Technical Report PTE2008

Transcription

1 Page 1 Distr.: Limited ENGLISH ONLY Technical Report PTE Proficiency Test Exercise (Manual 3M Split Sample Geometry) for Radionuclide Laboratories Supporting the Network of IMS Radionuclide Stations Prepared by: Emerenciana B. Duran and Barbara Nadalut Engineering & Development Section, IMS, PTS/CTBTO emerenciana.duran@ctbto.org

2 Page 2 Final Report PTE2008 Summary This report contains the results of the Proficiency Test Exercise (PTE) organized in 2008 by the IMS Engineering and Development Section for radionuclide laboratories which support the network of radionuclide stations for verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). These laboratories have an important role in corroborating the results of routine analysis of a sample from an IMS station, in particular to confirm the presence of fission products and/or activation products. PTE2008 is part of a continuing performance evaluation programme for these laboratories within the scope of their CTBT-specific activities. PTE2008 involved the analysis by each laboratory of a spiked reference sample of the split 3M manual type. Of the 16 radionuclide laboratories listed in the Treaty, only BRL04 did not participate in the exercise. The results of the 15 laboratories participating to the exercise are included in this report. The results of the exercise are summarized as follows: Metrics Acceptance Limit Number of Compliant Laboratories % of Total Participants 10 Certified laboratories Correct identification of nuclides 80% % False positives 10% % Deviation of activity reported from reference value for all the major nuclides (%D A ) Accuracy of activity reported for the major nuclides (u-test) Deviation in activity concentration reported from reference value for all the major nuclides (%D C ) <15% 9 90% <3 8 80% <15% 9 90% Accuracy of the activity concentration <3 8 80% reported for the major nuclides (u-test) Overall rating of Pass 80% 8 80% 5 Uncertified laboratories Correct identification of nuclides 80% 5 100% False positives 10% 1 20% Deviation of activity reported from reference value for all the major nuclides (%D A ) Accuracy of activity reported for the major nuclides (u-test) Deviation in activity concentration reported from reference value for all the major nuclides (%D C ) <15% 2 40% <3 3 60% <15% 2 40% Accuracy of the activity concentration <3 2 40% reported for the major nuclides (u-test) Overall rating of Pass 80% 1 20%

3 Page 3 CONTENTS 1 Introduction Materials DESCRIPTION OF MATERIALS Calibration samples Composition Preparation of Standardized Mixed Nuclide Solution Reference Samples Blank samples QUALITY ASSURANCE TESTING OF THE MATERIALS Homogeneity tests Dispatch of Samples and Reporting of Results by Participants Evaluation of Results Percent deviation in activity (% D A ) Accuracy of the activity reported (u-test) Uncertainty ratio Results and Discussion CORRECT IDENTIFICATION OF NUCLIDES Major nuclides Minor nuclides FALSE POSITIVES AND CONTAMINANTS ACTIVITY RESULTS Major nuclides % Deviation (%D A ) Accuracy of results (u-test) Uncertainty ratios Minor Nuclides % Deviation (%D A ) Accuracy of results (u-test) Uncertainty ratios Summary of Results for Minor Nuclides....Error! Bookmark not defined. 5.4 ACTIVITY CONCENTRATION Major nuclides % Deviation (%D C ) Accuracy of results (u-test) Uncertainty ratios Summary of Results for the Major Nuclides (Activity Concentration) UNCERTAINTY BUDGETS Summary and Conclusions Acknowledgements.,.40 References APPENDIX I HOMOGENEITY TESTS...43 APPENDIX II INDIVIDUAL PARTICIPANT'S RESULTS 48

4 Page 4 1. Introduction Since 2000, an annual proficiency test exercise (PTE) has been organized by the International Monitoring System (IMS) for radionuclide laboratories which support the network of radionuclide stations for verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). These laboratories support radionuclide stations by: (i) corroborating the results of routine analysis of a sample from an IMS station, in particular to confirm the presence of fission products and/or activation products, (ii) providing more accurate and precise measurements and (iii) clarifying the presence or absence of fission products and/or activation products in the case of a suspect or irregular analytical result from a particular station (CTBT/WGB/TL- 11/5/Rev. 10). The PTE is a means of ensuring that the radionuclide laboratories are capable of achieving and do achieve on an ongoing basis, the level of accuracy of nuclide identification and measurement required to reliably confirm or verify spectral data from radionuclide stations. ISO (Section 5.9) also recommends participation in proficiency tests as a laboratory s way of assuring the quality of its test and calibration results. This report contains the results of the proficiency test exercise organized in 2008 (PTE2008) by the IMS Engineering and Development Section. CTBT/PTS/INF.96/Rev.7 which is the document approved by WGB as basis in the certification of radionuclide laboratories, requires radionuclide laboratories to participate regularly in intercomparison and proficiency test exercises organized by the PTS. The results are used in monitoring laboratory performance of certified laboratories within the scope of their CTBT-specific activities. These are also used in the certification of radionuclide laboratories. Fifteen of the 16 radionuclide laboratories listed in the Treaty participated in PTE2008. BRL04 (Brazil) did not participate in PTE2008. The sample type is the MANUAL3M filter used for the manual particulate station, which is the most common among the 3 major types of systems used in the radionuclide particulate network. The other two types of particulate stations are both automatic systems, i.e. the ARAME (Automatic Radionuclide Air Monitoring Equipment) and RASA (Radionuclide Aerosol Sampler and Analyzer). As with previous exercises, the primary objective of the 2008 exercise is to assess the capability of the laboratories to identify radionuclides in a reference sample and traceably quantify the levels of these radionuclides. In addition, for each exercise, there are other specific objectives or determinations that the exercise was designed to accomplish. The types of samples used and other more specific objectives of previous exercises to date are listed in Table 1. Table 1. Sample Types and Specific Objectives of Previous PTEs PTE Sample Type Specific Objectives 2000 Compressed filter for manual stations (disk with a diameter of 70 mm and a height of 5 mm) 2001 RASA filter (polypropylene, 6 sheets, each sheet 12 x 50 cm 2 ) 2002 ARAME filter (glass fibre; sheet dimensions are 82 x 84 mm 2, 15 sheets to a sample, stacked one on top of each other. On each sheet, there is a circular exposed area with a diameter of 77 mm, area 46.6 cm 2 ) Final Report PTE2008 Determine the minimum detectable activity (MDA) for 140 Ba with a blank compressed filter Determine the MDA for a set of nuclides detected, this time in the reference samples Evaluate decision techniques employed by the laboratories to establish critical levels (L c ) for assessing detection limits 2003 Gamma spectrum generated by a Monte Demonstrate the usefulness of a spectrum-

5 Page 5 PTE Sample Type Specific Objectives Carlo simulation (VGSL*) based on an analysis of a real measurement M manual compressed disk (3M polypropylene filter sheet of dimensions cm 2 ) only exercise in assessing laboratory proficiency and quality of analytical results Main difference to previous tests with spiked samples was the lower activity levels (as low as 25 mbq) VGSL gamma spectrum Test the software for analysis of results for a VGSL gamma spectrum 2006 RASA Filter Test the software for analysis of results for a spiked sample 2007 Compressed filter for manual stations (disk with a diameter of 50 mm and height of 4 mm) 2008 Compressed filter for manual stations (disk with a diameter of 50 mm and height of 4 mm) *VGSL Virtual Gamma Spectroscopy Laboratory Test reporting time under conditions similar to when a sample is sent from a station Provide a test for nuclides which were relatively problematic in previous PTEs The National Physical Laboratory (NPL), United Kingdom was contracted by the PTS to prepare and distribute blank and reference samples in the MANUAL3M geometry to the participants and to submit a report which provides details on sample preparation and quality assurance of the sample. The calibration sample supplied to the participants has certified activity values that were traceable to UK national standards of radioactivity. This traceability to UK national standards in turn provides traceability at an international level to the ultimate reference point of all measurements, the SI reference value maintained by the Bureau International des Poids et Mesures (BIPM) (Gilligan and Harms, 2008). The radionuclides in the calibration sample were selected to enable energy and efficiency calibrations in the energy range of 46.5 to 1836 kev. The reference samples had nuclides selected from the list of fission and activation products relevant to the CTBT (CTBT/WGB/TL-2/40). Nuclide and activity selection took into consideration the following factors: (a) Even trace quantities of CTBT relevant nuclides can have verification value; thus, it might be useful for activities of some major nuclides to be made intentionally low but above detection limits in order to test for sensitivity; (b) Half-lives of the nuclides should be sufficiently long enough for decay not to present any problems within the timescale of the PTE and taking into account the low activity levels of some nuclides; (c) At least one multi-line nuclide is included in order to test for true coincidence or cascade summing correction. The results were analyzed according to criteria based in part on CTBT-PTS-INF.96-Rev.7 and previous PTEs. False positives have always been evaluated in previous PTEs, but in PTE2007 and also now in PTE2008, an acceptance limit was included. The overall rating of Pass or Not Pass introduced in PTE2007 was also maintained, in an effort to gauge overall laboratory performance and trigger preventive or corrective actions by the laboratory, where

6 Page 6 necessary. These criteria for evaluating acceptability of analysis are based on CTBT-PTS- INF.96-Rev.7 (Section 7.11) and pertain mainly to: Correct identification of radionuclides in the sample; Accuracy and precision of the results when comparing the laboratory results with the certified values. Final Report PTE2008

7 Page 7 2. Materials 2.1. DESCRIPTION OF MATERIALS PTE2008 is a test of the measurement and analytical method of participating laboratories for the MANUAL3M filter geometry. The MANUAL3M filter materials (3M 5379 BMF polypropylene filter sheet) has dimensions of approximately 57cm x 46 cm. Manual particulate stations in the IMS network use one sheet per process cycle and expose the entire filter area apart from a 1.5cm wide strip around the filter perimeter (Figure 1). After sampling, the supporting scrim is removed and the filtration media is compressed to form a cylindrical sample of diameter 50mm and height of approximately 4mm. The resulting disk is then placed in a PVC container and this sample type is referred to as MANUAL3M (Figure 2). Figure 1. MANUAL3M filter in an air sampler used in stations: each filter has one filtration sheet and one support layer. Figure 2. MANUAL3M compressed sample geometry The UK National Physical Laboratory (NPL) prepared the samples used in PTE2008 according to the terms of reference and a contract with the PTS. As part of the contract, NPL submitted a report to the PTS entitled, Provision of MANUAL3M type Filter Samples for the 2008 CTBTO Proficiency Test Exercise Involving Radionuclide Laboratories Supporting

8 Page 8 the IMS Network of Radionuclide Stations (Gilligan and Harms, 2008). This report discusses the preparation and quality assurance of the samples that were provided to the participants. Parts of the report are extracted or summarized in Sections to For PTE2008, NPL prepared two types of samples, all in MANUAL3M geometry: (a) reference and (b) blank Reference Samples Composition To prepare the reference samples, single radionuclide solutions were prepared, standardized and then combined and diluted as necessary. The dilutions were validated using a high resolution γdetector and by liquid scintillation counting. This was performed in accordance with established procedures that have been independently accredited by the United Kingdom Accreditation Service (UKAS) for the production of solution standards of radioactivity. The radionuclides included were standardized in an ionization chamber that had been calibrated by solutions that were standardized by coincidence counting techniques. Five nuclides which proved problematic in previous exercises were selected, namely: 54 Mn, 58 Co, 125 Sb, 133 Ba and 141 Ce These nuclides which were intentionally selected and added to the reference mixture were considered as major nuclides. The nuclides 59 Fe, 57 Co and 60 Co were present as contaminants in the 58 Co stock solution. The nuclide 139 Ce was present as a contaminant in the 141 Ce stock solution. All the contaminant nuclides were standardized with high resolution gamma ray spectrometry (Gilligan and Harms, 2008). These contaminants are considered as minor nuclides and although evaluated, are not considered in the over-all rating Preparation The filter material (3M 5379 BMF) was supplied by the CTBTO as sheets with approximate dimensions of cm 2. The reference sample was prepared by the drop wise addition of a known quantity of a standardized mixed nuclide solution ( 54 Mn, 58 Co, 125 Sb, 133 Ba and 141 Ce) onto a polypropylene filter sheet, using plastic pycnometer. The mass in grams dispensed to the filter was determined by difference weighing of the plastic pycnometers using a 6 decimal-place balance. The activity was dispensed into an area with a length of 54 cm and a width of 43 cm using a regular distribution pattern of approximately 2 cm between each drop. Subsequently, the filter was air dried, folded and compressed to produce a disk with a diameter of 50 mm and a thickness of approximately 5 mm. The disk was then packaged and sealed (using high flexibility cyanoacrylate glue) in a PVC container. The standard uncertainty associated with the filter sample preparation (i.e. dispensing the solution and drying, folding and compressing the filter) was estimated to be 0.5% (Gilligan and Harms, 2008). Table 3 shows the specific activities (reference time of November 1, 2008 at 1200 UTC) of the 9 radionuclides in the mixed solution which was used to spike the reference samples. The Final Report PTE2008

9 Page 9 reported uncertainties were based on standard uncertainties multiplied by a coverage factor of k=2 (approximately 95% confidence level). Table 3. Mixed Solution Used to Spike the Reference Samples (nuclides in bold letters were major nuclides of interest). Nuclide Activity Concentration (mbq g -1 ) 54 Mn 55.98(16) 57 Co 4.00(26) 58 Co 532.6(36) 59 Fe 0.282(50) 60 Co 1.82(8) 125 Sb 276.6(14) 133 Ba 54.62(38) 139 Ce 1.974(28) 141 Ce 532(6) Blank Samples NPL produced a total of 18 compressed blank filters. Each of the participating radionuclide laboratories was provided with one blank sample. The samples were given a unique number (B1 to B18) corresponding to the order of preparation. The filters were compressed using a brand new compression die and were prepared in an inactive laboratory to minimize the possibility of contamination. One of the blank filter samples (B1) was measured on a low background, high efficiency gamma spectrometer known at NPL as Arthur. This detector is a high purity germanium n- type crystal with a relative efficiency of 108 % at 1332 kev. The filter was measured for one week and no radionuclides were detected. Minimum detectable activities will of course vary with gamma energy and gamma emission probability. The contaminant nuclide 60 Co (with an activity below 4 mbq) was detected in reference filter samples using the contamination check measurements.

10 Page QUALITY ASSURANCE TESTING OF THE MATERIALS Homogeneity Tests The variation in the distribution of the activity in the samples during preparation can affect the accuracy and precision of measurements for some filters and geometries. The MANUAL3M sample is provided as a compressed sample that can be counted directly; this means that uncertainties owing to geometry and sample homogeneity in this sample are not so relevant. The samples were measured on NPL s gamma spectrometer known as Galahad. This detector has a high purity germanium n-type crystal with a relative efficiency of 70 % at 1332 kev. A jig was used to ensure that the samples were positioned in a reproducible manner in the center of the end cap. The details of the homogeneity tests are provided in Appendix 1. For the low activity measurements in the reference sample, it was only possible to observe any effects (due to inhomogeneity) for two nuclides ( 58 Co and 125 Sb). A 1% uncertainty component was added in quadrature to the uncertainty of the certified activity values to take into account source inhomogeneity. The measured effects of the sample inhomogeneity were estimated by NPL to be lower than 2%. Final Report PTE2008

11 Page Dispatch of Samples and Reporting of Results by Participants Like PTE2007, there were no short-lived nuclides in the reference sample. The shortest lived nuclide in the reference samples was 141 Ce (T 1/2 = days). The interval between dispatch and sample receipt ranged from 1 to 10 days with 15 out of 16 laboratories receiving their sample in less than a week. BRL04 received its sample after 10 days. The participants were asked to analyze all the radionuclides present in the reference sample by gamma spectrometry. Like in PTE2007, this PTE actually simulated the same conditions as when a sample is sent from a station to be analyzed. There is no specified activity reporting date and time. For a typical station sample, the laboratory uses the Radionuclide Laboratory Report (RLR) for reporting results. The activity reported is in Bq, decay corrected to the start date and time of acquisition while the activity concentration is in Bq/m 3, decay corrected to the sampling period, taking into account decay during spectral acquisition, sampling and the period between sampling and acquisition (IDC Rev. 6). In the analysis of results, PTS decay corrected the activities reported by the participants to the reference time provided by NPL which was 1200 UTC on 1 November The start of collection of the sample was given to participants as 1200 UTC on 30 October 2008 (sample collection stop is 1200 UTC on 31 October 2008). Like in PTE2007, only simple decay correction for activity concentration was needed, since there were no parent-daughter pairs in the radionuclide mix. The participants were asked to use a sample volume of 12,000 m 3 with zero uncertainty and to assume constant concentration and deposition during sampling. Certified laboratories were required to use the IDC3.4.1 Rev.6 format for all messages and reporting. A minirlr template was provided for uncertified laboratory RLR messages as per previous PTEs. Uncertified laboratories were also asked to send measurements of blank, calibration and reference samples in IMS2.0 format if feasible, otherwise in Canberra Genie2k (.cnf), Ortec GammaVision (.spc) or Ortec InterWinner (.spe) format.

12 Page Evaluation of Results Laboratory results were evaluated based on criteria used in previous PTEs and at least in part, on CTBT-PTS-INF.96-Rev.7. After considering historical criteria for statistical tests used in the evaluation of results, acceptance limits have been set in PTE2007 (Table 4) as a means of evaluating performance and subsequently triggering internal laboratory corrective action procedures and discussion of results with the PTS on an individual basis. In PTE2007, an acceptance limit of 10% for false positives which was suggested in PTE2003 has been adopted. During the Radionuclide Laboratories Workshop in 2004, it was emphasized that correct identification of radionuclides is the basis for the credibility of the work of the laboratories in support of Treaty verification. It was also concluded at the Workshop that without realistic uncertainty, a measurement result is of little value (Workshop Report, 2004). Thus, in PTE2008 (as with PTE2003, PTE2004 and PTE2007), uncertainty ratios were also included in interpreting further the u-test results. In addition, the over-all rating of Pass or Not Pass introduced in PTE2007, based on the ratio of correctly identified nuclides with acceptable u-test and deviation reported by a laboratory to the total number of major nuclides in the reference sample, was also adopted in PTE2008. This ratio should be 80% for an over-all rating of Pass (Table 4). As there are only 5 major nuclides in this exercise, the criterion adopted was that a participant should have no more than 1 false positive in order to be eligible for a Pass mark. This actually represents 20% of the total number of major nuclides. Table 4. Acceptance Criteria for Results Parameters Acceptance Limit APPLIED IN PTE2008: Correct identification of major nuclides 80% Deviation of activity reported from reference value (% D A ) < 15% Accuracy of activity reported (u-test) < 3 Deviation in activity concentration reported from reference value (%D C ) < 15% Accuracy of the activity concentration reported (u-test) < 3 False positives 1 Over all rating Pass 80% In the evaluation of results, all the decimal places or significant figures, as reported either by NPL or the laboratory, are used. NPL generally gives the reference values to 4 significant figures. Participants report their results using 4 to 7 decimal places. However, the data presented in the tables in this report are rounded-off to two decimal places. The statistical tests or parameters used in the evaluation of results are described below. Final Report PTE2008

13 Page ACTIVITY RESULTS Percent Deviation in Activity (% D A ) Deviation in the activity reported by the laboratory from the reference value was calculated according to equation (1) Alab Aref % A 100 A D (1) ref where: D A = deviation A lab = activity reported by the participant A ref = activity reported by NPL Accuracy of the Activity Reported (u-test) The accuracy of the activity reported by the laboratory was determined by applying the u-test, the result of which is expressed by a number u (also referred to as zeta): Alab Aref u (2) u u 2 lab 2 ref where: A lab = activity reported by the participant A ref = activity reported by NPL u lab = combined standard uncertainty reported by the participant = combined standard uncertainty reported by NPL u ref Laboratories provided results with uncertainties expressed as either combined standard uncertainties with a coverage factor of k = 1, or expanded uncertainties with k = 2. The u-test used uncertainties with k = 1, and thus some laboratory uncertainties were adjusted to obtain consistent coverage factors assuming: U u c (3) k where: u c = standard uncertainty U = expanded uncertainty k = coverage factor

14 Page Uncertainty Ratio and the Interquartile Range Method (IQR) Further to the u-test, relative uncertainty ratios were also calculated as a measure of precision. The relative uncertainty ratios was calculated using the participant s and NPL (as reference) u nuclide activity uncertainties expressed as percentages u Outliers in the uncertainty ratios were determined using the interquartile range (IQR) method. The IQR is a measure of statistical dispersion and is equal to the difference between the third and first quartiles. A value was considered as an outlier if it was outside the range of Q 1 k (Q 3 -Q 1 ) to Q 3 + k (Q 3 -Q 1 ) where: Q 1 = lower quartile Q 3 = upper quartile k = 3 A value of 3 was selected for k, which means that the result has a large deviation from the median (roughly around 4σ) to be considered an outlier. A smaller value of k would have made the test more stringent. lab ref % % ACTIVITY CONCENTRATION RESULTS Evaluation of activity concentration results were performed using identical methods and acceptance criteria as those listed above for activity results. 5. Results and Discussion 5.1. CORRECT IDENTIFICATION OF NUCLIDES Major nuclides Nine nuclides were present in the reference sample (Table 5). However, four of these are contaminants of single nuclide sources. Only the five nuclides which were intentionally added to the sample were considered as major nuclides, namely: 54 Mn, 58 Co, 125 Sb, 133 Ba and 141 Ce. Participants must report these nuclides with acceptable accuracy and precision. Fourteen of the 15 participating laboratories correctly identified all the major nuclides (Table 5). The only major nuclide that was not identified by one laboratory was 141 Ce. Final Report PTE2008

15 Page 15 Table 5. Nuclides Present in the Reference Samples (Nuclides in Bold Letters are Major Nuclides). Nuclides Major Nuclide? Number of Laboratories Reported % of Laboratories Reported 54 Mn Yes 58 Co Yes 125 Sb Yes 133 Ba Yes 141 Ce Yes 57 Co No 59 Fe 1 7 No 60 Co 6 40 No 139 Ce 9 60 No Minor nuclides (impurities) The nuclides 59 Fe, 57 Co and 60 Co were present as contaminants in the 58 Co stock solution. As with previous PTEs, the results of these nuclides were also evaluated, but not included in the overall rating. Among the minor nuclides, 57 Co and 139 Ce were identified by majority of participants (Table 5). 59 Fe and 60 Co were of low activity with large uncertainties and were below the MDAs of most, if not all, participants. Only one participant identified 59 Fe and six identified 60 Co. Table 6 shows how many of the minor nuclides present in the sample were identified by each participant. Table 6. Percentage of Minor Nuclides Identified by Laboratories Laboratory Number of Minor Nuclides % of Minor Nuclides Reported Reported ARL AUL ATL CAL FIL FRL JPL NZL GBL USL CNL ILL ITL RUL ZAL FALSE POSITIVES AND CONTAMINANTS In the preliminary analysis of the PTE2008 results, there were several instances of false positives consisting of many nuclides that were reported by the participants (Table 7) and not by NPL. False positives involving CTBT relevant fission products could lead to wrong

16 Page 16 categorization of samples. This may also be indicative of potential problems in operational performance such as cross-contamination of samples in a laboratory or misidentification of peaks. Careful analysis of the false positives was made in collaboration with NPL and with feedback from the laboratories, with the following results: The reported 65 Zn was probably a misidentification of the gamma emission of 65 Cu(n,n) reaction ( kev); 56 Co may be a misidentification of 214 Bi background line (1238 kev); 210 Pb and 40 K may be the result of erroneous background subtraction; 137 Cs was an erroneous peak identification; one of the two laboratories which identified 131 I admitted a possible sample contamination at the laboratory; 233 Pa was present in one sample and was detected by the involved laboratory; 249 Cf was present as a contaminant in some samples and was reported by two laboratories; 241 Am was reported by 6 laboratories and identified later by one more laboratory in a reanalysis of PTE2008 spectrum. Six out of 7 activity results provided by the laboratories for 241 Am were below NPL s declared MDA (approximately 20 mbq). After receiving the feedback to preliminary results from the laboratories and from NPL, PTS decided to re-classify 233 Pa, 241 Am and 249 Cf as present in some of the samples and categorized them as possible nuclides. Table 7. Other Nuclides Identified by the Participants in the Reference Sample (Nuclides in Bold Letters may be present in some samples and were categorized as possible) Nuclide No. of Laboratories that Identified the Nuclide 7 Be 2 40 K 2 65 Zn 1 56 Co 1 84 Rb 1 88 Y 1 99m Te 1 99 Mo 1 110m Ag Sn Sb I Cs Pb Pa Am Cf 2 Final Report PTE2008

17 Page 17 Table 8. False Positives and contaminants Laboratory False Positives Contaminants/Possible Nuclides Number of False Positives ARL AUL02 65 Zn 1 ATL Am 0 CAL Sn - 1 FIL Am 0 FRL Am 0 JPL Am, 249 Cf 0 NZL Cs 233 Pa 1 GBL Am 0 USL CNL06 7 Be, 56 Co, 84 Rb, 88 Y, 110m Ag, 127 Sb, 131 I - 7 ILL ITL10 7 Be, 99 Mo, 99m Te, 137 Cs 241 Am 4 RUL13 7 Be, 40 K, 137 Cs, 210 Pb 249 Cf 4 ZAL14 40 K, 131 I - 2

18 Page ACTIVITY RESULTS Major Nuclides Percentage Deviation (%D A ) The percentage deviation of reported activity from the reference value (see Section 4.2.1) for the major nuclides is shown in Table 9. Nine of the 10 certified laboratories and 2 out of 5 uncertified laboratories met the acceptance criterion of <15% deviation from the reference value for at least 80% of major nuclides. Among the major nuclides, 54 Mn and 141 Ce were the nuclides with the highest failure rate, i.e., with four participants overestimating the activity by >15% (Figure 3). All participants were able to meet the %D A criterion for 125 Sb. As can be seen from Figure 3, most of the results of the participants are within ± 10% of their respective reference values. Table 9 shows that some of the results tend to be overestimates (e.g. AUL02 and CAL05) while others tend to be underestimates (e.g. USL16). The largest spread was observed for 133 Ba, most likely due to problems with coincidence summing correction. AUL02 has since implemented corrective actions, i.e., by performing a new calibration using a new source and reviewing its method for coincidence summing corrections. Table 9. Percentage Deviation of Reported Activity from the Reference Value for each Major Nuclide (Values in Red Exceed Acceptance Limit) Laboratory Mn-54 Co-58 Sb-125 Ba-133 Ce-141 ARL AUL ATL CAL FIL FRL JPL NZL GBL USL CNL ILL ITL RUL ZAL Mean 4.69 ± ± ± ± ± Mean excluding AUL ± ± ± ± ± Final Report PTE2008

19 Page 19 Figure 3. Percentage Deviation of activity results from respective reference values for each major nuclide. Figure 4.a shows a plot of the mean %D A for each major nuclide, based on %D A results from all laboratories. The mean %D A is close to zero for 58 Co, 125 Sb and 133 Ba. The dispersion of values for 133 Ba is large, because of the result from AUL Ce values tend to be overestimates. Figure 4.a. Mean %D A of activity results among participants for the major nuclides.

20 Page 20 A reanalysis of the data excluding AUL02 results led to a decrease in the dispersion of values and different mean values; the mean %D A becomes negative for 125 Sb and 133 Ba, very close to zero for 58 Co and positive for 54 Mn and 141 Ce (Table 9). Figure 4.b. Mean %D A of activity results among participants for the major nuclides excluding AUL02 values. Final Report PTE2008

21 Page Accuracy of Results (u-test) The accuracy of the activity reported was determined using the u-test discussed in Section A result is considered acceptable if its deviation from the reference value is less than three times the combined uncertainty of the two values. The results are shown in Figure 5. Ten laboratories met the acceptance criterion of u-test < 3 for all major nuclides. The overall result for each nuclide is good, with most u-test values clustered below A u-test value of 1.64 indicates that the participant and NPL s values do not differ significantly (α=0.1). Four labs failed the u-test for 141 Ce, however. Figure 5. u-test results of each participant for each major nuclide (activity).

22 Page Uncertainty Ratios The acceptance criterion of a u-test result was further refined by factoring in relative uncertainty ratios (Table 10). It has been argued that applying relatively large uncertainties can make the u-test less stringent. As with previous PTEs, no upper limit for the relative uncertainty ratio was defined as this is nuclide-specific. Instead, outlier tests were used for rejecting extremely large uncertainties. To test whether the uncertainties applied by the participants are realistic or not, a simple approach based on IQR was applied (Section 4.3). The purpose of this test is to determine whether the relative uncertainty ratio is significantly different from the other uncertainty ratios in the data set. The uncertainty ratio was <10 in 82% of the data. JPL11 had the lowest uncertainty ratios while USL16 and GBL15 had comparatively higher ratios. The mean relative uncertainty ratio was lowest for 141 Ce and highest for 133 Ba. Table 10. Relative Uncertainty Ratios of the Activity Results for the Major Nuclides Laboratory Mn-54 Co-58 Sb-125 Ba-133 Ce-141 ARL AUL ATL CAL FIL FRL JPL NZL GBL USL CNL ILL ITL RUL ZAL Mean 7.53 ± ± ± ± ± 1.87 A plot of the u-test result versus the relative uncertainty ratio is shown in Figures 6.1 to 6.5, sorted by nuclide. In general, the values are clustered at the low end of the u-test value axis and somewhat spread out along the uncertainty ratio axis, indicating good accuracy and reasonable precision. There was only one outlier, which was for 125 Sb. If this value is excluded, then the mean uncertainty ratio for 125 Sb in Table 10 is 6.84 ± 2.93 instead of 7.58 ± 4.05.

23 Figure 6.1. u-test value versus uncertainty ratio for 54 Mn activity results. Page 23

24 Page 24 Figure 6.2. u-test value versus uncertainty ratio for 58 Co activity results. Figure 6.3. u-test value versus uncertainty ratio for 125 Sb activity results. Final Report PTE2008

25 Page 25 Figure 6.4. u-test value versus uncertainty ratio for 133 Ba activity results. Figure 6.5. u-test value versus uncertainty ratio for 141 Ce (activity).

26 Page Summary of Results for Major Nuclides (Activity) The criterion for evaluating the accuracy of the results was then further refined as follows: a u-test value of < 3 and an uncertainty ratio not significantly higher than the other uncertainty ratios is taken to mean that the result is in agreement. A u-test value of < 3 and an uncertainty ratio significantly higher than the other uncertainty ratios in the data set is taken to mean that the result is probably in agreement. If the u-test value exceeds the value of 3, then the result is taken as discrepant from the reference value, regardless of the value of the uncertainty ratio. A summary of the results for the major nuclides is shown in Table 11. The results of the u-test were not changed except for 125 Sb with one result probably in agreement because of the relatively large uncertainty. Table 11. Summary of Evaluation of Activity Results for Major Nuclides Number of Not Nuclides Reported Number of Results Pass u-test Number of Results Agree Number of Results Probably Agree Number of Results Discrepant Number of Results Pass Deviation Test Mn Co Sb Ba Ce Minor Nuclides Percentage Deviation Table 12 shows the %D A for the minor nuclides. Minor nuclides mean that these are impurities or contaminants present in the reference sample. The reference values given by NPL for these minor nuclides average was 0.141(25) mbq for 59 Fe, 2.00(13) mbq for 57 Co, 0.91(4) mbq for 60 Co and 0.987(14) mbq for 139 Ce. These levels are very close to or below the MDAs of most participating laboratories for these nuclides. Fourteen participants identified 57 Co; the mean %D A for 57 Co was ± Nine participants identified 139 Ce, providing a mean %D A of ± Six participants were able to identify 60 Co, but all of them provided %D A values exceeding 15%. Only one participant was able to identify 59 Fe but the %D A was far exceeding 15%. Thus, the results for 59 Fe and 60 Co were not included and only the results of 57 Co and 139 Ce were evaluated. Between these two nuclides and at these low activities, 57 Co results were better, with 11 out of 14 results passing the %D A test (Table 12). Five out of nine 139 Ce results exceeded 15%D A. The %D A of participant s activity results from respective reference values for these two minor nuclides is shown in Figure 7. The results on the minor nuclides were not included in the overall rating. Final Report PTE2008

27 Page 27 Table 12. Percentage Deviation of Reported Activity from the Reference Value for some Minor Nuclides. The results for 59 Fe and 60 Co were not included Laboratory Co-57 Ce-139 ARL AUL ATL CAL FIL FRL JPL NZL GBL USL CNL ILL ITL RUL ZAL Mean ± ± 35.13

28 Page 28 Figure 7. %D A of participant s activity results from respective reference values for 57 Co and 139 Ce. In Figure 8 the results of mean percentage deviation and its standard deviation (1 ) are reported for the two minor nuclides 57 Co and 139 Ce. Figure 8. Mean %D A of activity results among participants for 57 Co and 139 Ce. Final Report PTE2008

29 Page Accuracy of Results (u-test) Results of the u-test for the minor nuclides are shown in Figure 9. Best results were obtained for 57 Co, with 13 out of 14 labs meeting the u-test acceptance limit. The 9 labs which reported 139 Ce also provided accurate results (u-test <3) for this nuclide. Figure 9. u-test results for the minor nuclides, 57 Co and 139 Ce Uncertainty Ratios The relative uncertainty ratios for the two minor nuclides are shown in Table 13. Higher average ratio was observed for 139 Ce (17.23 ± 9.75). The IQR method was applied to determine whether a participant s relative uncertainty ratio is significantly different from the other uncertainty ratios in the data set. No outliers in the results of the minor nuclides were observed with this method. Table 13. Relative Uncertainty Ratios (in %) of the Activity Results for Minor Nuclides Laboratory Co-57 Ce-139 ARL AUL ATL CAL

30 Page 30 FIL FRL JPL NZL GBL USL CNL ILL ITL RUL ZAL Mean 2.18 ± ± 9.75 The plots of the u-test result versus the relative uncertainty ratio are shown in Figure 10.a for 57 Co and in Figure 10.b for 139 Ce. Better results were obtained with 57 Co than 139 Ce. Figure 10.a u-test value versus uncertainty ratio for the minor nuclide 57 Co activity results. Final Report PTE2008

31 Page 31 Figure 10.b u-test value versus uncertainty ratio for the minor nuclide 139 Ce activity results Summary of Results for Minor Nuclides There were four impurities present in the reference sample which were quantified by NPL and regarded in this report as minor nuclides. Only the results of two nuclides were evaluated since 59 Fe was identified by only one participant with an uncertainty too large for any meaningful result while only 6 participants were able to identify 60 Co. A summary of the results for these two nuclides is shown in Table 14. In general, good results were obtained, even though the activities of these two nuclides were very low. Table 14. Summary of Evaluation of Activity Results for Minor Nuclides Nuclides Number of Reported Number of Results Agree Number of Results Pass u- Test Number of Results Probably Agree Number of Results Discrepant Number of Not Reported Number of Results Pass Deviation Test Co Ce

32 Page ACTIVITY CONCENTRATION PTE2008 simulated the same conditions as for a routine station sample. The minimum requirement for airflow rate for particulate stations is 500 m 3 /h which yields a total of 12,000 m 3 during a 24 hour collection period. Participants were asked to use a sample volume of 12,000 m 3. For simplicity, participants were also asked to assume a sampling volume uncertainty of zero and constant deposition and concentration during sampling.. Calculation of activity concentration is thus straightforward, requiring division of activity by volume and simple decay correction of activity results to the date and time of collection. Thus, evaluation of the results for activity concentration based on criteria described in Section 4 is expected to yield similar results as those for activity Major Nuclides % Deviation (%D C ) As shown in Table 15, similar results were obtained for activity concentration as for activity. 141 Ce and 58 Co are the two shortest lived among the five major nuclides. The method of decay correction and detection sensitivity for short-lived nuclides, which could affect the accuracy of measurements in consideration of the low activities present in the samples, was performed with good accuracy by almost all laboratories. Note that for Level 5 samples from stations, there can also be considerable delay between the measurement of the sample and its verification by the laboratory, which could impact also the accuracy of measurements of short-lived nuclides. The results for activity concentration are similar to those for activity for the majority of laboratories. In the case of RUL13, the results for activity concentration show negative bias, which was not the case for activity results. This might be due to a misunderstanding of the reference date and time used by the laboratory for decay correction. Table 15. Percentage Deviation of Reported Activity Concentration from the Reference Value for Each Major Nuclide (Values in Red Exceed Acceptance Criterion) Laboratory Mn-54 Co-58 Sb-125 Ba-133 Ce-141 ARL AUL ATL CAL FIL FRL JPL NZL GBL USL CNL ILL ITL RUL ZAL Mean 0.81 ± ± ± ± ± Final Report PTE2008

33 Page 33 Table 15 and Figure 11 show the %D c results of the individual laboratories. AUL02 showed positive bias in their results. Figure 11. %D C of activity concentration results of participants from respective reference values for each major nuclide Accuracy of the Results (u-test) The accuracy of activity concentration results was determined using the u-test. The results are similar to activity, with the exception of the results for RUL13 laboratory. The u-test acceptance limit for minor nuclides activity results and activity concentration results was met by all participants. (Figure 12).

34 Page 34 Figure 12. u-test results of each participant for each major nuclide (activity concentration) Summary of Results for the Major Nuclides (Activity Concentration) Evaluation of the activity concentration measurements according to established criteria led to similar results as those for activity, with the exception of RUL13. It was observed, however, that there was greater spread among participants results. A summary of the evaluation of the results for these radionuclides is shown in Table 16. Table 16. Summary of Evaluation of Activity Concentration Results for Major Nuclides. Nuclides Number of Results Pass u-test Number of Results Agree Number of Results Probably Number of Results Discrepant Number of Not Reported Number of Results Pass Deviation Test Agree Mn Co Sb Ba Ce Final Report PTE2008

35 Page Minor nuclides Evaluation of the concentration results for minor nuclides yielded results that were similar to those for activity Summary of Results for the Minor Nuclides (Activity Concentration) As for activity results, only the results of 57 Co and 139 Ce were evaluated, since the other nuclides were either not identified by all participants or the uncertainties were too large for any meaningful result. A summary of the activity concentration results for the minor nuclides is shown in Table Fe was not included, as it was reported by only one laboratory. 59 Fe is the shortest-lived among the minor nuclides. Table 17. Summary of activity concentration results for minor nuclides. Nuclides Number of Reported Number of Results Pass u- Test Number of Results Agree Number of Results Probably Agree Number of Results Discrepant Number of Not Reported Number of Results Pass Deviation Test Co Ce UNCERTAINTY BUDGETS CTBT/PTS/INF.96/Rev. 7 considers the following as the minimum sources of uncertainty: Net count rate, including the uncertainties of both spectrum and detector background Radioactive decay of the nuclide. This may become significant if the decay time is large with respect to the half-life or if there is a large uncertainty in the nuclide half-life. Detection efficiency; calibration source. Gamma ray emission probability. Correction for cascade summing (if necessary). Assessment of the relative contributions of the above uncertainties is difficult to gauge without in-depth analysis of the participant reports, and may only be possible from those submitting standard RLRs unless spectra are also analysed. In selecting the nuclides for PTE2008, multi-line nuclides were included to test for true coincidence or cascade summing correction. This phenomenon can lead to significant signal loss when measuring in high efficiency geometries and short source-to-detector distances and

36 Page 36 hence, underestimation of activity. This problem may have affected some of the 125 Sb and 133 Ba results (Tables 9 and 15). To test whether the uncertainties applied by the participants are realistic or not, a simple approach based on IQR was applied (Section 4.3). Interpretation of the results of the u-test were further refined according to the results of this test. Only one outlier was observed with the IQR method, indicating the uncertainties applied are reasonably acceptable REPORTING TIME A specific objective of this PTE is to evaluate timeliness of reporting. CTBT/PTS/INF.96/Rev.7 specifies that the radionuclide laboratory shall have procedures to meet the specific requirements of the PTS for content of reports and reporting time. Further, that in the ongoing evaluation of performance of certified laboratories, particular attention will be given to timeliness in the conduct and reporting of analyses, adherence to IMS message formats and continued observance of certified procedures. Table 18 shows the date of sample receipt and submission of the final RLR (certified laboratories) or minirlr (uncertified laboratories). Reporting time in days is the difference between the date and time of sample receipt based on the SAMACK and the date and time of submission of the final RLR or minirlr. NPL original dispatch date of end of October was delayed due to homogeneity and contamination tests which they performed on all samples. In view of this delay and participation by many labs at the workshop, reporting date deadline had been extended to 15 December Ten laboratories met the reporting time deadline of 15 December Reporting time ranged from about 19 days to 78 days. Table 18. Reporting time of final results by laboratories Laboratory Sample Receipt Final RLR Reporting Time (days) ARL01 17/11/ /02/ AUL02 17/11/ /12/ ATL03 18/11/ /12/ BRL04 21/11/ CAL05 19/11/ /12/ FIL07 17/11/ /12/ FRL08 21/11/ /12/ JPL11 14/11/ /12/ NZL12 17/11/ /12/ GBL15 17/11/ /12/ USL16 17/11/ /12/ CNL06 15/11/ /12/ ILL09 16/11/ /12/ ITL10 17/11/ /12/ RUL13 14/11/ /01/ ZAL14 13/11/ /12/ Final Report PTE2008

37 Page 37 Summary and Conclusions A summary of the PTE2008 results for activity measurements according to participants is shown in Table 19. All participants identified correctly all the major nuclides, except for one participant which missed to report 141 Ce which is one of the major nuclides. Eleven out of fifteen participants passed the u-test (u < 3) for activity results for all major nuclides. When the uncertainty ratio was taken into account, one result was determined to probably agree with its reference value. The acceptance criterion of 10% for the number of major nuclides for false positives introduced in PTE2007 was also applied in PTE2008, rounding the number obtained to the upper integer. Four participants reported false positives consisting of a total of two or more nuclides. The Overall rating of Pass or Not Pass introduced in PTE2007 was also applied in PTE2008 to gauge overall laboratory performance and as a trigger for corrective action. To obtain a Pass rating, a participant must correctly identify at least 80% of major nuclides with good accuracy (u-test and %deviation results are within acceptance limits) and have not more than 1 false positive, if any. Nine out of fifteen participants obtained a Pass overall rating. Three participants had a Not Pass rating even though they passed the u- test and deviation test for all major nuclides (both activity and activity concentration) because they reported two or more false positives. AUL02 and JPL11 failed the u-test, based on combined standard uncertainty (k=1). JPL11 had the lowest uncertainty ratios. Ten out of fifteen participants met the reporting time deadline of 15 December Lack of homogeneity was not a problem for the Manual 3M compressed cylinder geometry. However, six participants measured 241 Am in their reference samples, providing in five out of six measurements results which were below the MDA valued stated by NPL. This was taken into account in the evaluation of results.

38 Page 38 Table 19. Summary of Activity Results for Major Nuclides According to Participants Laboratory Number of Major Nuclides Reported PTE Rating: Correct Identificat ion Number of Major Nuclides Pass u- Test PTE Rating: Accuracy based on u-test Number of Major Nuclides Agree with Certified value Number of Major Nuclides Probably Agree Number of Major Nuclides Discrepant Number of False Nuclides Reported PTE Rating: False Positives Reported Number of Major Nuclides Pass Deviation Test PTE Overall Rating ARL01 5 Pass 5 Pass Pass 4 Pass AUL02 5 Pass 2 Fail Pass 2 Fail ATL03 5 Pass 5 Pass Pass 5 Pass CAL05 5 Pass 5 Pass Pass 5 Pass CNL06 5 Pass 5 Pass Fail 5 Fail FIL07 5 Pass 5 Pass Pass 5 Pass FRL08 5 Pass 5 Pass Pass 5 Pass ILL09 5 Pass 5 Pass Pass 5 Pass ITL10 5 Pass 3 Fail Fail 3 Fail JPL11 5 Pass 3 Fail Pass 5 Fail NZL12 5 Pass 5 Pass Pass 5 Pass RUL13 5 Pass 4 Pass Fail 2 Fail ZAL14 4 Pass 1 Fail Fail 2 Fail GBL15 5 Pass 5 Pass Pass 5 Pass USL16 5 Pass 5 Pass Pass 5 Pass Total : Total %: Final Report PTE2008

39 Page 39 Table 20. Summary of Activity Concentration Results According to Participants Laboratory # Major Nuclides Reported # Major Nuclides Pass u-test # Major Nuclides Agree PTE Rating: Major Nuclides Identified PTE Rating: Major Nuclides u-test # Major Nuclides Probably Agree # Major Nuclides Discrepant # False Nuclides Reported PTE Rating: False Positives Reported Number of Major Nuclides#Pass Deviation Test PTE Overall Rating ARL01 5 Pass 5 Pass Pass 4 Pass AUL02 5 Pass 2 Fail Pass 2 Fail ATL03 5 Pass 5 Pass Pass 5 Pass CAL05 5 Pass 5 Pass Pass 5 Pass CNL06 5 Pass 5 Pass Fail 5 Fail FIL07 5 Pass 5 Pass Pass 5 Pass FRL08 5 Pass 5 Pass Pass 5 Pass ILL09 5 Pass 5 Pass Pass 5 Pass ITL10 5 Pass 3 Fail Fail 3 Fail JPL11 5 Pass 3 Fail Pass 5 Fail NZL12 5 Pass 5 Pass Pass 5 Pass RUL13 5 Pass 0 Fail Fail 0 Fail ZAL14 4 Pass 1 Fail Fail 2 Fail GBL15 5 Pass 5 Pass Pass 5 Pass USL16 5 Pass 5 Pass Pass 5 Pass Total : Total %:

40 Page 40 The overall performance of laboratories in PTE2008 based on acceptance limits which were set for several established metrics is summarized in Table 21. The individual participant results are presented in Appendix II. Fifteen of the sixteen laboratories listed in the Treaty participated in this exercise. Table 21. Overall Performance of Laboratories Based on Acceptance Criteria Metrics Acceptance Limit Number of Compliant Laboratories % of Total Participants Correct identification of nuclides 80% % False positives % Deviation of activity reported from reference value for all major nuclides (% D A ) < 15% 11 73% Accuracy of activity reported for all major nuclides (u-test) < % Deviation in activity concentration reported from reference value for all major nuclides (%D C ) Accuracy of the activity concentration reported for all major nuclides (u-test) < 15% 11 73% < % Overall rating of Pass 80% 9 60% Over-all rating of Pass among certified labs 8 80% Over-all rating of Pass among non-certified labs 1 20% Final Report PTE2008

41 Page 41 Acknowledgements The PTS acknowledges the contribution of the National Physical Laboratory of the United Kingdom as its contractor for sample preparation and distribution among participants. Parts of its report that was submitted to the PTS were used in this report. The PTS gratefully acknowledges the radionuclide laboratories which participated in PTE2008.

42 Page 42 References CTBT-PTS-INF.96-Rev.7, Certification of radionuclide laboratories. Gilligan, C. and Harms, A., Provision of MANUAL3M type Filter Samples for the 2007 CTBTO Proficiency Test Exercise Involving Radionuclide Laboratories Supporting the IMS Network of Radionuclide Stations. ((NPL Report to the PTS dated January 2008). Gilligan, C., Harms, A., Collins, S. and Pearce, A., RASA filter proficiency test exercise (NPL, Report to the PTS dated January 2007) Harms, A., Gilligan, C., Chari, K., Collins, S., Jerome, S., Johansson, L., MacMahon, D., Pearce, A. and Watkins, N., Proficiency test exercise for radionuclide laboratories supporting the network of IMS radionuclide stations (PTE 2004). (NPL Report to the PTS dated June 2005). IDC Revision 6, Formats and protocols for messages (IDC/PTS Documentation). Karhu, P., McWilliams, E., Werzi, R., De Geer, L. and Plenteda, R. Proficiency test for radionuclide laboratories supporting the network of IMS radionuclide stations (PTE 2003), Analysis of a VGSL-simulated gamma spectrum based on a real measurement of a nuclear event, including an experiment on the feasibility of fast preliminary reporting for high-priority samples. (Final Report, 2004) PTS Workshop Report Report of the 2004 Radionuclide Laboratory Workshop held in Strassoldo (Udine), Italy, August 23-25, The Decay Data Evaluation Project (DDEP) Final Report PTE2008

43 Page 43 Appendix I Homogeneity Testing of the Reference Samples The aim of the homogeneity testing was to observe and quantify the effects caused by any variation in the activity distribution in the sources. The results give an indication of the effects that sample inhomogeneity might have on a typical measurement performed on the end cap of a high efficiency high purity germanium gamma spectrometer. The samples were measured on NPL s gamma spectrometer know as Galahad. This detector has a high purity germanium n-type crystal with a relative efficiency of 70 % at 1332 kev. A jig was used to ensure that the samples were positioned in a reproducible manner in the center of the end cap. For each nuclide in each sample, the decay-corrected count rates per unit mass of solution xi or xj with their corresponding counting uncertainties ui and uj were determined. The between-sample variance was determined by measuring all samples once (n = 18), while the measurement variance was determined by measuring a single sample m times (m = 8). The homogeneity uncertainty was calculated as the difference between the sample variance and either; (i) the measurement variance or; (ii) the mean of the internal uncertainties (whichever was larger). The internal uncertainties were dominated by the counting uncertainties. In case the between-sample variance was smaller than either the measurement variance or the internal uncertainty squared, the value of the relative homogeneity uncertainty is zero. u bb i x x i n 1 i xi Relative standard deviation between samples u meas j x j x j x j m m 2 Relative standard deviation for a single sample u j u int 100 mean Internal uncertainty (counting and peak fitting) x j u 2 hom u u or u u u Relative homogeneity uncertainty 2 bb 2 meas 2 hom 2 bb 2 int

44 Page 44 where: n number of samples tested x i decay-corrected count rate per unit mass for sample i u bb relative standard deviation of x i m number of measurements on single selected sample x j decay-corrected count rate per unit mass for sample j u meas relative measurement uncertainty u int mean of the relative uncertainties of x j u hom relative homogeneity uncertainty (cps g 1 ) (cps g 1 ) (%) (%) Table I.1. Reference Samples Nuclide u bb (%) u meas (%) u int (%) u hom (%) 54 Mn * 0 58 Co * Sb * Ba * Ce * * values used for calculating u hom In all cases the relative homogeneity uncertainty was less than 2%. With a 100,000 s measurement, it can be seen from table 4.2 that the uncertainty on the peak area for some nuclides is considerable. Longer measurement times, which may reveal any hidden inhomogeneity, were not possible within the time schedule. Table I.2. Peak Areas for a typical 100,000 s measurement on the NPL detector Galahad at the reference date Nuclide Peak area 54 Mn 631(36) 58 Co 5778(85) 125 Sb 1519(60) 133 Ba 668(36) 141 Ce 9448(121) The deviations of decay-corrected count rate per unit mass from the mean value for each sample (X08300-X08317) are plotted in figures I.1.1 to I.1.5 for the major nuclides individually, and in figure I.2. globally. The error bars are based on peak area uncertainties. Final Report PTE2008

45 Page 45 Figure I.1.1. Deviation of decay-corrected count rate per unit mass from the mean value for each sample (X08300-X08317) for 54 Mn. Figure I.1.2. Deviation of decay-corrected count rate per unit mass from the mean value for each sample (X08300-X08317) for 58 Co.

46 Page 46 Figure I.1.3. Deviation of decay-corrected count rate per unit mass from the mean value for each sample (X08300-X08317) for 125 Sb. Figure I.1.4. Deviation of decay-corrected count rate per unit mass from the mean value for each sample (X08300-X08317) for 133 Ba. Final Report PTE2008

47 Page 47 Figure I.1.5. Deviation of decay-corrected count rate per unit mass from the mean value for each sample (X08300-X08317) for 141 Ce. Figure I.2. Deviation of decay-corrected count rate per unit mass from the mean value for each sample (X08300-X08317) for all the nuclides.