Radiochim. Acta 2015; 103(12): 825–834
Celia Venchiarutti*, Zsolt Varga, Stephan Richter, Rozle Jakopič, Klaus Mayer, and
Yetunde Aregbe
REIMEP-22 inter-laboratory comparison: “U Age
Dating – determination of the production date of
a uranium certified test sample”
DOI 10.1515/ract-2015-2437
Received May 3, 2015; accepted July 28, 2015; published online
September 14, 2015
Abstract: The REIMEP-22 inter-laboratory comparison
aimed at determining the production date of a uranium
certified test sample (i.e. the last chemical separation date
of the material). Participants in REIMEP-22 on “U Age Dating – Determination of the production date of a uranium
certified test sample” received one low-enriched 20 mg
uranium sample for mass spectrometry measurements
and/or one 50 mg uranium sample for 𝛼-spectrometry
measurements, with an undisclosed value for the production date. They were asked to report the isotope amount ratios 𝑛(230 Th)/𝑛(234 U) for the 20 mg uranium sample and/or
the activity ratios 𝐴(230 Th)/𝐴(234 U) for the 50 mg uranium
sample in addition to the calculated production date of the
certified test samples with its uncertainty. Reporting of the
231
Pa/235 U ratio and the respective calculated production
date was optional.
Eleven laboratories reported results in REIMEP-22. Two of
them reported results for both the 20 mg and 50 mg uranium certified test samples. The measurement capability
of the participants was assessed against the independent
REIMEP-22 reference value by means of z- and zeta-scores
in compliance with ISO 13528:2005. Furthermore a performance assessment criterion for acceptable uncertainty
was applied to evaluate the participants’ results. In general, the REIMEP-22 participants’ results were satisfactory.
This confirms the analytical capabilities of laboratories to
*Corresponding author: Celia Venchiarutti, European Commission,
Joint Research Centre (JRC), Institute for Reference Materials and
Measurements (IRMM), Retieseweg 111, 2440 Geel, Belgium,
e-mail: [email protected]
Stephan Richter, Rozle Jakopič, Yetunde Aregbe: European
Commission, Joint Research Centre (JRC), Institute for Reference
Materials and Measurements (IRMM), Retieseweg 111, 2440 Geel,
Belgium
Zsolt Varga, Klaus Mayer: European Commission, Joint Research
Centre (JRC), Institute for Transuranium Elements (ITU), Postfach
2340, 76125 Karlsruhe, Germany
determine accurately the age of uranium materials with
low amount of ingrown thorium (young certified test sample). The Joint Research Centre of the European Commission (EC-JRC) organised REIMEP-22 in parallel to the preparation and certification of a uranium reference material
certified for the production date (IRMM-1000a and IRMM1000b).
Keywords: Age dating, thorium, uranium, nuclear forensics, inter-laboratory comparison, quality control.
1 Introduction
Nuclear forensics is a key element of nuclear security aiming at the identification and characterisation of seized
nuclear material, such as uranium or plutonium, to reestablish the history of the nuclear material of unknown
origin. By applying advanced analytical techniques, the
isotopic composition, the chemical impurities and the
macro- or microstructure of the nuclear material can be determined [1]. The potential and advantages of the “age” determination of the material has been successfully demonstrated [1, 2]. The “age” of a nuclear material refers to its
production date, i.e. the time elapsed since the last chemical separation of the daughter nuclides from the parent U
or Pu radionuclides [3, 4]. During its production, the nuclear material is chemically purified from impurities including radioactive decay products. However, up to now,
no certified age dating reference materials existed for the
validation of mass spectrometric or radiometric methods,
which in combination with the proper uncertainty evaluation [5], are required to characterise intercepted nuclear
material, establishing its age and origin without ambiguity. This determined origin can be then verified against the
declared origin of the seized material and therefore provide the necessary evidence for nuclear safeguards and
nuclear forensic investigations in a court of law.
The European Commission Joint Research Centre Institute for Reference Materials and Measurements (JRCIRMM) is a renowned producer of certified reference maUnauthenticated
Download Date | 6/14/17 4:49 PM
826 | C. Venchiarutti et al., REIMEP-22 inter-laboratory comparison
terials (CRMs) and of quality control/conformity assessment tools such as inter-laboratory comparisons (ILCs)
supporting nuclear safeguards and security. In this context, the two EC-JRC institutes, JRC-IRMM and JRC-ITU (Institute for Transuranium Elements) joined efforts to produce uranium reference materials certified for the production date suitable to serve as a reference material for
method validation in ‘age dating’ of uranium materials.
These CRMs, called IRMM-1000a (20 mg uranium) and
IRMM-1000b (50 mg uranium), were prepared from a lowenriched uranium solution by a complete chemical separation of thorium from uranium at a well-known time with
subsequent monitoring of the ingrowth of the daughter nuclides in the purified material, which confirmed the very
high Th separation efficiency. Before release, units from
these CRMs were used as Proficiency Test (PT) items for
the Regular European Inter-laboratory Measurement Evaluation Programme, REIMEP-22 “U Age Dating – Determination of the production date of a uranium certified test sample” [6].
REIMEP-22 was organised, according to ISO/IEC
17043:2010 [7], in support to the Nuclear Forensics International Technical Working Group (ITWG). The ITWG is an
international network of nuclear forensics experts, including nuclear scientists, law enforcement and regulators,
contributing to advances in nuclear forensics through
a variety of activities, such as comparative material
analysis exercises, guidelines and best practices. Beyond
the ITWG, network laboratories or institutions in the field
of nuclear and environmental sciences also participated
in REIMEP-22. Participants received one 20 mg and/or one
50 mg uranium certified test sample with an undisclosed
value for the production date, depending on the applied
measurement technique. They were asked to report the
two parent/daughter pairs: 234 U/230 Th (compulsory) and
235
U/231 Pa (optional) to determine the production date
of the uranium certified test sample and its associated
uncertainty. Participants were requested to apply their
routine measurement procedures and to complete a questionnaire on the measurement procedures applied in their
laboratories.
Fourteen laboratories registered for REIMEP-22; three
laboratories could not report their results due to technical problems. Eleven laboratories reported results; among
those, two laboratories submitted results for both 20 mg
and 50 mg uranium certified test samples. Thirteen results
were reported for the 234 U/230 Th ratio. A specific lab code
per participant was attributed to each of the thirteen results. Six out of the eleven participating laboratories were
members of the ITWG and are involved in the measurements of nuclear forensics samples.
This paper presents the results reported by REIMEP22 participants, the evaluation of the participant performances and discusses the questionnaire and participants’
feedback in order to gain insight in the current techniques
applied in age dating and the expertise applicable in the
field of nuclear forensics.
2 Materials and methods
2.1 Preparation of REIMEP-22
The REIMEP-22 certified test samples were prepared in the
framework of the production and certification of the reference materials, IRMM-1000a and IRMM-1000b, in compliance with ISO Guide 34 [8]. They were produced from
a low-enriched uranium solution (with a relative mass
fraction 𝑚(235 U)/𝑚(U) of 3.6%) after chemical separation
of thorium decay products from the material, at a wellknown time. The production date was then confirmed by
measuring the ingrown 230 Th in the material. The methodology by Varga et al. [9, 10] was the analytical method used
for the production of the certified test samples. The resulting purified uranium solution was dispensed into precleaned PFA (perfluoroalkoxy alkane) vials to produce 161
units in two sizes: 20 mg (IRMM-1000a) and 50 mg uranium (IRMM-1000b) in dried uranyl-nitrate form.
2.2 Assignment of the reference value
The reference value is the carefully recorded date and time
of the last chemical separation and corresponds to the
complete removal of thorium from the original uranium
material. The reference value is the production date expressed as dd/mm/yyyy with an expanded uncertainty in
days and is based on the 230 Th/234 U radiochronometer.
A complete uncertainty budget was established in accordance with the ‘Guide to the Expression of Uncertainty in
Measurement’ (GUM) [11].
The completeness of the separation of thorium from
the uranium was assessed during the confirmation and homogeneity assessments carried out in compliance with ISO
Guide 34, ISO Guide 35:2006 [12], and ISO 13528 [13] as part
of the certification of the reference material. Detailed results of these assessments are described in Venchiarutti et
al. [14]. The confirmation study demonstrated the successful purification of the uranium material (resulting in an expanded uncertainty of 0.17 d, 𝑘 = 2), whereas the homogeneity study using one-way analysis variance (ANOVA)
showed that the REIMEP-22 certified test samples were
Unauthenticated
Download Date | 6/14/17 4:49 PM
C. Venchiarutti et al., REIMEP-22 inter-laboratory comparison |
considered sufficiently homogeneous for the purpose of
this PT with an expanded uncertainty for homogeneity of
7.8 d (𝑘 = 2). As a result, the REIMEP-22 reference value,
which corresponds to the production date of the uranium
test sample based on the 230 Th/234 U radiochronometer is:
09/07/2012 (as 9 July, 2012) with an uncertainty, at the time
of the REIMEP-22 ILC, of 7.8 d (𝑘 = 2) based on the confirmation and homogeneity study results during the certification of the candidate reference materials.
3 Discussion
3.1 Measurements results
Nine results were reported for the 𝑛(230 Th)/𝑛(234 U)
amount ratios in the 20 mg uranium certified test samples and four for the 𝐴(230 Th)/𝐴(234 U) activity ratios in
the 50 mg uranium certified test sample. In addition, two
laboratories reported the 𝑛(231 Pa)/𝑛(235 U) amount ratios
in the 20 mg uranium certified test sample, as well as
827
the production dates and associated uncertainties. Participants were requested to report three replicates of the ratio
and the average value normalised to a common reference
date specified by the ILC organiser as 06/03/2013 (6 March,
2013). This enabled the evaluation of the measurement results without any data treatment by the ILC organiser. This
approach leaves the responsibility for the reported result
with the participant and enables to identify directly any
differences in the reported ratio results.
The participants’ results are presented with their respective lab codes in Figures 1–2 and Table 1. All the results
are displayed as reported by the participants, i.e. with uncertainties with coverage factors of either 𝑘 = 1 or 𝑘 = 2;
however laboratories’ results that were reported as standard uncertainties with 𝑘 = 1 are clearly identified in the
figures and in the text. The average (amount and activity)
ratios in the figures are reported for the reference date of
06/03/2013.
For the results of the 20 mg uranium certified samples
analysed by the mass spectrometry, four participants reported production date values that agreed well with the
Fig. 1: Reported results for the 20 mg
uranium certified sample a) for
production dates (squares), as
dd/mm/yyyy and uncertainty in day
as stated by participants (i.e. 𝑘 = 1 or
𝑘 = 2), with reference value 𝑋ref on
09/07/2012 (full line) and its
expanded uncertainty (dotted lines)
as described in Section 2.2. b) for the
average 𝑛(230 Th)/𝑛(234 U) amount
ratios (diamonds) on 06/03/2013
with uncertainties as stated by
participants (i.e. 𝑘 = 1 or 𝑘 = 2). The
asterisks in the lab codes legend
indicate values reported by
laboratories with uncertainties 𝑘 = 1
(standard uncertainty). The lab codes
given by the PT organisers were
10249, 10246, etc and are now
presented as L49, L46, etc.
Unauthenticated
Download Date | 6/14/17 4:49 PM
828 | C. Venchiarutti et al., REIMEP-22 inter-laboratory comparison
Fig. 2: Reported results for the 50 mg
uranium certified sample a) for
production dates (squares), as
dd/mm/yyyy and uncertainty in day
(i.e. 𝑘 = 1 or 𝑘 = 2), with reference
value 𝑋ref on 09/07/2012 (full line)
and its uncertainty (dotted lines) as
described in Section 2.2. b) for the
average 𝐴(230 Th)/𝐴(234 U) amount
ratios (diamonds) on 06/03/2013
with uncertainties as stated by
participants (i.e. 𝑘 = 1 or 𝑘 = 2). Same
legend for lab codes as in Figure 1.
The asterisks indicate values
reported by laboratories with
uncertainties 𝑘 = 1 (standard
uncertainty).
reference value (Figure 1a), whereas all the other participants reported systematically production dates that corresponded to a younger age than the known age, i.e.
the known time elapsed between the reference value
(09/07/2012) and the reference date (06/03/2013). This shift
towards younger age might result from an incomplete re-
Table 1: Reported production date (as dd/mm/yyyy) and uncertainties in day based on the average measured 𝑛(231 Pa)/𝑛(235 U) amount
ratios on 06/03/2013 with uncertainties as stated by participants
(i.e. 𝑘 = 1 or 𝑘 = 2) for the measurements of the 20 mg uranium
certified sample.
𝑋ref 1
L46
L52
1
Production dates (± day)
𝑛(231 Pa)/𝑛(235 U) amount ratios
09/07/2012 ± 7.8 (𝑘 = 2)
11/04/2012 ± 22 (𝑘 = 2)
23/07/2012 ± 27 (𝑘 = 1)
–
(8.86 ± 0.59) × 10−10 (𝑘 = 2)
(6.10 ± 0.30) × 10−10 (𝑘 = 1)
Note that REIMEP-22 is not certified for the production date based on
this radiochronometer, but only on the 230Th/234 U radiochronometer.
Therefore, the reference value of 09/07/2012 is only given in this figure as indicative value.
covery (due to loss) of thorium in the REIMEP-22 samples,
prior to the addition of the Th spike in the samples [9].
All participants stated to report the 𝑛(230 Th)/𝑛(234 U)
amount ratios for the reference date of 06/03/2013. Therefore, the analysis of the reported average 𝑛(230 Th)/𝑛(234 U)
ratios with respect to the resulting production dates in
Figure 1b should allow to depict any bias in the reported results. A lower 𝑛(230 Th)/𝑛(234 U) amount ratio
should correspond to a younger age (production date after
09/07/2012), whereas a higher 𝑛(230 Th)/𝑛(234 U) amount
ratio would result in an older age of the sample (production date before 09/07/2012). The latter can be observed
from the participants’ results as shown in Figure 1a and 1b.
Two participants with laboratory codes L42 and L43 had reported ratios contradicting the associated calculated production dates. When calculating the production date from
the average 𝑛(230 Th)/𝑛(234 U) amount ratio as reported
by laboratory L43 on the reference date, one would derive a production date of about one year before the certified test sample was actually produced. On the other
hand, for the participant L42, neither the reported average
𝑛(230 Th)/𝑛(234 U) amount ratio nor the production date
agree. The reported/measured average 𝑛(230 Th)/𝑛(234 U)
Unauthenticated
Download Date | 6/14/17 4:49 PM
C. Venchiarutti et al., REIMEP-22 inter-laboratory comparison
amount ratio is far too high compared to the amount ratio expected in the REIMEP-22 sample leading to a positive bias of ca. 238% in the resulting age. Whereas the
reported production date leads to a negative bias of ca.
49% in the calculated age compared to the known age. It
is very likely a calculation error in combination with an inadequate preparation of the sample prior to measurement
(e.g. spiking) or insufficient noise or abundance sensitivity
correction for 230 Th in the mass spectrometric measurements. However, the participant reported that the 230 Th
noise correction was the major contribution to the final uncertainty budget.
For the results of the 50 mg uranium certified samples
measured by the 𝛼-spectrometry (Figure 2a), two participants L58 and L54 reported production dates that agreed
within uncertainties with the REIMEP-22 reference value,
though L54 reported a larger uncertainty than L58. Laboratory L57 reported a value close to the reference value and
standard uncertainty while L59 reported a production date
that significantly deviates from the reference value (Figure 2a). The good agreement of the reported average ratios
for the 𝐴(230 Th)/𝐴(234 U) amount ratios with the resulting
production dates in Figure 2a and 2b confirmed that all the
participants reported correctly their average activity ratios
for 06/03/2013. In general, the reported uncertainties for
the 𝛼-spectrometry are larger than those for the mass spectrometry measurements (Figs. 1 and 2). The relative uncertainties for the mass spectrometric measurements on
the 20 mg certified test samples are in the range of 2%
to 15%, whereas they are within 8 to 30% in the case of
𝛼-spectrometry measurements for the 50 mg certified test
samples.
The reported production dates for REIMEP-22 (Figure 2a) estimated by 𝛼-spectrometry do not appear to display a systematic shift towards younger age as observed
for the mass spectrometry results (Figure 1a). In the past,
a negative bias in the 𝛼-spectrometry results was observed
by Wallenius et al. [15], resulting in younger ages than
the known ages. The absence of such bias in the REIMEP22 𝛼-spectrometry may be due to the use of both techniques by some participants. Indeed, laboratories L54 and
L58 (Figure 2) participated as well in the measurements
of the 20 mg sample with mass-spectrometry (identified
by L50 and L48 respectively in Figure 1), while laboratory
L57 measured the sample with TIMS and 𝛼-spectrometry.
This could have influenced the way how these participants treated the 50 mg sample prior to 𝛼-spectrometry
measurements, but, as can be seen from the difference in
the reported results of laboratories L54-L50 and L58-L48
in Figure 2-Figure 1, these participants reported indepen-
| 829
dently the production dates based on their 𝛼-spectrometry
or mass spectrometry measurements, respectively.
Most REIMEP-22 participants reported uncertainties
according to the Guide for Quantifying Measurement Uncertainty (GUM) [11] issued by the International Organization for Standardization as ISO 99:2005. Six laboratories reported expanded uncertainties with a coverage factor 𝑘 = 2 and three others reported standard uncertainties
(𝑘 = 1) according to the GUM [11]. One laboratory reported
uncertainties with 𝑘 = 2 using another standard for the
quantification of uncertainty (here the GOST R-ISO-57252-2002), while another laboratory propagated the analytical uncertainties (with a coverage factor 𝑘 = 2 and using
a Student factor for the average ratio).
As can be seen from Figures 1 and 2 some participants
underestimated the uncertainties associated with their
measurements and the production date. On the one hand,
laboratories L45, L50, L47, and L42 reported results with
a significant deviation from the reference value (Figure 1a).
Their uncertainties did not reflect these biases and there
is no overlap with the certified range. On the other hand,
laboratory L52 (Figure 1a and b) reported only a standard
uncertainty (𝑘 = 1) for the measurement of the 20 mg uranium certified test sample. Therefore, although the reported production date did not deviate much from the reference value, the difference is significant. The same can be
observed in Figure 2a for the measurement of the 50 mg
uranium certified test sample for laboratory L57, which
also underestimated the uncertainty by reporting the production date with a standard uncertainty (𝑘 = 1), instead
of reporting the expanded uncertainty with a coverage factor 𝑘 = 2, as it was done for the average 𝐴(230 Th)/𝐴(234 U)
amount ratio.
The reporting of the 𝑛(231 Pa)/𝑛(235 U) amount ratios
or 𝐴(231 Pa)/𝐴(235 U) activity ratios was optional since
the REIMEP-22 reference value corresponds to the production date based on the 230 Th/234 U radiochronometer and not the 231 Pa/235 U radiochronometer. The verification of the completeness of the 231 Pa from its mother
235
U in the material was beyond the scope of the IRMM1000 certification project [10, 14]. Nevertheless, two participants reported the 𝑛(231 Pa)/𝑛(235 U) amount ratio measured by mass spectrometry (Table 1) and the derived production date [16]. Laboratory L46 reported only one value
for one replicate due to technical problems. Laboratory
L52, reported an average 𝑛(231 Pa)/𝑛(235 U) amount ratio
with an associated production date, which confirmed the
production date of the certified test sample, which is for
the 231 Pa/235 U radiochronometer only given as indicative value (Table 1). Most of the other participants stated
that they had neither experience in protactinium measureUnauthenticated
Download Date | 6/14/17 4:49 PM
830 | C. Venchiarutti et al., REIMEP-22 inter-laboratory comparison
ments, nor they had an appropriate and validated measurement procedure to measure the 𝑛(231 Pa)/𝑛(235 U) ratio
in the REIMEP-22 samples. As there are only two reported
production dates based on the 231 Pa/235 U radiochronometer, it is difficult to conclude the REIMEP-22 reference value
is also applicable for this radiochronometer. However the
result of laboratory L52 is in good agreement with the
REIMEP-22 reference value (Table 1), and may indicate that
the separation of 231 Pa from its mother 235 U might have
been complete.
2
𝑢 2
𝑢
𝑧 2
( ) − ( ref ) = ( lab )
𝐶
𝜎̂
𝜎̂
3.2 Evaluation of laboratory performances
The ITWG does not recommend quality goals or performance criteria to the network laboratories that could have
been used in REIMEP-22 to assess the laboratories measurement capabilities. It was therefore agreed to evaluate the measurement performance in REIMEP-22 by means
of z- and zeta-scores in compliance with ISO 13528 [13].
The zeta-scores were calculated for each laboratory results reported with an uncertainty and give an indication
of whether the estimate of the uncertainty is consistent
with the laboratory’s deviation from the reference value as
given in Section 2.2 [17].
In this paper, the authors suggest 5% of the known age
of 464.2 d of the material on 16/10/2013 as relative standard deviation (𝜎̂) for proficiency assessment [13]. 𝜎̂ was
set to 23.2 d. The same criterion was used in REIMEP-22
for the homogeneity assessment [17]. The 𝑧-score provides
an indication whether a laboratory is able to perform the
measurement in accordance with the 𝜎̂. The scores are expressed as follows:
zeta =
𝑧=
𝑥lab − 𝑋ref
√𝑢2ref + 𝑢2lab
𝑥lab − 𝑋ref
𝜎̂
the laboratory (𝑢lab ) was calculated as the reported uncertainty divided by the coverage factor. Deviation from the
reference value and tendencies of laboratories to underestimate their uncertainties are discussed in Figure 3 using
the “Naji plot” as a straightforward graphical tool to evaluate participants’ results [18]. The two scores are combined
as expressed in Eq. (3), which display the participants results/performances by means of z-scores (𝑥-axis) with respect to the acceptable uncertainty (𝑦-axis) in areas delimited by |zeta| ≤ 2 and 𝑢min ≤ 𝑢lab ≤ 𝑢max with 𝑢min = 𝑢ref ,
and 𝑢max = 2 ⋅ 𝜎̂.
(1)
(2)
where 𝑥lab is the result reported by a participant (based
on their measurement), 𝑋ref is the certified reference value
(assigned value), 𝑢ref is the standard uncertainty of the reference value and 𝑢lab is the standard uncertainty reported
by a participant.
Both scores can be interpreted as: satisfactory performance for |score| ≤ 2, questionable performance for 2 <
|score| ≤ 3 and unsatisfactory performance for |score| > 3.
An unsatisfactory laboratory performance may be
caused by an underestimated uncertainty or by a large
deviation from the reference value. Since all the laboratories participating in REIMEP-22 provided uncertainties
with a coverage factor (𝑘), the standard uncertainty of
(3)
Results fall in the Naji plot within areas defined in Figure 3
by two parabolas (𝐶 = 2 delimits the performance criteria domain for |zeta| = 2 and 𝐶 = 3 the one for |zeta| = 3)
delimiting the different performance criteria domains. Results falling in the area delimited by 𝑢max and |zeta| ≤ 2
are satisfactory and results in the area delimited by 𝑢max
and |zeta| ≤ 3 are questionable.
Figure 3 shows that five results (L43, L46, L48/L58 and
L49) fall within the area corresponding to satisfactory performances, i.e. that their reported value and its uncertainty falls well within the range of the acceptable uncertainty based on a 𝜎̂ = 0.05𝑋ref ; even though laboratory L46 reported an uncertainty that may be underestimated (smaller than 𝑢ref , Figure 3). Two laboratories L52
and L57 reported questionable results. Among the laboratories having reported satisfactory or questionable results, five are part of the ITWG (L48 and L58 represent the
same laboratory having measured both REIMEP-22 samples). It is interesting to see that laboratories L49 and L43
considered themselves as not very experienced in Th-U
mass spectrometry measurements, yet they reported satisfactory results. Moreover, laboratories L42, L47 and L48 reported that they did not have a routine measurement procedure in place to measure such low amount of Th samples
and had to set-up completely new methods to analyse the
REIMEP-22 sample(s) with mass spectrometry. The results
for laboratory L48 are therefore very encouraging and confirm that the newly developed analytical procedures are
suitable for this kind of measurements. For the two other
laboratories, the determination of low amount of Th in the
certified samples remains an analytical challenge. Concerning the analysis of the 50 mg samples, only laboratory
L58 reported to be experienced in the radiometry measurements of Th-U samples; experience that is confirmed in
Figure 3 by the satisfactory laboratory performance within
reported uncertainty.
Unauthenticated
Download Date | 6/14/17 4:49 PM
C. Venchiarutti et al., REIMEP-22 inter-laboratory comparison
| 831
Fig. 3: Naji plots of all REIMEP-22 participants’
results for the measurements of the 20 mg
uranium test samples (squares) and of the
50 mg uranium test samples (triangles). The
ITWG laboratories are identified by open
symbols. Two laboratory results (not ITWG) for
the 50 mg uranium test samples fall outside
the scale of this plot and are indicated by
dashed arrows.
While results using 𝛼-spectrometry have larger but realistic uncertainties, the majority of the laboratories using mass spectrometry underestimated the uncertainty
(|zeta| > 2 in Figure 3). This clearly shows the importance
of providing reasonable and realistic measurement uncertainties, which do not have to be necessarily as small
as possible. The questionable performance of laboratory
L57 indicates that the calculation of realistic uncertainties might be an issue. This laboratory possibly underestimated the uncertainties on the reported ratios or/and production dates, as may have done laboratory L45. In addition, laboratories L46 and L47 reported an expanded combined uncertainty smaller than the expanded uncertainty
of the reference value (Figure 3).
Five out of the eleven REIMEP-22 participants reported
results that met the laboratory performance criteria set by
the PT organisers (based on the scores and acceptable uncertainty), by reporting values and expanded uncertainties within the range of the expanded uncertainty of the
reference value.
4 Discussions on the participants’
answers to the questionnaire
4.1 Analytical methods used for REIMEP-22
samples
Participants used from 0.1 mg up to almost 6 mg of sample for one aliquot/replicate for the mass spectrome-
try measurements and up to 15 mg of sample for the
𝛼-spectrometry measurements. Three laboratories using
mass spectrometry technique did not perform any chemical separation prior to measurements. Other participants
applied a chemical treatment by dissolving the samples in
nitric or hydrochloric acid followed by a separation using
TEVA® resin or anion exchange (e.g. AG 1-X8). Some participants used co-precipitation with lanthanides to separate Th from U before 𝛼-spectrometry measurements. All
the participants applied isotope dilution for the determination of the amount or activity of Th and U in the samples. For the mass spectrometry measurements, seven out
of the nine participants used Multi-Collector ICP-MS (MCICP-MS) for the U/Th measurements while two other laboratories used Sector-Field ICP-MS (SF-ICP-MS). For the
measurement of the 50 mg uranium certified samples, all
the participants used 𝛼-spectrometers. Laboratories L59
and L57 measured Th by 𝛼-spectrometry and U by Thermal Ionisation Mass Spectrometry (TIMS). Certified reference materials were used for instrument calibration, mass
bias and abundance sensitivity correction. Furthermore,
CRMs were used for the quantification of in-house spikes
(229 Th ).
4.2 The components of the uncertainty
budgets
For the determination of the production date based on
the (amount or activity) 230 Th/234 U ratio, all participants
reported that the major contribution to the final uncerUnauthenticated
Download Date | 6/14/17 4:49 PM
832 | C. Venchiarutti et al., REIMEP-22 inter-laboratory comparison
tainty came from the thorium determination, whether using mass spectrometry or 𝛼-spectrometry. This is mainly
due to the low amount of thorium present in the relatively “young” REIMEP-22 certified test samples. For the
mass spectrometry analysis, the low 230 Th signal was often the problem, since it could then be hindered by background or high abundance sensitivity if 232 Th was used
as spike for the isotope dilution. Participants reported as
well uncertainty related to the calibration of the Th spikes
against other reference materials, which is mostly due
to the low availability of thorium certified reference materials. For the 𝛼-spectrometry measurements, the 230 Th
counting statistics as well as the estimation of the detection efficiencies (if no isotope dilution was used) may be
the reasons for the higher uncertainties observed for 𝛼spectrometry results.
For the Pa-U analysis of the REIMEP-22 samples,
though no detailed measurement procedure was given,
both participants indicated that the separation yield of Pa
and the 233 Pa spike calibration were the major contributors to the final uncertainty on the reported values.
Since the thorium determination was identified as the
major component of the final uncertainty on the production date, we may expect a reduction with ageing of the uncertainty on the 230 Th determination and 230 Th/234 U ratio
of the REIMEP-22 certified test sample [5]. With increasing
amount of 230 Th in the material with time, the activity ratio will tend to unity and the measurements of the 230 Th
should be achievable by most of the laboratories. However,
other components of uncertainties, such as the half-lives
of 234 U or 230 Th , may then become more significant with
time [5].
4.3 Use of half-lives and molar masses
REIMEP-22 participants were asked to report the half-lives
(in years) and molar masses (g mol−1 ) with associated uncertainties (with a coverage factor 𝑘 = 2) used in their calculations. Pommé et al. [5] have shown recently that halflives and their uncertainties play a significant role in the
final calculation of the age of the radioactive material and
its uncertainty. This is of high importance in nuclear forensic science. Therefore, the REIMEP-22 organisers explicitly
asked participants to report as well the bibliographic references for these values to get an overview of the commonly used half-lives within the nuclear forensic community.
The values for the half-lives and the expanded uncertainties as reported by the participants [17] indicate that
participants in REIMEP-22 used the 234 U half-lives pro-
vided by DDEP-BIPM [19] and by Cheng et al. [20], while
more references were cited for the 230 Th half-life. There
seems to be a possible misuse of the uncertainties associated with these values. Laboratories L48, L50 and L57
reported the same half-life values as given in DDEP-BIPM
[19], but L48 reported associated uncertainties that were
twice the uncertainty value reported by the two other laboratories [17]. Similarly, L46, L49, L42 and L47 reported similar half-life values as those given in Cheng et al. [20], while
L42 reported uncertainties based on [21] that were twice
the uncertainty value reported by the three other participants [17].
For the values published in [19], it appears that the expanded uncertainties as reported by L48 are correct, i.e.
with 𝑘 = 2, so that the two other laboratories reported in reality standard uncertainties with 𝑘 = 1. On the other hand,
L42 may have considered that the uncertainties for the
230
Th and 234 U half-lives provided in [20, 21] were standard uncertainties instead of expanded uncertainties with
𝑘 = 2. Therefore, the correct half-lives and their respective
expanded uncertainties are summarised in Table 2.
More variations in the reported values were observed
for molar masses than for half-lives, even though it has
less influence on the final results [17]. Even if most of the
reported values agreed well on the three first digits, the
last significant digits were very different. Among the few
reported uncertainties associated with molar masses, L48
reported an uncertainty for the 230 Th molar mass twice
that of L50 and more than twice that of L47 [17]. Finally,
the correct molar masses and uncertainties for 𝑀(234 U)
and 𝑀(230 Th) in g mol−1 are those reported by laboratory L48, based on Audi et al. [22] and are respectively:
234.0409521 ± 40 × 10−7 and 230.0331338 ± 38 × 10−7
(𝑘 = 2).
This study clearly indicates that harmonisation of
the half-life and molar mass values from bibliographic
sources/nuclear data references within the nuclear forensic community is necessary for a more accurate and robust
determination of the age (production date) of a uranium
sample and of its associated uncertainty. The references
provided in this paper shall be used as a starting point
towards the harmonisation of these values. The nuclear
forensics community shall participate in or even initiate
Table 2: Half-lives (in years) and expanded uncertainties (with 𝑘 = 2)
as from literature.
References
DDEP-BIPM [19]
Cheng, H., et al. [20]
𝑇1/2 (234 U)
𝑇1/2 (230 Th)
2.4550 × 105 ± 1200
2.4525 × 105 ± 490
7.538 × 104 ± 600
7.569 × 104 ± 230
Unauthenticated
Download Date | 6/14/17 4:49 PM
C. Venchiarutti et al., REIMEP-22 inter-laboratory comparison
the evaluation of the existing half-life values, especially
when new values are published. For instance, Cheng et al.
[23] recently published updated 234 U and 230 Th half-life
values (as 2.45620×105 ±260 and 7.5584×104 ±110 a), to
replace the previous ones ([20], see Table 2), improved due
the use of new gravimetrically prepared reference materials for mass spectrometric measurements and a justified
new choice of secular equilibrium materials. The new 234 U
half-life value closely approaches the DDEP-BIPM value
([19], see Table 2) and has a much smaller uncertainty.
However, the new 234 U and 230 Th half-life values could
only be derived by using the old measurement of the 238 U
half-life by Jaffey et al. [24], the uncertainty of which might
be underestimated. The value by Jaffey et al. shall either
be combined with a more realistic commonly agreed uncertainty, or new measurements for the 238 U half-life shall
be performed (on-going at IRMM and elsewhere). Subsequently by using the new equilibrium 234 U/238 U and
232
Th/238 U values provided by [23], the 232 Th and 234 U
half-lives could be re-calculated with improved uncertainties. As a conclusion, a critical evaluation and harmonisation of published half-life values used for nuclear forensics
shall be performed at international level by suitable organisations like BIPM, the IAEA, or dedicated nuclear forensics organisations, such as the ITWG.
5 Conclusions and outlook
REIMEP-22 offered a unique opportunity to laboratories involved in nuclear forensic to demonstrate that their measurement results for the characterisation of the age of
uranium materials are fit for the intended purpose and
within the required measurement uncertainties for stateof-practice sample analysis.
The challenge for REIMEP-22 participants was to measure 230 Th/234 U in a young uranium sample containing
a low amount of thorium. Most of the participants in
REIMEP-22 performed well using mass spectrometry or 𝛼spectrometry. The spread of results was larger for the measurements performed by 𝛼-spectrometry than those performed by mass spectrometry. REIMEP-22 confirmed the
quality of the analytical capabilities of the participating
laboratories to determine the production date of similar
intercepted uranium materials. However there is room for
improvement in the estimation and reporting of measurement uncertainties. Moreover, discrepancies were identified among participants using different half-lives and molar masses and the corresponding uncertainties from bibliographic references. Attention should be brought to the
reporting of results and determination of the associated
| 833
uncertainties. Proper harmonisation of nuclear reference
data, such as half-lives, should be ensured within the nuclear forensics community.
The additional results reported by two of the participants indicate that the separation of 231 Pa from its mother
235
U might be complete in the certified test sample and
that the 231 Pa/235 U radiochronometer could be used to determine the production date. This needs to be further confirmed by additional measurements.
Acknowledgement: The authors are very grateful to
Adrian Nicholl and Judit Krajko (both at EC-JRC-ITU) and
Monika Sturm (former EC-JRC-IRMM and now at IAEASGAS) for their technical help with the preparation and
certification of the certified test samples. The authors
would like to thank the EC-JRC-IRMM colleagues: Theo
Droogmans, Frederik Van der Straat and Andreas Fessler
and Marleen Peetermans who helped with the logistics
of REIMEP-22. The authors are also very thankful to the
internal reviewer at EC-JRC-IRMM, Piotr Robouch and an
anonymous peer-reviewer for their helpful and constructive comments.
References
1. Mayer, K., Wallenius, M., Varga, Z. (2013). Nuclear Forensic
Science: Correlating Measurable Material Parameters to the
History of Nuclear Material. Chem Rev 113: 884–900.
2. Varga, Z., Suranyi, G. (2007). Production date determination of
uranium-oxide materials by inductively coupled plasma mass
spectrometry. Anal Chim Acta 599: 16–23.
3. Wallenius, M., Mayer, K. (2000). Age determination of plutonium material in nuclear forensics by thermal ionisation mass
spectrometry. Fresenius J Anal Chem 366: 234–238.
4. Morgenstern, A., Apostolidis, C., Mayer, K. (2002). Age Determination of Highly Enriched Uranium: Separation and Analysis
of 231 Pa. Anal Chem 74: 5513–5516.
5. Pommé, S., Jerome, S. M., Venchiarutti, C. (2014). Uncertainty
propagation in nuclear forensics. J Appl Radiat Isotopes 89C:
58–64.
6. EC-JRC-IRMM Webpage for REIMEP Inter-Laboratory Comparisons. https://ec.europa.eu/jrc/en/interlaboratorycomparisons/REIMEP.
7. ISO/IEC 17043:2010 (2010). Conformity assessment – General
requirements for proficiency testing.
8. ISO Guide 34:2009 (2009). General requirements for the competence of reference material producers.
9. Varga, Z., Nicholl, A., Wallenius, M., Mayer, K. (2012). Development and validation of a methodology for uranium radiochronometry reference material preparation. Analytica
Chimica Acta 718: 25–31.
10. Varga, Z., Venchiarutti, C., Nicholl, A., Krajkó, J., Jakopič, R.,
Mayer, K., Richter, S., Aregbe, Y. (2015). IRMM-1000a and
IRMM-1000b uranium reference materials certified for the
Unauthenticated
Download Date | 6/14/17 4:49 PM
834 | C. Venchiarutti et al., REIMEP-22 inter-laboratory comparison
11.
12.
13.
14.
15.
16.
17.
production date Part I: Methodology, preparation and reference value. Journal of Radioanalytical and Nuclear Chemistry.
DOI 10.1007/s10967-015-4227-x.
ISO (2005). Guide to the Expression of Uncertainty in Measurement http://www.bipm.org/en/publications/guides/gum.html.
ISO Guide 35:2006 (2006). Reference materials – General and
statistical principles for certification.
ISO 13528:2005 (2005). Statistical methods for use in proficiency testing by inter-laboratory comparisons.
Venchiarutti, C., Varga, Z., Richter, S., Nicholl, A., Krajkó, J.,
Jakopič, R., Mayer, K., Aregbe, Y. (2015). IRMM-1000a and
IRMM-1000b: uranium reference materials certified for the
production date based on the 230 Th/234 U radiochronometer
Part II: Certification. Journal of Radioanalytical and Nuclear
Chemistry. DOI 10.1007/s10967-015-4368-y.
Wallenius, M., Morgenstern, A., Apostolidis, C., Mayer, K.
(2002). Determination of the age of highly enriched uranium.
Anal Bioanal Chem 374: 379–384.
Eppich, G., et al. (2013). 235 U-231 Pa age dating of uranium materials for nuclear forensic investigations. J Anal At Spectrom 28:
666–674.
Venchiarutti, C., Varga, Z., Richter, S., Nicholl, A., Krajkó, J.,
Jakopič, R., Mayer, K., Aregbe, Y. (2015) REIMEP-22 U Age Dating – Determination of the production date of a uranium certified test sample. JRC Science and Policy report. EUR
18.
19.
20.
21.
22.
23.
24.
27124. DOI 10.2787/843376. https://ec.europa.eu/jrc/en/
interlaboratory-comparisons/REIMEP.
Robouch, P., Younes, N., Vermaercke, P. (2003). PTB IT-10, pp.
149–157.
DDEP-BIPM (2004–2013). Table of radionuclides. Monographie
BIPM-5. Retrieved from http://www.nucleide.org/DDEP_WG/
DDEPdata.htm4.
Cheng, H., Edwards, R. L., Hoff, J., Gallup, C. D., Richards, D. A.,
Asmerom, Y. (2000). The half-lives of uranium-234 and
thorium-230. Chemical Geology 169: 17–33.
Nucleonica 2007–2014, developed under a License of the European Atomic Energy. (European Commission) http://www.
nucleonica.net/.
Audi, G., Wapstra, A. H., Thibault, C. (2003). The AME2003
atomic mass evaluation – (II). Tables, graphs and references.
Nucl Phys A 729: 337–676.
Cheng, H., Edwards, L., Shen, C.-C., Polyak, V. (2013). Improvements in 230 Th dating, 230 Th and 234 U half-life values, and U-Th
isotopic measurements by multi-collector inductively coupled
plasma mass spectrometry. Earth Planet Sci Lett 371–372, 82–
91.
Jaffey, A. H., Flynn, K., F Glendenin, L. E., Bentley, C.,
Essling, A. M. (1971). Precision Measurement of Half-Lives
and Specific Activities of 235 U and 238 U. Phys Rev C 4(5), 1889–
1906.
Unauthenticated
Download Date | 6/14/17 4:49 PM