Elemental characterization of the Avogadro silicon crystal WASO 04 by neutron activation analysis

Abstract

Analytical measurements of the 28 Si crystal used for the determination of the Avogadro constant are essential to prevent biased results or under-estimated uncertainties. A review of the existing data confirms the high-purity of silicon with respect to a large number of elements. In order to obtain a direct evidence of purity, we developed a relative analytical method based on neutron activation. As a preliminary test, this method was applied to a sample of the Avogadro crystal WASO 04. The investigation concerned twenty-nine elements. The mass fraction of Au was quantified to be (1.03 ± 0.18) × 10 -12 . For the remaining twenty-eight elements, the mass fractions are below the detection limits, which range between 1 × 10 -12 and 1 × 10 -5 .

Instrumental neutron activation analysis of an enriched 28Si single-crystal

“…The method was preliminary tested on a sample of the Avogadro natural silicon crystal WASO 04. The investigation concerned twenty-nine elements and reached detection limits ranging between 1 pg/g and 10 g/g [12].…”

Accurate measurements of the Avogadro and Planck constants by counting silicon atoms

“…Silicon occurs in three isotopes - 28 Si, 29 Si, and 30 Si. Therefore, a molar mass measurement to within a 10 −8 M accuracy requires that the fractions of the minority isotopes 29 Si and 30 Si -about 0.05 and 0.03, respectivelyare determined to within a relative accuracy better than 10 −5 .…”

“…Therefore, a molar mass measurement to within a 10 −8 M accuracy requires that the fractions of the minority isotopes 29 Si and 30 Si -about 0.05 and 0.03, respectivelyare determined to within a relative accuracy better than 10 −5 . However, insuperable difficulties have impaired the efforts to achieve this accuracy.…”

“…There is a general consensus that the contamination by most of the elements is significantly smaller than 10 −9 parts of an Si atom. In order to gain a direct evidence of purity, an analytical method based on neutron activation is being developed to exclude trace contaminations [29]. Microdefects in dislocation-free Si crystals are related to both vacancies and self-interstitial atoms.…”

Neutron activation analysis of the30Si content of highly enriched28Si: proof of concept and estimation of the achievable uncertainty

“…x( 28 Si)/(mol mol -1 ) x( 29 Si)/(mol mol -1 ) x( 30 Si)/(mol mol -1 ) AVO28 parts PTB [6] 0.999 957 50 (17) 0.000 041 21 (15) 0.000 001 290(40) 5, 8 PTB [7] 0.999 957 26 (17) 0.000 041 62 (17) 0.000 001 120(06) 5, 8 NRC [8] 0.999 958 79 (19) 0.000 040 54 (14) 0.000 000 670(60) 5, 8 NIMIJ [9] 0.999 957 63(07) 0.000 041 20 (14) 0.000 001 180(69) 5, 8 NIST [10] 0.999 957 703(38) 0.000 041 229(37) 0.000 001 069(06) 5, 8 Table 1. Measured isotopic compositions of the AVO28 material with their associated combined standard uncertainties.…”