ISOTOPE-DILUTION MASS-SPECTROMETRY OF B-10 AND (LIF)-LI-6 REFERENCE DEPOSITS USED FOR THE DETERMINATION OF THE NEUTRON LIFETIME

The accurate characterization of B-10 and 6LiF reference deposits is of the highest importance in absolute neutron counting, e.g. for the determination of the lifetime of the free neutron [J. Byrne et al., Phys. Rev. Lett., 65 (1990) 289]. A series of B-10 and 6LiF deposits, with different surface densities, were prepared on silicon substrates by vacuum deposition and analyzed by isotope dilution mass spectrometry (IDMS), subsequent to their relative characterization by neutron induced charged particle counting and determination of their surface area. The IDMS results were performed to calibrate the (n,alpha) measurements. They carry a total uncertainty (1s equivalent) on the number of B-10 atoms in the deposit of 0.48% on Series I (without chemical purification) and 0.58% on Series II (with chemical purification). The corresponding uncertainty on the number of Li-6 atoms in the deposit is 0.25%. The systematic components of these uncertainties are 0.12% on the B-10 content and 0.21% on the Li-6 content. An intercomparison on the basis of the count rates of the (n, alpha) measurements, the surface areas and the IDMS results shows an excellent agreement between the results for B-10 and Li-6.