Influence of detector size and air gap on the sensitivity of Bonner spheres

The results of a Monte Carlo calculation on the response factors of some Bonner spheres where the central detector is a spherical BF3 counter or LiI detector of varying sizes, are discussed. The best microscopic cross section data available in literature were used for all the constituents of the moderator and detectors. Anisotropy of neutron scattering was taken into account. Calculations show that in the case of a 10 inches sphere, counts due to epithermal neutrons with a central BF3 counter were only 5% of the total counts whereas for a 3 inches diameter polythene sphere epithermal neutrons contributed about 25% of the total counts. Where a LiI scintillator is kept at the centre of a moderating sphere, necessarily an air gap is present between the scintillator and the moderator. The influence of the air gap on the response of the system is studied. (B.G.W.)