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

Abstract

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.

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

“…In summary, the results obtained in this study point out that the purity level of the new material is even higher than that of the material used for the latest determination of the Avogadro constant, called AVO28. 9 In detail, with the exception of W, reduced amounts of Cr, Co, Cu, Ga, As, Br, La, Au and no traces of Sc, Mn, Fe, Zn, Sb, Ir were detected. For many of the remaining elements, the analysis achieved detection limits of 1 order of magnitude lower.…”

“…With respect to point defects, previous Avogadro silicon materials were measured using positron lifetime and copper decoration to determine vacancies. , In addition, infrared spectroscopy was used to quantify carbon, oxygen, boron, nitrogen, and INAA to probe a large number of the remaining elements. − …”

“…In this study, the k 0 -method of standardization was applied to determine the largest possible number of elements using a monoelemental standard. A full description of the fundamental principles of the k 0 -method can be found elsewhere, while the measurement model we adopted to estimate the mass fraction of a target isotope is the eq 4 reported in a earlier work together with details on the experimental parameters …”

Naturally occurring 32Si and low-background silicon dark matter detectors

“…Over the last 25 years a research program -often collectively referred to as the "Avogadro Project" -has focused on redefining the kilogram using monocrystalline silicon that is highly enriched in 28 Si [54][55][56][57][58][59], encompassing a variety of materials and metrological techniques for achieving an improved, lower-uncertainty definition of the kilogram mass. The use of a single crystal of 28 Si with ultra-high purity, both chemically and isotopically, has the following benefits for metrology: i) Minimizing chemical impurities reduces uncertainty in lattice defects and total crystal mass [60]; ii) Minimal lattice defects and the physical regularity of the crystal structure allow for high-precision atom counting [55]; and iii) Having high isotopic purity (and thus a single atomic mass) further reduces uncertainty when estimating the total mass of an enriched crystal [61]. This research program is made possible via gaseous ultracentrifugation followed by chemical reduction to metal [55,62].…”

Quantification of the Void Volume in Single-Crystal Silicon

“…The presence of contaminants was extensively investigated using infrared spectrometry and neutron activation analysis and corrected for or demonstrated to be negligible. 2,3 The crystal used was grown in vacancy mode; the vacancy concentration, N vac = 3.3(1.1) × 10 14 cm −3 , was measured by positron lifetime spectroscopy. 4,5 Typical grown-in defects are voids formed by vacancy aggregation; cavities bigger than 50 nm in diameter were excluded by laser scanning topography.…”