Short BF3 Counters and Their Use with Spherical Moderators: Physical Characteristics and Choice of Useful Thicknesses of the Moderator

The anisotropy of standard BF3 counters placed in cylindrical moderators is considerable, and in scattered neutron fluxes may lead to measurement errors of as much as a factor of 2. To eliminate this defect, the authors designed a short BF3 counter having both satisfactory sensitivity and a small useful volume. The exact dimensions of this volume were determined by studying the collection regions (through analysis of the electric field present in the counter). These counters are placed in paraffin spheres whose useful thickness varies according to whether fluence or dose-equivalent measurements are to be made. Tine paper describes the limits of the method and discusses the overall accuracy of measurement for a number of spectra commonly obtained at the installations, to clarify the terms in which the results are interpreted. The attractiveness of these counters, compared with lithium-iodide scintillators, for measuring neutrons in the presence of gamma radiation is illustrated by an assessment of the maximum electron-energy loss in the two instruments. (author)