MASS ASSAY AND UNIFORMITY TESTS OF BORON TARGETS BY NEUTRON BEAM METHODS

The areal densities (g/cm 2) of enriched boron and lithium fluoride ta rgets were compared to those of reference deposits from the Central Bu reau for Nuclear Measurements (CBNM, Geel, Belgium) by charged-particl e counting at the NIST Research Reactor. The uniformity of the target thicknesses was also investigated by scans with a small-diameter, inte nse neutron beam. The neutron uniformity measurements agree satisfacto rily with previously reported interferometric results and no pattern o f uniformity variation except the expected, small radial dependence wa s found. In a separate series of measurements, the boron areal density ratios of a number of the deposits were determined using a new double ionization chamber. The new double ionization chamber permits a compa rison of two targets directly without a beam monitor and without sensi tivity to the exact placement of the samples. The new device needs onl y a very simple data acquisition system and requires less neutron beam intensity than the previously employed systems with arrays of surface barrier detectors. The ionization chamber counting losses from the co mbined effects of self-absorption and sub-threshold pulse-height forma tion were determined by comparison with ratio measurements made by the conventional surface barrier detector technique.