Optimized geometry for bulk coal PGNAA with external moderation of the source neutrons

A simulation study of a PGNAA system for the analysis of coal with a ²⁵²Cf neutron source has been carried out using the MCNP code to investigate the effect on the system response of variable thickness, density and composition of the sample when the source is placed at the center of a moderating sphere of polyethylene. Results for different radii of the moderating sphere are reported. The results show that the geometry of the measuring arrangement can be chosen so that the γ count rate/wt % of an arbitrary element (including hydrogen) is independent both of the hydrogen content of the coal, ω_H, expressed as a mass fraction, and of the bulk density, d, or volume hydrogen content of the sample ν_H, at least in the range of ω_H-values commonly found in bituminous coals. The system response, defined as the magnitude of the output signal/wt % of an arbitrary element in the coal composition is then independent of composition and bulk density of the coal sample.