Applied Radiation and Isotopes 72 (2013) 68–72

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Applied Radiation and Isotopes

0969-80

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n Corr

E-m

journal homepage: www.elsevier.com/locate/apradiso

Development of liquid scintillation based 4pb(LS)-g coincidencecounting system and demonstration of its performance bystandardization of 60Co

D.B. Kulkarni a,n, R. Anuradha a, Leena Joseph a, B.S. Tomar b

a Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai 400 085, Indiab Radio Analytical Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India

H I G H L I G H T S

c A 4pb(LS)-g coincidence counting system has been developed in the laboratory.c The performance of the system is demonstrated by standardizing a 60Co solution.c Three methods viz. 4pb(LS)-g, 4pb(PC)-g coincidence counting system, and CIEMAT/NIST method are used.c The activity concentrations obtained by different methods are comparable within uncertainty limits.

a r t i c l e i n f o

Article history:

Received 29 March 2012

Received in revised form

24 September 2012

Accepted 28 September 2012Available online 7 October 2012

Keywords:

4pb(PC)-g coincidence counting system

4pb(LS)-g coincidence counting system60Co

CIEMAT/NIST

43/$ - see front matter & 2012 Elsevier Ltd. A

x.doi.org/10.1016/j.apradiso.2012.09.024

esponding author. Fax: þ91 222 550 5151.

ail address: devak@barc.gov.in (D.B. Kulkarni

a b s t r a c t

A single-vial, single-PMT 4pb(LS)-g coincidence counting system has been developed at the Radiation

Safety Systems Division, BARC. It has advantages of simple sample preparation, higher counting

efficiency and the absence of self absorption over the conventional proportional counter based

4pb(PC)-g coincidence counting system. The performance of the system is demonstrated by standar-

dizing a 60Co solution using the 4pb(LS)-g coincidence counting system, 4pb(PC)-g coincidence

counting system and CIEMAT/NIST method and comparing the results obtained by each method.

The detection efficiency of liquid scintillation counter of the 4pb(LS)-g coincidence counting system

was varied by color quenching, by chemical quenching and by varying the bias voltage applied to the

LSC PMT. For the proportional counter based 4pb(PC)-g coincidence counting system the detec-

tion efficiency was varied by source self absorption. The activity concentrations obtained using the

4pb(LS)-g coincidence counting system, the 4pb(PC)-g coincidence counting system and the

CIEMAT/NIST method are comparable within the uncertainty limits.

& 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Coincidence counting is a powerful and absolute method for themeasurement of activity of radionuclides, and has been in use forover 60 years (Dunworth, 1940; Campion, 1959; Baerg, 1966;Herceg, 1973). The technique, with its variants, enables the directmeasurement of activity of most radionuclides, without any specificvalue being assumed for branching ratios, decay scheme parametersand detector efficiencies. It is directly applicable to all radionuclidesdecaying by the simultaneous emission of two or more radiations,such as b–g, a–g, electron capture-g, g–g etc. The Radiation SafetySystems Division (RSSD), BARC maintains the 4pb–g coincidencecounting system as a primary standard for radioactivity measure-ments. The system consists of a 4p proportional counter for beta

ll rights reserved.

).

detection and a NaI(Tl) scintillation detector for gamma detection.The detailed description of the system can be found elsewhere(Anuradha et al., 2005). A liquid scintillation counter can be used inthe 4pb–g coincidence counting system instead of a proportionalcounter and it has advantages of simple sample preparation, highercounting efficiency and the absence of self absorption. A new singlevial 4pb(LS)-g coincidence counting system has been developed inthe laboratory. The description of the system and its performance ispresented in this paper.

2. Experimental

2.1. 4pb(LS)-g coincidence counting system

A single vial, single photomultiplier tube (PMT) 4pb(LS)-gcoincidence counting system has been developed in this laboratoryand consists of a liquid scintillation counter placed over a 300 �300

530

540

550

560

Nβ

(CPS

)

D.B. Kulkarni et al. / Applied Radiation and Isotopes 72 (2013) 68–72 69

NaI(Tl) detector as shown in Fig. 1. The liquid scintillation counterconsists of a cylindrical counting chamber made of PVC. The ETL9235 PMT is attached to the chamber from one side perpendicularto its principal axis (i.e. Z axis). A liquid scintillation vial is placed ina brass container which is coated from inside with TiO2 paint whichacts as a reflector. An annular brass shutter is also provided so as toisolate the PMT from the counting chamber while replacing asample. The entire system is shielded with 5 cm thick annular leadshield. The entire assembly is placed inside a light tight enclosure soas to avoid light leakage. The PMT anode is connected to the inputstage of a low noise preamplifier. The preamplifier output is shapedand amplified using a double delay line amplifier. The amplifieroutput is fed to a single channel analyzer (SCA) in the integralmode. A discriminator of 1 V is applied to reject the noise pulses.The gamma detector is a 300 �300 NaI(Tl) scintillator integral assem-bly. Its output is shaped and amplified using a spectroscopyamplifier. The output of the amplifier is fed to a SCA operated inthe normal mode. The outputs of the liquid scintillation counter(LSC) channel SCA and the NaI(Tl) channel SCA are fed to acoincidence analyzer developed in the laboratory. A coincidenceresolving time of 2 ms is used. The three pulses are fed to anAdvantech PCI1780 counter timer card. The software for acquiringthe data was developed in Visual Basic 6. The input data are fedthrough a user friendly GUI, after which data are acquired as per theconfiguration data provided by the user. The required calculationsare carried out with the help of software and all the data are storedin the hard disk for final analysis.

4 6 8 10 12 14 16520

Volume of the scintillator (ml)

Fig. 2. Observed count rate in the beta channel versus volume of scintillator.

2.2. Standardization of 60Co using 4pb(LS)-g coincidence

counting system

The performance of the newly developed system was demon-strated by standardizing 60Co and comparing the results witha 4pb(PC)-g coincidence counting system, which is the primary

Fig. 1. Drawing of the detector system of 4

standard maintained in the laboratory and CIEMAT/NIST measure-ments. The volume of scintillator used was optimized by measur-ing a sample with different amount of scintillator varying from5–15 ml. The observed count rate in the beta channel versus thevolume of scintillator is almost constant over the entire volumerange as shown in Fig. 2. A scintillator volume of 5 ml was used forthe source preparation so as to reduce the amount of organic liquidwaste. Six samples of 60Co solution in the form of CoCl2 in 0.1 NHCl were prepared in the 20 ml low potassium liquid scintillationvials with 5 ml Ultima Gold scintillator. The amount of radioactivesolution added was in the range of 10 mg to 25 mg. The standar-dization was carried out by efficiency extrapolation technique

pb(LS)-g coincidence counting system.

D.B. Kulkarni et al. / Applied Radiation and Isotopes 72 (2013) 68–7270

(EET) (Campion, 1959; Baerg, 1966). The beta efficiency (eb) for asample was varied using three methods, namely:

440

1.

Figvar

(17

Chemical quenching using nitromethane.

2.

438

/Nc

Color quenching using methyl orange solution in Ultima Goldscintillator.

Nγ

3.

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35

434

436Nβ*

(1-εβ)/εβ

Fig. 4. Plot of Nb�Ng/Nc versus (1�eb)/eb for sample no. 7584 using efficiency

variation by color quenching. (Fitting equation: Y¼(433.9970.69)þ(20.4449773.02)�X.

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40430

432

434

436

438N

β*N

γ/Nc

(1-εβ)/εβ

Fig. 5. Plot of Nb�Ng/Nc versus (1�eb)/eb for sample no. 7589 using efficiency

variation by EHT variation. (Fitting equation: Y¼(432.1070.34)þ(14.5571.37)�X.

Varying the bias voltage applied to the PMT.

2.2.1. Efficiency variation by chemical quenching

Two samples (Sample nos. 7585 and 7586) were counted byvarying the efficiency of the liquid scintillation counter usingchemical quenching. The sample was first counted without addingthe quenching agent. Subsequently one drop of the nitromethane(app. 10 mg) was added to the sample and was counted again afterthorough mixing. The procedure was repeated with the successiveaddition of a drop of the nitromethane. The plot of Nb�Ng/Nc versus(1�eb)/eb for sample no. 7585 is shown in Fig. 3, where Nb, Ng andNc are the true count rates of the beta, gamma and coincidencechannel respectively and eb is the beta efficiency measured from theratio of Nc and Ng eb for the sample was varied in the range of 75% to89%. The activity concentration obtained by extrapolation of (1�eb)/eb to zero for sample nos. 7585 and 7586 was 434.9271.16 kBq/gand 433.5071.34 kBq/g respectively as on reference date of 15thMarch, 2011 at 06:30 GMT.

2.2.2. Efficiency variation by color quenching

Two samples (Sample nos. 7584 and 7588) were measured byvarying the efficiency of the liquid scintillation counter usingcolor quenching. The procedure used was similar to that used inthe Section 2.2.1 except that methyl orange dye solution wasadded instead of nitromethane. The plot of Nb�Ng/Nc versus(1�eb)/eb for sample no. 7584 is shown in Fig. 4 where eb variedin the range of 75% to 89%. The activity concentration obtained byextrapolation of (1�eb)/eb to zero for sample nos. 7584 and 7588was 433.9671.74 kBq/g and 435.4371.37 kBq/g respectively ason reference date.

2.2.3. Efficiency variation by changing the bias voltage applied

to the PMT

Two samples (Sample nos. 7587 and 7589) were measuredusing the efficiency variation achieved by varying the bias voltage

0.00 0.05 0.10 0.15 0.20 0.25 0.30

434

436

438

440

Nβ*

Nγ/

Nc

(1-εβ)/εβ

. 3. Plot of Nb�Ng/Nc versus (1�eb)/eb for sample no. 7585 using efficiency

iation by chemical quenching. (Fitting equation: Y¼(434.9270.63)þ

.3072.84).

applied to the PMT of the liquid scintillation counter. The biasvoltage applied to the PMT was varied from 680 V to 800 V insteps of 20 V and resulted in an efficiency variation from 60% to86%. The plot of Nb�Ng/Nc versus (1�eb)/eb obtained for sampleno. 7589 using bias voltage variation is shown in Fig. 5. Theactivity concentration obtained by extrapolation of (1�eb)/eb tozero for sample nos. 7587 and 7589 was 431.4271.74 kBq/g and432.1071.37 kBq/g respectively as on reference date.

The activity concentrations obtained using the 4pb(LS)-gcoincidence counting system employing various efficiency varia-tion methods are shown in Fig. 6 along with the associateduncertainties. The average of the activity concentration obtainedby all these methods was 433.5671.44 kBq/g as on the referencedate. The coverage factor is 1 for all the quoted uncertainties.

2.3. 4pb(PC)-g coincidence counting system measurements

The 4pb(PC)-g coincidence counting system is maintainedas the primary standard for radioactivity measurements in thelaboratory. The equivalence of the 4pb(PC)-g coincidence count-ing system with the other international laboratories has beenalready established by participating in the international referencesystem (SIR) of Bureau International Des Poids Et Mesures (BIPM).

7584 7585 7586 7587 7588 7589430

431

432

433

434

435

436

437

Act

ivity

Con

cent

ratio

n of

60C

o (k

Bq/

g)

Source Numbers

Fig. 6. The activity concentrations obtained using 4pb(LS)-g coincidence counting

system employing various efficiency variation methods (here: square denotes the

data of efficiency variation by color quenching. Triangle denotes the data of

efficiency variation by chemical quenching. Circle denotes the data of efficiency

variation by EHT variation. Solid line represents the average activity concentration

of all the sources and dashed lines represent an uncertainty in the average activity

concentration).

Table 1

Uncertainty budget for 60Co activity concentration measurements using 4pb(LS)-gcoincidence counting system for source no. 7584 at coverage factor k¼1.

Source of uncertainty Relative standard uncertainty (%)

Counting statistics 0.10 Type A

Extrapolation 0.16 Type A

Background 0.01 Type A

Weighing 0.02 Type B

Dead time 0.26 Type B

Resolving time 0.24 Type B

Half life 0.00 Type B

Combined standarduncertainty

0.40

Table 2

Uncertainty budget for 60Co activity concentration measurements using 4pb(PC)-gcoincidence counting system method at coverage factor k¼1.

Source of uncertainty Relative standard uncertainty (%)

Counting statistics 0.13 Type A

Extrapolation 0.16 Type A

Background 0.02 Type A

Weighing 0.03 Type B

Dead time 0.18 Type B

Resolving time 0.16 Type B

Half life 0.00 Type B

Combined standarduncertainty

0.32

D.B. Kulkarni et al. / Applied Radiation and Isotopes 72 (2013) 68–72 71

The description of the system can be found elsewhere (Anuradhaet al., 2005). The efficiency extrapolation technique (Campion,1959; Baerg, 1966) was used for the standardization.

The source preparation involves, deposition of radioactivesolution of CoCl2 in 0.1 N HCl on 15 mg cm�2 VYNS films, coatedwith 20 mg cm�2 gold using the pycnometer method. Teflon wasdeposited on the substrate as a seeding agent for uniformdistribution of the source. The counting efficiency of the propor-tional counter was varied by source self absorption, efficiency wasvaried in the range of 78% to 90%. For this purpose the inactiveCoCl2 solutions in 0.1 N HCl with concentration in the range of50 mg/ml to 1000 mg/ml were used. The sources were dried in adesiccator. The dried sources were sealed with another goldcoated VYNS film. Accurate source weighing was carried out withSartorius MC21S micro balance. The activity concentra-tion obtained by this method was 432.8271.39 kBq/g as onreference date.

2.4. Standardization by CIEMAT/NIST method

Standardization was carried out using the CIEMAT/NISTmethod (Garcia-Torano et al., 1991; Grau Malonda and Garcia-Torano, 1982; Gunther., 1994). The original solution used for thestandardization by the coincidence method was first diluted sothat the activity concentration was such that the sources pre-pared for the measurements in the liquid scintillation counterwas having a count rate less than 10,000 CPM. Nine samples of60Co solution in the form of CoCl2 in 0.1 N HCl were preparedin the 20 ml low potassium liquid scintillation vials with15 ml Ultima Gold scintillator and 1 ml water. The amount of CoCl2solution added was in the range of 12 mg to 17 mg. A set of 3Hquenched standards already prepared for 131I standardization wereused. The detail description regarding the set of 3H quenchedstandards can be found elsewhere (Kulkarni et al., 2011). The 60Cosamples were counted along with the 3H standards in the PackardTRI-CARB 2900 TR Liquid Scintillation Analyzer with two photo-multipliers operated in coincidence. The counting efficiency of 3Hversus FOM and 60Co versus FOM was calculated using the freewareprogram CN2003 (Gunther, 2002), developed by Gunther of thePhysikalisch-Technische Bundesanstalt (PTB), Germany, which is

available on the ICRM Liquid Scintillation Counting Working Groupwebsite hosted by the Laboratoire National Henri Becquerel (LNHB)(/http://www.nucleide.org/ICRM_LSC_WG/icrmsoftware.htmS).Two betas of end point energy 318.2 keV and 1491.4 keV and twogammas of energy 1173.2 keV and 1332.5 keV were used for thecalculation of the detection efficiency in the program. The nucleardecay data used for the calculation was taken from the Decay DataEvaluationProject (DDEP) and was obtained from the website hosted by theLNHB (/http://www.nucleide.org/DDEP.htmS). The 60Co countingefficiency (eb) corresponding to the measured tSIE values was in therange of 93% to 95%. The measured count rate for each source wasdecay corrected to the reference date. The activity concentrationwas obtained by plotting the graph of decay corrected count rate ofthe samples versus (1�eb) and extrapolation of (1�eb) to zero. Theactivity concentration obtained by the CIEMAT/NIST method was437.1571.62 kBq/g as on reference date, after applying the dilutioncorrection.

3. Results and discussion

The detail uncertainty budgets for standardization using4pb(LS)-g coincidence counting system, 4pb(PC)-g coincidencecounting system and the CIEMAT/NIST method are shown inTables 1–3 respectively. The activity concentrations obtainedusing 4pb(LS)-g coincidence counting system, 4pb(PC)-g coin-cidence counting system and the CIEMAT/NIST method are shownin Fig. 7. This shows that the results obtained using differentmethods are comparable within the uncertainty limits. Thisdemonstrates that the performance of the newly developed4pb(LS)-g coincidence counting system is comparable with theprimary standard (4pb(PC)-g coincidence counting system) main-tained in the laboratory and CIEMAT/NIST method. The equiva-lence of the 4pb(PC)-g coincidence counting system with theother international laboratories have been already established byparticipating in the international reference system (SIR) of Bureau

Table 3Uncertainty budget for 60Co activity concentration measurements using the

CIEMAT/NIST method at coverage factor k¼1.

Source of uncertainty Relative standard uncertainty (%)

Counting statisticþBG 0.13 Type A

Extrapolation 0.33 Type A

Weighing 0.04 Type B

Half life 0.01 Type B

Uncertainty due to tracer activity 0.05 Type B

Quench indicating parameter 0.01 Type B

Dilution correction 0.02 Type B

Combined standard uncertainty 0.36

1 2 3426

428

430

432

434

436

438

440

442

444

Act

ivity

Con

cent

ratio

n of

60C

o (k

Bq/

g)

Fig. 7. The activity concentrations obtained using 4pb(LS)-g coincidence counting

system, 4pb(PC)-g coincidence counting system and the CIEMAT/NIST method

(here: 1 denotes the activity concentration of 60Co obtained using 4pb(LS)-gcoincidence counting system, 2 denotes the activity concentration of 60Co obtained

using 4pb(PC)-g coincidence counting system, and 3 denotes the activity concentra-

tion of 60Co obtained using the CIEMAT/NSIT method. Solid line represents the

average activity concentration obtained using all the methods and dashed lines show

1% band above and below the average activity concentration).

D.B. Kulkarni et al. / Applied Radiation and Isotopes 72 (2013) 68–7272

International Des Poids Et Mesures (BIPM). Thus equivalence ofthe newly developed 4pb(LS)-g coincidence counting system withthe other international standards can be established through the4pb(PC)-g coincidence counting system.

4. Conclusion

The results of the 60Co activity measurements using the4pb(LS)-g coincidence counting system are comparable with theother standardization methods and hence demonstrates its per-formance. The results obtained are in excellent agreement withother standards maintained in the laboratory. Hence 4pb(LS)-gcoincidence counting system can be an alternative methodsuitable to standardize radionuclides with acceptable precisionin the laboratory. There are also several advantages with this4pb(LS)-g coincidence counting system such as simple processof source preparation, high counting efficiency, absence ofself-absorption, and applicability of various counting methods.

Acknowledgment

The authors would like to express their thanks and gratitude toDr. A.K.Ghosh, Director, Health Safety and Environment Group,and Dr. D.N. Sharma, Associate Director, Health Safety andEnvironment Group and Head Radiation Safety Systems Division,Bhabha Atomic Research Centre, Trombay, Mumbai for theirconstant encouragement and support.

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