The accurate assay of radionuclides, i.e. unstable radioactive nuclides, is essential in several applications: environmental monitoring, nuclear forensics, safeguards, irradiated samples characterization for burnup assessment, radiotherapy, radioactive reference material fabrication or process control monitoring. For example, uranium and plutonium accountancy is of prime importance in the nuclear fuel cycle and for safeguards purposes. Isotope Dilution associated with Mass Spectrometry measurement (ID-MS) is a powerful tool for quantification and is based on the internal calibration principles. The International Atomic Energy Agency (IAEA) considers ID-MS one of the most accurate methods in terms of the International Target Values (ITVs) for measurement uncertainties applied to safeguarding nuclear material, and ID-MS is listed as a primary method [1, 2].

The Commission for the Establishment of Analytical Methods (CETAMA) of the French Alternative Energies and Atomic Energy Commission (CEA) aims to improve measurement quality in the nuclear field. One CETAMA working group is dedicated to ID MS, bringing together several national and international scientific bodies (CEA, French Institute for Radiation Protection and Nuclear Safety (IRSN), Joint Research Centres (Europe), IAEA, Belgian Nuclear Research Centre (SCK CEN)) sharing theirs expertise in radionuclides analysis by ID MS. This cooperation, which took place over the period 2019-2024, resulted in the publication of a jointly edited critical review with definitions of best practices [3].

This presentation will highlight the good laboratory practices recommended by the members of the ID MS working group. A careful implementation is required to achieve the best performance for ID MS: this includes the weighing procedure, the spike management and the isotopic homogenization. An optimum of the (sample – spike) amount ratio is required to obtain the best accuracy. This is obtained when the (sample – spike) mixture isotope ratio (R_M) is equal to the square root of the product of the sample (R_S) and spike (R_T) isotope ratio: R_M = √(R_S×R_T). Furthermore, the choice of isotope dilution tracer is crucial: the corresponding isotope must be very low in abundance in the sample and not be affected by isobaric interferences. A content of at least 90% of the isotope dilution isotope in the tracer solution is recommended. The ID MS uncertainty can be evaluated using the quadratic cumulation method (recommended in GUM [4]) and the Monte Carlo method (GUM-S1 [5]): approaches yield identical. As calculations are easy and quick with R or Python softwares, the Monte Carlo method is recommended. To conclude, two applications regarding the measurement of ²²⁶Ra in the environment and the measurement of actinides in irradiated UAmO₂ transmutation pellets will be taken as examples [4, 5].

References

[1] STR368 (Revision 1.1), IAEA report, 2022

[2] M. J. T. Milton, Metrologia, 2001, 38(4), 289-296

[3] A. Quemet, J. Anal. At. Spectrom, 2024, 39, 1665.

[4] JCGM, JCGM 100:2008

[5] JCGM, JCGM 101:2008

[6] M. Boudias, Microchemical Journal, 2024, 110971.

[7] A. Quemet, Journal of Radioanalytical and Nuclear Chemistry, 2022, 331, 1051-1061.