Prompt (n,γ) Mass Measurements for the AVOGADRO Project

The aim of the AVOGADRO project is to replace the kilogram artefact by a high-purity, perfect single crystal of natural or isotope-enriched silicon. The isotopic composition and the impurities of the silicon crystal must, therefore, be known with highest possible accuracy and precision. The only method to obtain all this information without destruction of the massive samples is prompt (n,γ)-spectrometry. The measurements are performed at a thermal neutron guide of the ILL (Institut Max von Laue Paul Langevin) in Grenoble, France. The spectrometry of γ-radiation emitted by a nucleus promptly after thermal neutron capture allows a highly precise determination of atomic mass differences, as well as the determination of isotope abundances leading to the molar mass. The uncertainties assigned to the results for the respective atomic masses determined by the mass differences amount to up to 10⁻¹⁰, while the molar mass of an isotope-enriched Si single crystal has so far been determined with an uncertainty of 1 ⋅ 10⁻⁴. A direct comparison (for example, relative value of isotope abundances determined by (n,γ)-spectrometry omitting the thermal neutron cross section) furnishes a value of 7 ⋅ 10⁻⁵. The final aim of the AVOGADRO project is to provide a well specified crystal, which allows a more accurate value of the Avogadro constant to be determined. This constant is the key input parameter for tabulated values of fundamental constants and for a new definition of the unit of mass - the kilogram itself. This revised version was published online in July 2006 with corrections to the Cover Date.