Optimation of PGNAA instrument design for cement raw materials using the MCNP code

Abstract

A simulation study of a PGNAA measuring arrangement with a²⁵²Cf neutron source for the characterization of cement raw materials was carried out using the MCNP code to investigate the effect on the system response of varying the bulk density ρ and the water contentv _w of samples of fixed dry composition. The source is placed at the centre of a lead cylinder of extemal radiusR _Pb. This cylinder is enclosed in a coaxial cylinder of polyethylene moderator of extermal radiusR _in. The sample material is confined to the space between an outer cylindrical surface of radiusR _out and the moderator cylinder. The first series of simulation runs was carried out for different sets of values ofR _in,R _out and ρ, and constantv _w. The system specific responseS (count rate per wt%) shows a broad maximum aroundT _M=R _in-R _Pb=4 cm andT _S=R _out-R _in=8.5 cm and a dependence on ρ that is almost linear in the region of the maximum. In a second series of runs the dependence ofS onv _w was studied for a geometry corresponding to a real experiment described elsewhere and for a geometry for which the density saturation effect is already apparent. It is shown that when the sensitivity of the system is optimized both ρ andv _w must be used as calibration parameters while in the low-sensitivity design regionv _w can be used as the only calibration parameter.