Mass assay and uniformity tests of boron targets by neutron beam methods

Abstract

The areal densities (g/cm 2) of enriched boron and lithium fluoride targets were compared to those of reference deposits from the Central Bureau for Nuclear Measurements (CBNM, Geel, Belgium) by charged-particle counting at the NIST Research Reactor. The uniformity of the target thickness was also investigated by scans with a small-diameter, intense neutron beam. The neutron uniformity measurements agree satisfactorily with previously reported interferometric results and no pattern of uniformity variation except the expected, small radial dependence was 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 comparison of two targets directly without a beam monitor and without sensitivity to the exact placement of the samples. The new device needs only 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 combined effects of self-absorption and sub-threshold pulse-height formation were determined by comparison with ratio measurements made by the conventional surface barrier detector technique.