Bilateral comparison between PTB and ENEA to check the performanceof a commercial TDCR system for activity measurements

Karsten Kossert a,n, Marco Capogni b, Ole J. Nähle a

a Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, D-38116 Braunschweig, Germanyb ENEA Istituto Nazionale di Metrologia delle Radiazioni Ionizzanti (ENEA-INMRI), C.R. Casaccia, Via Anguillarese 301, I-00123 Rome, Italy

H I G H L I G H T S

� The TDCR counter from Hidex Oy was tested at PTB and ENEA.� The studies comprised linearity checks and investigation of adjustments.� A bilateral 89Sr comparison was organized to compare two Hidex counters.

a r t i c l e i n f o

Keywords:Commercial TDCR counterBilateral comparison89Sr

a b s t r a c t

The only commercial TDCR counter from Hidex Oy (Finland), comprising three photomultiplier tubes,was tested at the two National Metrology Institutes (NMIs) PTB and ENEA. To this end, the two NMIspurchased a Hidex 300 SL TDCR counter (METRO version) each and carried out various tests at theirlaboratories. In addition, the two institutions agreed to organize a bilateral comparison in order toacquire information on the reproducibility of the results obtained with the counters. To achieve this, PTBprepared some 89Sr liquid scintillation samples, which were first measured in various counters at PTBand then shipped to ENEA for comparative measurements.

The aim of this paper is to summarize the findings on the counter characteristics and adjustments. Inaddition, the results of the bilateral comparison between PTB and ENEA are presented and the resultsfrom various commercial counters using the CIEMAT/NIST efficiency tracing and the TDCR method arediscussed.

& 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Within the scope of European Metrology Research Programme(EMRP) project JRP ENG08 “Metrology for new generation nuclearpower plants” (MetroFission) the triple-to-double coincidenceratio (TDCR) counter from Hidex Oy (Finland) had to be testedby PTB and ENEA. It was required that the test should comprise aninvestigation of the applicability of this 3-phototube counter forreliable activity measurements. To this end, the two NMIs PTB andENEA each purchased a Hidex 300 SL “METRO” TDCR counter andcarried out various tests at their laboratories. The tests compriseda careful investigation of counter linearity, high voltage (HV) andthreshold adjustment as well as measurements with severalradionuclides which were first standardized with a custom-builtTDCR system at PTB. After several changes of the electronics (madeby Hidex Oy) and with correct adjustments, the results were found

to be satisfactory, indicating the suitability of the counter forreliable activity measurements.

However, such an investigation is useful for characterizing oneindividual counter, but it does not necessarily indicate that otherHidex 300SL “METRO” counters will have comparable characteristics.

Thus, the two institutes agreed to organize a bilateral compar-ison in order to acquire information on the reproducibility of thecounters. This yielded valuable information on adjustments andfurther useful information for other potential users of this counter.

2. Investigation of the Hidex counter at PTB

The outcome of the investigation of the Hidex TDCR carried out atPTB has already been published (Wanke et al., 2012). The publicationincludes a comprehensive report on the performance of the counteras well as valuable information on the instrument settings. In thefollowing, some important aspects are summarized.

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n Corresponding author. Tel.: þ49 5315926110; fax: þ49 5315926305.E-mail address: karsten.kossert@ptb.de (K. Kossert).

Please cite this article as: Kossert, K., et al., Bilateral comparison between PTB and ENEA to check the performance of a commercialTDCR system for activity measurements. Appl. Radiat. Isotopes (2014), http://dx.doi.org/10.1016/j.apradiso.2014.01.008i

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2.1. Counter linearity

The system linearity depends on the correct determination ofthe dead time and, thus, a linearity test is a good check for acorrect dead-time measurement, which is crucial for correctactivity determination.

Samples containing 241Am with expected counting ratesbetween about 167 s�1 and 16700 s�1 were prepared usingUltima Gold™ AB as a liquid scintillation (LS) cocktail. The activityconcentrations of the 241Am solutions can be traced back toprimary standards of PTB. Since weighed aliquots of 241Am wereused, the sample activity and the expected counting rate can becalculated. A counting efficiency of 100% was assumed, which is avery good approximation for alpha emitters in liquid scintillationcounting with low quenching (Kossert et al., 2009).

After iterative improvement of the electronics (see Wanke et al.(2012) for details), the counter was found to deviate from linearityas shown in Fig. 1. From this plot it can be seen that the deviationsare below 0.1% when the counting rates are below 2000 s�1.This can be regarded as acceptable for most applications. Thesystem (non-)linearity is comparable to that of other commercialLS counters.

2.2. Instrument settings

Optimized settings of the coincidence resolving time and thethreshold are crucial for the TDCR method. The threshold needs tobe set below the single electron peak, since the TDCR method isbased on the assumption that the number of created photoelec-trons follows a Poisson distribution and that single electrons arecounted (Broda et al., 2007). In the Hidex counters it is, however,not possible to adjust the threshold directly for the individualphotomultiplier tubes (PMTs), which are of type Electron Tubes9102KA. It is only possible to set a common threshold for all threePMTs. Thus, the following procedure was applied at PTB (Wankeet al., 2012) and is also recommended to other users:

1. The setting of the bias voltage (HV) for the three multipliers iscarried out with the aid of the counting protocol “HVset”. Theprocedure requires an unquenched 14C LS sample. The principleof the HV adjustment is to yield the same QPE (quenchparameter external) value for the three channels. The QPEvalue is defined as the 99th percentile of all counts in thespectrum. The goal of the HV adjustment is to obtain the samegain (same QPE value) for all three PMTs. For this part of theprocedure a preliminary threshold is set to discriminate noise,

fine-tuning is done in step 2. In the latest version of thecounter, an automatic HV adjustment is also possible.

2. The common threshold can then be adjusted by use of theHidex software. To this end, a rather high value (e.g. 100; unitsunknown) should be used at the beginning. When measuring abackground sample, this should yield very low counting rates.The threshold parameter can then be reduced using smalldecrements. At low values (about 30 for the system at PTB)the counting rate should significantly increase, since thesystem will also count noise events. The threshold should beadjusted to be as low as possible, but high enough to avoid thesudden increase due to noise.

Another important parameter is the coincidence resolving timewhich can also be defined in the software. In the Hidex system thevalues can be set between 0 ns and 100 ns, although the meaningof 0 ns resolving time is unclear to the authors of this article. As anexample, the activity concentration of a 99Tc solution was deter-mined with various coincidence resolving times, tc. Some resultsare shown in Fig. 2. It can be seen that the results are almostconstant when tc is 40 ns or larger. For tc¼40 ns the results for theactivity concentration were in very good agreement with theresults of PTB’s homemade TDCR system (Nähle et al., 2010). ThePTB counter is equipped with a MAC3 coincidence module(Bouchard and Cassette, 2000) which also uses a coincidenceresolving time of 40 ns. The influence of the coincidence resolvingtime was also reported by other authors (Steele et al., 2009; Bobinet al., 2012) and to date, there has been no consensus about thecorrect adjustment. For low-energy beta emitters, like 63Ni, therole of tc is very important and higher values (e.g. 200 ns) wereproposed (Bobin et al., 2012). This subject will certainly be aninteresting future research topic.

3. Investigation of the Hidex counter at ENEA

The Hidex 300 SL “Metro” TDCR counter was introduced atENEA in February 2011. The parameter setting suggested by themanufacturer was applied to carry out the measurements withactivity standards of pure beta-emitting sources. The ENEAfocused its attention on the influence of the coincidence resolvingtime tc on the activity measurements by taking into account twokinds of radionuclides: one low-energy beta emitter (63Ni) andone high-energy beta emitter (90Sr). The sources were preparedfrom solutions which are traceable to primary activity standards ofENEA-INMRI.

Fig. 1. System linearity check—deviations of the measured counting rates from theexpected ones in the measurement of 241Am. The bars indicate the standarddeviation of three or more repetition measurements.

Fig. 2. The variation in the coincidence resolving time tc has significant influenceon the determined activity concentration of 99Tc. Values for tco10 ns do not followthe trend and are lower and, thus, not visible in the plot. The measurements werecarried out with the Hidex TDCR counter at PTB. The bars indicate the standarddeviation of eight or more repetition measurements.

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3.1. 63Ni

A set of 63Ni sources prepared from the same solution, withdifferent masses and different levels of quenching agent (CCl4),were measured in the ENEA Hidex counter for different values of tcranging from 15 ns to 100 ns. The data were analyzed by using theCEA software code TDCR07 (Cassette, 2010). The final results ofactivity concentration a as function of tc are shown in Fig. 3.

An activity concentration of (12.6670.10) kBq/g was thenassigned (for tcZ35 ns) on the reference date (30th March 2011,07:00 CET), which is in good agreement with the result of(12.5670.07) kBq/g that was determined by means of theCIEMAT/NIST method. For the latter method the samples weremeasured with a Packard Tricarb 3100TR LS counter.

3.2. 90Sr

In a similar manner as discussed in the previous section, a set of90Sr sources was prepared. Strontium-90 decays to 90Y and bothradionuclides were in radioactive equilibrium. They were consid-ered as pure beta emitters for the computation of the efficiency bymeans of the TDCR07c Fortran code (Cassette, 2009). The finalresults of activity concentration measured with the Hidex counterare shown in Fig. 4 as a function of the coincidence resolvingtime, tc.

An activity concentration of (8.4470.05) kBq/g was thenspecified (for tcZ20 ns) on the reference date (30th March 2011,07:00 CET), which is in good agreement with the result of(8.4870.04) kBq/g that was obtained by means of the CIEMAT/NIST method using the Packard Tricarb 3100TR LS counter.

4. Bilateral comparison with Sr-89

4.1. Comparison scheme

The main intention of the comparison was to obtain informa-tion on the accuracy and repeatability of activity standardizationsmade using the Hidex counter. To this end, it is necessary to have areliable basis for the activity determination. In our case this wasensured by using well-established techniques: The TDCR methodwith a homemade system at PTB and the CIEMAT/NIST 3H-efficiency tracing method with various commercial counters inboth laboratories.

The proposed comparison scheme which had been accepted byboth participating laboratories is shown in Table 1. According tothis scheme, the same LS sources were measured in both labora-tories and, consequently, differences due to different primarystandardization (NMI references) or due to different source pre-paration techniques can be excluded. Moreover, the requirementto make at least one measurement with the Hidex counter underdefined conditions (step No. 5 in Table 1) assures that potentialdifferences are not due to different instrument settings.

The radionuclide 89Sr was selected since it is relevant fornuclear power plants. Due to the high maximum beta energy ahigh counting efficiency is achieved in LS counting and, conse-quently, a low model dependence can be expected. The half-life ofT1/2¼50.57(3) d (Bé et al., 2004) is long enough for such a bilateralcomparison. Week 0 corresponds to the start of the comparisonwhich was in October 2011.

The recipe for the source preparation can be seen in Table 2.The inactive components were added to standard PerkinElmer20 mL glass vials. The vials were then closed, shaken andcentrifuged before and after the addition of weighed portions ofthe active solution. The nominal activity concentration of the 89Srsolution was about 19 kBq/g resulting in expected counting ratesin the order of 3000 s�1 with low statistical uncertainties. SampleNo. 1 served as a background sample. Masses of active solutionplus the corresponding uncertainty, as well as information onpotential photon-emitting impurities, were provided by PTB.

The analysis of raw data from both labs with the same program(step No. 9 in Table 1) was planned to exclude differences dueto the usage of different models. However, since the modeldependence is very low for 89Sr, both participants agreed to skipthis step.

After the sources were returned to PTB, they were againmeasured (step No. 7 in Table 1). In addition, a second sampleseries, which was not shipped to ENEA, was also measured again.From the results it can be concluded that the LS samples werestable and that no activity was lost during shipping or during themeasurements at ENEA.

4.2. Results of the bilateral comparison

At ENEA, the 89Sr LS samples were measured in a 15-year-oldPackard Tricarb 3100 TR counter and in the Hidex counter. For thelatter detector the adjustments of PMT thresholds and HV settingswere made by the manufacturer and the coincidence resolvingtime was set to 40 ns.

The Tricarb data were used to apply the CIEMAT/NIST techni-que (Grau Malonda, 1999). To this end, ENEA made use of the PTBprogram CN2004 (Günther, 2004a, 2004b) which includes aparameterized function to describe the ionization quenching. Thisfunction has been determined by the author of CN2004 using aconstant kB¼0.0075 cm/MeV and Birks’ function as realized in theKB program (Los Arcos and Ortiz, 1997). The efficiency tracing wasrealized using LS samples with 10 mL Ultima Gold and weighedportions of about 10 mg of a 3H solution from the French

Fig. 3. Nickel-63 activity concentration as a function of the coincidence resolvingtime, tc. The measurements were carried out with the Hidex TDCR counter at ENEA.The bars indicate the total standard uncertainty of the activity concentration.

Fig. 4. Strontium-90 activity concentration as a function of the coincidenceresolving time (tc). The measurements were carried out with the Hidex TDCRcounter at ENEA. The bars indicate the total standard uncertainty of the activityconcentration.

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designated institute for radionuclide metrology: the BNM-LNHB(now LNE-LNHB). The expanded relative uncertainty of the 3Hactivity concentration was estimated to be 2% with and expandingfactor k¼2.

The measurement data from the Hidex counter were analyzedwith the TDCR technique (Broda et al., 2007) using the TDCR07ccode (Cassette, 2009). Potential differences in the response of thethree PMTs, often referred to as PMT asymmetry, are also takeninto account, using the minimization algorithm as described byBroda et al. (2007). Again a constant kB¼0.0075 cm/MeV wasused. It should be noted that the parameterizations of the electronstopping powers in the two codes used by ENEA are different; butit is expected that this causes only a negligible difference for high-energy beta emitters like 89Sr. Moreover, the composition of the 3HLS samples is not the same as that of the 89Sr samples (seeTable 2). However, due to the high counting efficiency of 89Sr thisis of minor importance.

At the PTB use was also made of two other commercialcounters for CIEMAT/NIST measurements: A 15-year-old WallacGuardian 1414 and a 5-year-old Tricarb 2800 TR. The datawere analyzed with a PTB code (unpublished) which includesroutines from the KB code (Los Arcos and Ortiz, 1997) tocompute the ionization quenching function. PTB used a constantkB¼0.0075 cm/MeV and used its own 3H standard with a relativestandard uncertainty of 1%.

The beta spectrum is computed with routines from EFFY4which is an updated version of EFFY2 (Garcia-Toraño and GrauMalonda, 1985). Decay data were taken from Bé et al. (2004). Forthe efficiency calculation, the decay scheme was slightly simplifiedassuming only the main beta transition with Eβ,max¼1495.1(22)keV, C(W)¼p2þq2 for the unique 1st forbidden transition and100% transition probability.

In addition, the homemade TDCR system (Nähle et al., 2010)and – of course – the Hidex counter were used. The latestmodifications of the custom-built TDCR counter of PTB as well assome analysis techniques were published recently (Nähle andKossert, 2011). An extension of the analysis software developedfor the CIEMAT/NIST method is used for the TDCR method and,

consequently, exactly the same corrections (e.g., decay correction,correction for decay during measurement) and models (e.g.,ionization quenching including electron stopping powers and kBvalue, nuclear decay data) are used. A very low PMT asymmetrywas observed and taken into account using the minimizationalgorithm as described by Broda et al. (2007). The asymmetrycorrection was, however, negligible. For the Hidex counter, theadjustments of PMT thresholds and HV settings were made asdescribed in Section 2. The coincidence resolving time was set to40 ns.

4.2.1. ImpuritiesA 85Sr impurity was detected by means of gamma ray spectro-

metry at PTB. The result, A(85Sr)/A(89Sr)¼1.31(3)10�3 at referencedate (1st November 2011, 0:00 CET), was reported to ENEA beforecompletion of the analysis. ENEA also made impurity measure-ments by means of an HPGe detector and confirmed the PTBresult.

In the analysis at PTB the LS counting rates were reducedaccordingly. To this end, the counting efficiencies of 85Sr areneeded, which were determined by means of a secondary stan-dard method (Kossert, 2006), i.e. the efficiencies were derivedfrom experimental data of a pure 85Sr solution with knownactivity. At PTB, LS spectra measured in October 2011 wereanalyzed to determine a potential 90Sr/90Y impurity. An upperlimit for such an impurity was estimated to be A(90Sr)/A(89Sr)o1.5�10�3. This is, however, not proof of the existence of a 90Srimpurity in the solution. A corresponding component wasincluded in the uncertainty budget of PTB. A repetition measure-ment was carried out in March 2012 (after completion of thecomparison) which led to a reduction in the upper limit to A(90Sr)/A(89Sr)o7.2�10�5.

To check the presence of pure beta emitters as impurities in the89Sr sources supplied by PTB, a series of the repeated count ratereadings at different starting times were performed at ENEA byusing the two liquid scintillation counters. The recorded data werethen analyzed by applying the method described by Capogni et al.(2008). No significant activities of radionuclides different from 89Srwere detected.

4.2.2. EvaluationFig. 5 and Table 3 show the individual results of the activity

concentrations and the standard uncertainties as stated by theparticipants. All results are in very good agreement since theuncertainty bars overlap. The results of the two Hidex countersare very similar and slightly higher than the other results. Usingthe homemade TDCR system of PTB as a basis, the results with the

Table 1Proposed comparison scheme.

No. Action Date

1 PTB prepares two sets of LS sources of the radionuclide 89Sr Week 02 The sources will be measured at PTB using established measurement equipment for primary standardization (LS with TDCR and CIEMAT/NIST

efficiency tracing)Week 1

3 The sources will be measured in the Hidex TDCR counter at PTB. At least one measurement will be carried out with defined instrument settingsaccording to an agreed protocol

Week 1

4 One set of sources will be shipped to ENEA Week 25 The sources will be measured in the Hidex TDCR counter at ENEA. At least one measurement will be carried out with defined instrument settings

according to an agreed protocolWeek 3

6 ENEA sends sources back to PTB Week 47 Test measurements at PTB of both sets of sources to confirm that sources are stable Week 58 ENEA submits results to PTB Week 69 ENEA also submits file(s) with measured raw data to PTB which then analyzes data of both laboratories with the same software Week 610 Joint report on the comparison including recommendations for other users After week

10

Table 2Recipe used for LS sample preparation. The first sample is used to measure thebackground.

Sample no. 1 2 3 4 5 6

Scintillator Ultima Gold 15 mLbidest. water 1 0.985 mLCH3NO2 (…50%) 0 0 10 20 40 70 μLMass of active solution – Approx. 150 mg

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Hidex counters are higher by about 0.25% (PTB) and 0.32% (ENEA),respectively. This effect can be explained by a slight non-linearityfound, i.e. deviations of the counting rates for high activities. Thiswas clearly shown in the studies made by PTB within this project(see Fig. 1 and Wanke et al. (2012)). From Fig. 1 it can be seen thatthe determined counting rates are too high by about 0.2%, whenthe expected counting rates are in the range of 2500–3000 s�1, asin the case of the 89Sr measurements presented here. This holds

for both the triple counting rate T and the double counting rate D.Consequently, the TDCR¼T/D value is correct and also the countingefficiencies εT and εD should be correct as well, but the activityA¼T/εT¼D/εD is too high by about 0.2%.

The uncertainty budgets for the activity concentration deter-mined at PTB and ENEA by means of LS counting are listed inTable 4. Although the interpretation and evaluation of individualuncertainty components are different between the two labs, theoverall uncertainties for the results obtained with commercialcounters are very similar. The lowest overall relative uncertaintywas obtained by PTB using the homemade TDCR system (Nähleet al., 2010; Nähle and Kossert, 2011), which is equipped with aMAC3 coincidence module (Bouchard and Cassette, 2000). Thisinstrument is well under control, since the thresholds can beadjusted correctly and the live-timed coincidence module wascarefully tested.

For the evaluation of the laboratories’ performances, the valueEn(k¼2) is calculated according to the equation

Enðk¼ 2Þ ¼ ðxi�xjÞffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffið2uiÞ2þð2ujÞ2

q ; ð1Þ

where xi, ui are the 89Sr activity concentration and its standarduncertainty (k¼1), respectively, obtained for the solution at ENEA.xj, uj are the 89Sr activity concentration and its standard uncer-tainty (k¼1), respectively, obtained for the solution at PTB.

The results xi and xj are considered to be in agreement if theabsolute value of En(k¼2) is lower than 1. The En values of any pair

Fig. 5. Activity concentration as determined with various LS counters at PTB (whitesymbols) and ENEA (black symbols). The bars indicate standard uncertainties.

Table 3Individual results for the activity concentration, a, and its standard uncertainty forvarious counters as reported by PTB and ENEA. The ENEA final result is theunweighted mean of the two individual results determined. The final result of PTBcorresponds to the unweighted mean of PTB’s CIEMAT/NIST result (mean of twocounters) and the homemade TDCR system.

Participant and counter a in kBq/g u(a) in kBq/g

PTB, Wallac 1414 18.712 0.040PTB, Tricarb 2800 TR 18.684 0.040PTB, TDCR 18.720 0.034PTB, Hidex “METRO” 18.767 0.051PTB, final result 18.709 0.034ENEA, Tricarb 3100 TR 18.70 0.07ENEA, Hidex “METRO” 18.78 0.07ENEA, final results 18.74 0.05

Table 4Uncertainty budgets as reported by the two participants. The stated values are relative standard uncertainties in %.

Uncertainty component ENEA, Tricarb ENEA, Hidex PTB, Wallac PTB, Tricarb PTB, TDCR PTB, Hidex

Counting statistics 0.02 0.02 0.02 0.02 0.01 0.01Weighinga 0.02 0.02 0.02 0.02 0.02 0.02Dead time 0.1 0.1 0.10 0.10 0.03 0.20Background 0.03 0.04 0.03 0.03 0.03 0.03Counting time 0.01 0.01 0.01 0.01 0.01 0.01Adsorption 0.03 0.03 0.05 0.05 0.05 0.05Impurities 0.05 0.05 0.15 0.15 0.15 0.15Tracer 0.01b 0.01e 0.01 0.01 n.a. n.a.Input parameters and statistical modelc 0.2 0.2 0.05 0.05 0.05 0.05Ionization quenching 0.2 0.2 0.01 0.01 0.01 0.01Interpolation from calibration curved – – – – 0.01 0.01Decay scheme parametersc 0.1 0.1 – – – –

Half-life (T1/2¼50.57(3) d) 0.1 0.1 0.05 0.05 0.04 0.04PMT asymmetry 0.1 0.03 0.03 0.02 0.02Low level discriminator 0.1 – – – – –

Quenching indicator (tSIE, SQP(E)) 0.05 – 0.01 0.01 n.a. n.a.Combined uncertainty 0.36 0.35 0.21 0.21 0.18 0.27

a The uncertainty for weighing was provided by PTB.b Tritium standard from BNM-LNHB; relative standard uncertainty assumed to be 2% for k¼2.c PTB has combined the uncertainty components assigned to “input parameters and statistical model” and “decay scheme parameters”.d At PTB, the uncertainty due to “interpolation from calibration curve” is already combined with the uncertainty component for the “tracer”.e Here the reported uncertainty is assigned to the influence of the kB value. In the initial report it was denoted as “tracer uncertainty” and then corrected by ENEA.

Table 5En(k¼2) values of any pair of results for the 89Sr activity concentration.

PTB-Tricarb

PTB-TDCR

PTB-Hidex

PTBfinal

ENEA-Tricarb ENEA-Hidex

ENEAfinal

PTB-Wallac 0.25 �0.07 �0.42 0.03 0.08 �0.41 �0.22PTB-Tricarb �0.34 �0.64 �0.24 �0.10 �0.59 �0.44PTB-TDCR �0.39 0.11 0.13 �0.38 �0.17PTB-Hidex 0.47 0.39 �0.07 0.19PTB final 0.06 �0.45 �0.26ENEA-Tricarb �0.40 �0.23ENEA-Hidex 0.23

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of results are similarly calculated and are listed in Table 5. As theresults of Table 3 already suggest, all absolute values of En aresignificantly lower than 1, which confirms that all results are ingood agreement.

Employing the convention used in CCRI(II) comparisons (see,e.g. Ratel (2005)), the degree of equivalence of the participants, Dij,was calculated according to

Dij ¼ xi–xj ð2Þ

where – in this case – xi and xj are the final results of bothlaboratories. The expanded uncertainty of Dij is given by

Uij ¼ 2ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðuiÞ2þðujÞ2

qð3Þ

Here it is assumed that the values xi and xj are not correlated.The result of the degree of equivalence is Dij¼0.031(121) kBq(k¼2). As the previous analysis suggests, the results of bothlaboratories are in excellent agreement.

Both participants also reported the individual results for eachLS sample which had different degrees of chemical quenching. Allresults were in good agreement and no outlier was identified(Fig. 6). A very low trend can be seen in Fig. 6a for the Hidexcounter. This trend cannot be attributed to the counter non-linearity since all sample had very similar counting rates. Anefficiency variation is very important because a variation in theresults would indicate a failure of the model or incorrect inputdata, e.g. due to a bad choice of the kB value or wrong nucleardata, such as the beta shape factor function. In general, also thepresence of radioactive impurities or instrument problems

could cause variations, but these effects were already properlyaccounted for.

5. Discussion of results

The comparison shows that both Hidex counters are in verygood agreement. Since we can rule out any reasons other thanthose alluded to before for causing a possible deviation betweenthe results (which might compensate a potential deviationbetween counters), we can assume that both counters act in avery similar manner. Of course this does not guarantee that allother Hidex “METRO” counters will have the same characteristicsand, thus, a few validation measurements are recommended. Suchmeasurements should at least comprise a linearity check of theinstrument (e.g., using 241Am) as described in Section 2 and morecomprehensively by Wanke et al. (2012).

The results also hint that the activity concentrations derivedfrom the measurements with the Hidex counters are slightlyoverestimated. This is probably acceptable for most measurementsin nuclear power plants, since the standard uncertainties are stillwell below 1%. From a radionuclide metrologist's perspective thisdiscrepancy is, however, not entirely satisfactory. As mentionedabove, the discrepancy can be explained by the non-linearity ofthe Hidex counter. This leads to too high activities at high countingrates. Our recommendation is to use the Hidex counter only forsamples with counting rates of 2500 s�1 or below.

-1

-0.75

-0.5

-0.25

0

0.25

0.5

0.75

1

0.992 0.9925 0.993 0.9935 0.994εD

(ai-a

mea

n)/

a mea

n in

%89Sr, Hidex-TDCR (PTB)

-1

-0.75

-0.5

-0.25

0

0.25

0.5

0.75

1

0.225 0.25 0.275 0.3 0.325 0.35 0.375 0.4εtracer

(ai-a

mea

n)/

a mea

n in

%

89Sr, Wallac Guardian 1414 (PTB)

-1

-0.75

-0.5

-0.25

0

0.25

0.5

0.75

1

0.9945 0.995 0.9955 0.996 0.9965 0.997εD

(ai-a

mea

n)/

a mea

n in

%

89Sr, TDCR (PTB)

-1

-0.75

-0.5

-0.25

0

0.25

0.5

0.75

1

0.2 0.225 0.25 0.275 0.3 0.325 0.35 0.375 0.4 0.425εtracer

(ai-a

mea

n)/

a mea

n in

%

89Sr, TriCarb 2800 TR (PTB)

Fig. 6. Residuals (ai�amean)/amean vs. double counting efficiency, εD, for TDCR counters (left) or corresponding 3H counting efficiency, εtracer, for CIEMAT/NIST counters (right),respectively. The bars indicate the standard deviation of the mean of several repetition measurements. They are often smaller than the squares and, thus, invisible inmost cases.

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Please cite this article as: Kossert, K., et al., Bilateral comparison between PTB and ENEA to check the performance of a commercialTDCR system for activity measurements. Appl. Radiat. Isotopes (2014), http://dx.doi.org/10.1016/j.apradiso.2014.01.008i

Acknowledgements

The authors are very grateful to Stefanie Hennig from PTB andMaria Letizia Cozzella from ENEA for the preparation of the LSsamples and to Rainer Dersch from PTB and Aldo Fazio from ENEAfor their careful determination of photon-emitting impurities.Finally, we wish to thank Oana Alexandra Rusu from Babes-Bolyai University (Cluj-Napoca, Romania), who participated inthe activity measurements of pure beta radionuclides carried outat ENEA within the framework of a research stage supported bythe EURODOC project, and Andrei Antohe from Horia HulubeiNational Institute of Physics and Nuclear Engineering (IFIN-HH,Romania), who supported this work as an RMG Researcher of theMetroFission Project. This work has been carried out within thescope of the EMRP Joint Research Project ENG08 “MetroFission”.

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Please cite this article as: Kossert, K., et al., Bilateral comparison between PTB and ENEA to check the performance of a commercialTDCR system for activity measurements. Appl. Radiat. Isotopes (2014), http://dx.doi.org/10.1016/j.apradiso.2014.01.008i