Giancarlo D’Agostino

30Si Mole Fraction of a Silicon Material Highly Enriched in 28Si Determined by Instrumental Neutron Activation Analysis

The latest determination of the Avogadro constant, carried out by counting the atoms in a pure silicon crystal highly enriched in (28)Si, reached the target 2 × 10(-8) relative uncertainty required for the redefinition of the kilogram based on the Planck constant. The knowledge of the isotopic composition of the enriched silicon material is central; it is measured by isotope dilution mass spectrometry. In this work, an independent estimate of the (30)Si mole fraction was obtained by applying a relative measurement protocol based on Instrumental Neutron Activation Analysis. The amount of (30)Si isotope was determined by counting the 1266.1 keV γ-photons emitted during the radioactive decay of the radioisotope (31)Si produced via the neutron capture reaction (30)Si(n,γ)(31)Si. The x((30)Si) = 1.043(19) × 10(-6) mol mol(-1) is consistent with the value currently adopted by the International Avogadro Coordination.

Measurement of the neutron flux parameters f and α at the Pavia TRIGA Mark II reactor

In this paper, evaluation of neutron flux parameters of TRIGA Mark II reactor in Pavia was carried out. For any of the three irradiation positions investigated, this work represented the first experimental evaluation of α. Moreover, in addition to α, values of other parameters such as f, Φth and Φe were also calculated and compared with the existent literature data from other TRIGA Mark II reactors and the Pavia’s facility. Results obtained in the present study represent a mandatory step ahead for future application of k0-Neutron Activation Analysis method (k0-NAA) at Pavia’s facility.

Purity of 28Si-Enriched Silicon Material Used for the Determination of the Avogadro Constant

At present, counting atoms in a one-kilogram sphere made of (28)Si-enriched silicon allows the determination of the Avogadro constant with the 2.0 × 10(-8) relative standard uncertainty required for the realization of the definition of the new kilogram. With the exception of carbon, oxygen, boron, nitrogen, and hydrogen, the claimed uncertainty is based on the postulation that the silicon material used to manufacture the sphere was above a particular level of purity. Two samples of the silicon were measured using instrumental neutron activation analysis to collect experimental data to test the purity assumption. The results obtained in two experiments carried out using different research reactor neutron sources are reported. The analysis confirmed that the silicon material was of sufficient purity by quantifying the ultratrace concentration of 12 elements and determining the detection limits of another 54 elements.

An uncertainty spreadsheet for the k0-standardisation method in Neutron Activation Analysis

This paper focuses on the use of the spreadsheet technique to set up the uncertainty budget for the k0-standardisation method in Neutron Activation Analysis. The adopted measurement model included most of presently recognized error sources and was written to limit the covariances between input quantities. The calculations were implemented in a worksheet file and tested in a multi-elemental analysis of a biological material. Besides, it was demonstrated that the k0-standardisation turns to the relative-standardisation when the monitor element corresponds to the analyte element. The developed worksheet is available and suitable for the analysis of other materials in different experimental conditions.

Measurement of the 30Si Mole Fraction in the New Avogadro Silicon Material by Neutron Activation and High-Resolution γ-Spectrometry

The use of new silicon single crystals highly enriched in 28Si recently produced for the upcoming redetermination of the Avogadro constant requires knowledge of their molar masses. The isotopic composition data are collected independently in different laboratories but all using the virtual element technique with multicollector inductively coupled plasma mass spectrometers. In this framework, the comparison of the results with an independent measurement of the amount of at least one of the depleted isotopes is useful to limit hidden systematic errors. To this aim, the 30Si mole fraction of a sample of the new material was measured using a relative measurement protocol based on instrumental neutron activation analysis. The protocol is similar to that previously applied with the AVO28 silicon material used for the last determination of the Avogadro constant value with the exception that unknown and standard samples are not coirradiated. The x(30Si) = 5.701 × 10-7 mol mol-1 estimate is close to the expected one and is given with a standard uncertainty of 8.8 × 10-9 mol mol-1. This value, if adopted, gives a contribution to the relative standard uncertainty of the Avogadro constant of 6.3 × 10-10.

Impurities in a 28Si-Enriched Single Crystal Produced for the Realization of the Redefined Kilogram

The practical realization of the unit of mass is possible by manufacturing a perfect one-kilogram sphere from a 28Si-enriched single crystal. The mass of the sphere can be determined in terms of a fixed value of the Planck constant by counting the number of silicon atoms in the core of the single crystal. To reach the target 2.0 × 10-8 relative standard uncertainty, the mass of the surface layer and the mass deficit due to point defects such as impurities and vacancies must be investigated and corrected for. A sample of a 28Si-enriched single crystal produced to test the possibility of obtaining material at a scale useful to the dissemination of mass standards was measured by instrumental neutron activation analysis to check the purity with respect to a large number of possible contaminant elements. The results collected in a neutron activation experiment performed with the high thermal neutron flux available at the 20 MW OPAL research reactor are described. The data collected in this study showed that the produced material has a purity level never achieved with silicon used to manufacture previous one-kilogram spheres.

Erratum to: The linkup of mono-elemental solutions to the SI using INAA: a measurement procedure and the achievable uncertainty

The possibility of using neutron activation analysis to link up a secondary to a primary mono-elemental solution was investigated. A procedure was developed for the determination of the ratio between the mass fractions of two solutions. The use of a monitor element was essential to limit the effect of the non-uniformity of the neutron flux during irradiation. The proposed procedure was tested in the case of two molybdenum solutions having the same mass fraction. Although the experiment did not reach the goal, possible ways are suggested to reach the target expanded uncertainty of 0.1 %.