NEUTRON REFLECTIVITY MEASUREMENTS OF TITANIUM-BERYLLIUM MULTILAYERS

Some of the best neutron supermirrors currently in production are made of alternating layers of nickel and titanium, with carbon added to th e Ni to eliminate preferential growth. Reflectivities of 75-95% in the theta(c)-2 theta(c) range are currently obtained from the Ni/C-Ti sys tem. The Ti-Be multilayer system is presented here as a possible alter native to the Ni/C-Ti system. Titanium is one of the few elements whic h has a negative neutron scattering length, while beryllium has a rela tively large positive scattering length. This makes for bilayer struct ures which have excellent neutron contrast, a necessary requirement of supermirror devices. Single-bilayer-spacing Ti-Be multilayers were pr epared by magnetron sputtering onto 2 mm thick silicon (001) substrate s. Depositions were made with 10-80 bilayers at two different bilayer thicknesses of approximately 90 and 110 Angstrom. Bulk layers of Ti an d Be were also prepared to determine the scattering length density of each deposition component. The samples were measured at the Intense Pu lsed Neutron Source (IPNS) at Argonne National Laboratory on the POSYI I reflectometer. First order Bragg peak reflectivities of up to 64% we re observed. Data from the two bulk depositions and one multilayer dep osition have been fitted using a genetic algorithm developed at IPNS. For the bulk depositions, the scattering length densities of Ti and Be depositions are - 2.2 x 10(-6) and 10.3 x 10(-6) Angstrom(-2), respec tively. In addition, results clearly indicate the presence of a 50 Ang strom thick oxide layer on the bulk Ti deposition. The neutron scatter ing length density profile obtained from the neutron reflectivity meas urements are in good agreement with XPS analysis of the oxide him whic h showed a mixture of TiO2 and Ti2O3.