curve

226Ra: measured data

222Rn+218Po+214Po ingrowth curve

rela

tive

activ

ity

time after separation [d]



ms tr

VkA 0

1* 21

0 5 10 15 20 250

10

20

30

40

50

226Ra

228Ra

210Pb

low

er li

mit

of d

etec

tion

[mB

q / L

]

counting time [h]



LSC detection limits, 2 l aliquot, quantitative adsorption on RadDisc filter,

counting time 6 h

• Ra-226: 3 mBq/liter• Ra-228: 20 mBq/liter• Pb-210: 15 mBq/liter



Conclusions• LSC is a powerful method to detect particle radiation

(alpha / beta) without the need of applying extensive sample preparation procedures

• LSC is highly useful to measure potentially volatile isotopes such as 3H and 14C

• New counter developments allow almost quantitative activity concentration determinations without (internal) standardization, i.e. application of TDCR-measurements using triple to double coincidence ratio measurements



Thank you for your attention

principles of liquid scintillation counting: theories and … · principles of liquid scintillation...

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