Prompt (n,gamma) mass measurements for the AVOGADRO project

The aim of the AVOGADRO project is to replace the kilogram artefact by a hi gh-purity, perfect single crystal of natural or isotope-enriched silicon. T he isotopic composition and the impurities of the silicon crystal must, the refore, be known with highest possible accuracy and precision. The only met hod to obtain all this information without destruction of the massive sampl es is prompt (n,gamma)-spectrometry. The measurements are performed at a th ermal neutron guide of the ILL (Institut Max von Laue Paul Langevin) in Gre noble, France. The spectrometry of gamma -radiation emitted by a nucleus pr omptly after thermal neutron capture allows a highly precise determination of atomic mass differences, as well as the determination of isotope abundan ces leading to the molar mass. The uncertainties assigned to the results fo r 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 cryst al has so far been determined with an uncertainty of 1.10(-4). A direct com parison (for example, relative value of isotope abundances determined by (n , gamma)-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 t abulated values of fundamental constants and for a new definition of the un it of mass-the kilogram itself.