metrology of radionuclides and chemical metrology

Cetama 50 years anniversary 05/25/2011 1

Metrology of radionuclides and chemical metrology

P. Cassette – CEA/DRT/LIST/LNHBC. Rivier – CEA/DEN/DRCP/CETAMA


• Measurand: mean number of spontaneous transitions of a radionuclide in a time interval

• Primary standardmeasurement standard established using a primary reference measurement procedure, (or created as an artifact, chosen by convention)

activity and uncertainty

• Secondary standardmeasurement standard established through calibration with respect to a

primary measurement standard for a quantity of the same kind

source with activity and uncertainty determined by relative measurement to primary standard

Radionuclide metrology: realization of the SI-unit Becquerel


Specificity of radionuclide metrology

• Radionuclides decay (sic!) so generally no possible permanent standard

• Diversity of:• Physical form (solid, liquid, gas, aerosols,…)• Activity range (generally from mBq to GBq)• Radioactive transitions (α, β, ec., sf.)• Associated de-excitations (none, γ, X, ce., Auger, 2nd order processes…)• Decay schemes (various daughter levels), metastable states• Half-lives (from ns to Ts)• Physical-chemical properties (stability, volatility,…)• Isotopic mixtures…• Daughters series

•The measurand cannot be directly observed but transitions are only measured through the emitted radiation (X, γ, α, e-, e+)

Cetama 50 years anniversary 05/25/2011

Traceability and Equivalence

TRACEABILITY

property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty

(VIM)

DEGREE OF EQUIVALENCE

the degree to which the value of a measurement standard is consistent with the key comparison reference value. This is expressed quantitatively by the deviation from the key comparison reference value and the uncertainty of this deviation.

(MRA)


International traceability, BIPM

Authority - Convention du Mètre (diplomatic treaty between 53 nations)

Operates - Consultative Committees (CCRI (II) for radionuclide metrology)

BIPM laboratories

CC members - National Metrology Institutes (NMIs)

The task of the Bureau International des Poids et Mesures (BIPM) is to ensure world-wide uniformity of measurements and their traceability to the International System of Units (SI).


CCRI(II) and SIR Key Comparisons

SIR (Système International de Référence)(for gamma emitters)

- high pressure, well-type ionization chamber- sources submitted at any time by an NMI- normalised to 226Ra reference sources

Project of extension to α and β emitters

CCRI(II) comparisons

- organised by CCRI(II), piloted by individual NMIs (with BIPM)- all samples from same solution- common dead-line for reporting- incorporated into SIR, where possible


BIPM, CCRI(II) and RMO Comparisons

BIPM takes part in, and organizes, international comparisons of national measurement standards, and it carries out calibrations for Member States.

Regional Metrology Organisations (RMOs) can also conduct radioactivity key comparisons – in accordance with CCRI(II) guidelines

APMP – Asia-PacificCOOMET – Euro-AsiaEURAMET - EuropeSIM – Inter-AmericaSADCMET – South Africa


Scheme for Key Comparisons

National metrology institute (NMI) participating in CIPM key comparisonsNMI participating in CIPM key comparisons and in (RMO) key comparisonsNMI participating in RMO key comparisonsNMI participating in ongoing BIPM key comparisonsNMI participating in a bilateral key comparisonInternational organization signatory to the MRA


Key Comparison Reference Value (KCRV) and Degrees of Equivalence

KCRV- (normally) based on arithmetic mean of all results

(can be modified on the advice of the Key Comparison Working Group and with the agreement of the CCRI(II) )

- best estimate of SI value

Degree of Equivalence- difference between KCRV and NMI result- differences between NMIs


Traceability and Equivalence

Bureau International des Poids et MesuresIntercomparisons

E Q U I V A L E N C E

T R A

C E A

B l L I T Y

LNHB NPL PTBBEV

SSL SSL

End user

End user

NMI

Secondary laboratory


BNM - LNHB

AIST (ex-E

TL)NRCC

PMH

PTBBARC

BEVBIPM

CIEMATCNEAENEA

IFINIR

A - meta

sIR

MMKRISSLN

MRI

NIM NPL

P3KRBIN

- Bata

n RC

CMISMUVNIIM

560

580

600

620

Arithmetic mean of the activity concentration : 582.0 Bq/mg; u = 1.4 Bq/mg

1 %

Trial comparison

Results of the full-scale international comparison of activity measurements of a solution of 152Eu

CIEMAT/NIST method LS-βγ coincidence counting 4πPC coincidence counting LS-βγ counting with computer discrimimination 4πPPC coincidence counting γ-counting with calibrated Ionizing Chamber 4πPC(β,e,x)-γ coincidence counting γ-counting with Ge detectors 4πγ with well-type NaI(Tl)-detectors 4πPPC-γ-Σ 4π CsI(Tl) well-type NaI(Tl) γ

Act

ivity

con

cent

ratio

n /

(Bq/

mg)

Laboratory

Example of result of a key comparison


MRA (Mutual Recognition Arrangement)

The MRA was drawn up by the International Committee of Weights and Measures (CIPM), under the authority given to it in the Metre Convention, for signature by directors of the NMIs of Member States of the Convention and Associates of the CGPM.

(October 1999)

Objectivesto establish the degree of equivalence of national measurement

standards maintained by NMIs;

to provide for the mutual recognition of calibration and measurement certificates issued by NMIs;

thereby to provide governments and other parties with a secure technical foundation for wider agreements related to international trade, commerce and regulatory affairs

BIPM Key Comparison Database (KCDB) includes KCRV & CMC


Supporting Comparisons for CMCs

All CMC entries need to be supported by a valid comparison (e.g.CCRI(II), RMO, supplementary, ….) or by other means

CMC tables contain over 150 nuclides

Period of validity of supporting comparisons - usually 10 years

Worst Case scenario

Some NMIs may need to participate in over

15 key comparisons every year

practical solution – GENERIC GROUPINGS


• Different physical detection principles and devices must be used, depending on radionuclide

• Counting efficiency should be ≈ 100 % with small corrections

> high-geometry (4π) methods< 100 %, but calculated with low uncertainty

> coincidence counting, defined solid angle counting

• The ‘ideal’ primary method is accurate, under statistical control, if possible independent of decay scheme parameters and not based oncalibrations with other radioactivity standards

Primary calibration methods


Example of primary measurement method: ADSA

Alpha-particlecounter with well-defined geometry

PIPS detector

source

distance tube


Example of primary measurement method coincidence counting

alpha detector

gamma detector

Measure of count rates in each detector along with coincidence rate

Να = Α εα Νγ = Α εγΝc = Α εα εγ

coinc

cNNN

A γα=

α

γ


Some requirements

• Detectors sensitive to one type of radiation onlyno gamma-ray detection in beta-counteralso no pickup of electronic noise !!

• εα and εγ must be independent and constantno directional correlation between α and γ→ use 4π beta detectorat least one of the efficiencies should be the same in all parts of the source

• No coincidences should be lostcoincidence window wide enough to avoid loss through time-jitter between α and γ signalscompensate for accidental coincidences and dead time


Suitable alpha detectors

• Need 4π geometryhigh ε reduces uncertainty of extrapolationno directional correlationno scattering/absorption correction outside source

• Proportional counters with thin sourceshigh gas gains & low dead timesalmost all charged particles escaping source are countedrequires suitable chemical form for thin stable solid source (not trivial

• Liquid scintillation countersdead times usually larger than PC (after-pulses …)higher εαγ interaction than PC (by ~ 10x)requires suitable chemical form to be stable, compatible with scintillant


Some specific points (example of Pu standardization)

• Pu is generally present as a mixture of isotopes> Mass spectrometry or α spectrometry is necessary to complete alpha

primary standardization (eg. by ADSA)> Pu isotopes have very different half-lives (e.g. Pu-241: 14 a,Pu-238: 88 a, Pu-239: 24 ka) so mass composition is not similar to activity composition)

> The problem of Pu-241: almost pure beta low-energy emitter (Eβmax = 21 keV), short half-life, daughter is an α emitter (Am-241).

- Global alpha activity of a Pu source can increase with time! - Notable Pu-241 activity can correspond to very small mass of Pu-241- Pu-241 is difficult to standardize (forbidden beta transition)

> So Pu standardization (and this is also the case for other radionuclide) really needs the support of chemical metrology…


Specificities of chemical analysis: References

The chemist needs to know the type of analytes he measures

⇒ need to develop as many highpurity standards as chemicalspecies

No single reference for the mole, unlikefor weight measurements

∞ of referencesneeded

Chemical species

Chemical references One-axis scale


Specificities of chemical analysis: measurement protocol complexity

Second main difficulty: Complexity of the measurement protocols, especially the sample preparation steps: extraction, digestion…

High purity reference materials, used alone, do not generallyguarantee the traceability of analytical results

⇒ Need for matrix reference materials

∞2 of referencesneeded

Chemical references Two-axis scale

Mat

rices

Chemical species

The NMIs can not cover all the analytical needs

⇒ Call for expert laboratories to provide Reference Materials


Availability of RMs

The number of commercially available RMs is estimated to be 20000, most of them coming from the metalurgy industry

www.comar.bam.de

10100 RMs registered in COMAR database

In the fields of environment, foodstuffs and life sciences,only 10% of the current needs for RMs are covered


Role of interlaboratory comparisons in the traceability chain

Other tools than high purity RMs and matrix RMs are needed to guarantee the comparability of analytical results:

Interlaboratory Comparisons

Currently, around 1200 PT schemes are recorded in the international EPTIS database

For water analysis, more than250 different parameters are covered by PTS while only

around 30 are covered by CRMs

www.eptis.bam.deConcentration en mg/L

0.12

0.13

0.14

0.15

0.16

0.17

0.18

0.19

0.20

0.21

0.22

labo 8 labo 18 labo 17 labo 9 labo 22 labo 14 labo 11 labo 19 labo 12 labo 4Laboratoires

Uranium concentration determinationNT DEN CETAMA 2009-02 – GT 18


General scheme for metrological traceability in chemical analysis

Standardization bodies :ISO, CEN, AFNOR

Metrology organizations :CCQM, EURAMET, NMIs

Reference Materialsproducers PTS providers

Field laboratories

ISO guide 34 Accreditation ISO 17043 Accreditation

ISO 17025 Accreditation

Standardizedmethods


Role of CETAMA in ensuring the comparability of nuclear chemicalanalysis

StandardizationParticipation in BNEN,

AFNOR and ISO commissions

CETAMA network

Reference materials(33 references)

- High purity standards and solutions (U, Pu and Np),- U matrix CRMs- Isotopic composition (U, Pu)

Interlaboratory comparisons- Proficiency tests (EQRAIN)- Method validation interlaboratory tests- « exploratory » tests


Example of a traceability chain for Pu chemical analysis in nitricsolutions: available references

Standardized method

NF ISO 8298:2001 « Determination of milligramamounts of plutonium in nitric acid solutions »

Reference materials

Pu metal MP2 certified for purity (by coulometry, titrimetry and « 100% - ∑impurities » approach)and isotopic composition

Proficiency Testing Schemes

EQRAIN Pu – Reference value determined by LAMMAN using gravimetricmethods and checked by primary methods (coulometry and/or isotope dilution by TIMS)


Primary reference methods

CCQM defined several potential primary reference methods to be used as a basis for metrological traceability in chemistry

zFIdt

zFQn ∫==

Gravimetry,

Titrimetry,

Differential scanning calorimetry,

Coulometryoulometry :: No chemical standard, the traceabilityis based on electrical measurementsand on Faraday’s law

Isotope dilutionCavity Ring Down SpectrometryNeutron Activation Analysis


Isotope dilution as a primary reference method for Pu analysis

Double isotope dilution mass spectrometry

m/zSample RX

m/zSpike RY

m/zSample blend RB

m/zCalibration solution RZ

m/zSpike RY

m/zCalibration blend RBc

Direct isotope dilution

Reverse isotope dilution

Double isotope dilution

239Pu

242Pu



Concentration of the calibration solution

Solutions masses

Isotope ratios Concentration of the blank

High purityCRM

Standard masses

Correction for mass biasneglected if conditions close to

exact matching are met

Double isotope dilution mass spectrometry

Blk

Zi

Xi

BcY

ZBc

XB

BY

YcX

YZc

ZXC

RR

RRRR

RRRR

mmmmCC −×

−−

×−−

×××

×=∑

∑



BcBRR = =

BYRR >>

Conditions for exact matching:

m/zSample blend RB

m/zSample blend RBc

Optimum values for the spike and blend whenconditions for exact matching are not perfectly met:

239Pu

242Pu

XBRR >>and

ZBcRR >>and


Example of a traceability chain for Pu chemical analysis in nitricsolutions: EQRAIN Pu (2006-2008)

Robust Mean: 4,1602 g/kgStandard uncertainty of the robust mean: 0,0052 g/kg

Pu Reference Value (LAMMAN): 4,1620 ±

0,0042 g/kg (k=2)

22reflab

reflab

uu

XX

+

−=ζ

-12 satisfactory results- 2 results giving a « warning signal »- 1 result giving an « action signal »

Detection of laboratory biasEQRAIN Pu 10

NT DEN CETAMA 2009-03 – GT 3


Example of a traceability chain for Pu chemical analysis

Standardization bodiesAFNOR / BNEN

Metrology organizationsIRMM

Reference Materials producersCETAMA MP2

(Pu purity : 0,9990 ± 0,0004 kg/kg)

PTS providersCETAMA EQRAIN Pu

(Ref value : 4,1620 ± 0,0042 g/kg)

Field laboratories

ISO guide 34 +ISO 17025 Accreditation ISO 17043 Accreditation

ISO 17025 Accreditation

NF ISO 8298:2001


Conclusions

• In nuclear industry (and related activities like environment survey, medical applications…) there is a need for radionuclide and chemical standards and for reference materials

• As these topics are highly sensitive, the quality of these standards must be very high and traceable. This traceability must be transparent and available (e.g. publication by BIPM)

• In both fields, there is a need of (high quality) reference materials

• Unfortunately there is a lack of accredited calibration laboratories in both fields and this could jeopardize the traceability chain

metrology of radionuclides and chemical metrology

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