Comparison between experimental data and Monte Carlo simulations for a short distance borehole logging probe

The paper contains the main results obtained in the field of borehole logging in coal prospecting using a very low activity gamma source disposed at a short distance from the detector. Such geometry is very suitable from three points of view: it is possible to use a very low activity gamma source of ≅2 - 3 MBq; - the scattered gamma spectrum contains information both about density and about the equivalent order number Zeq; - it is possible to detect the unscattered gamma line and to use it for gain stabilisation. We have used the MCNP-4A Monte Carlo simulation code to simulate the scattered spectra. Calculations were performed for different materials like: coal, sand, carbon, water. A cylindrical geometry was adopted. The influence of the detection system was also taken into consideration. Monte Carlo simulations were compared with experimental data obtained in a cylindrical vessel where a NaI(Ti) scintillation logging probe was placed. The gain stabilisation was achieved by using the direct radiation peak. Three simulated spectra are presented showing the influence of Zeq parameter on the low energy part of the spectrum. The spectra were obtained at the same density and the same number of particle histories. The dependence on the electron density is much simpler, the amplitude throughout the whole spectrum varying like 1/ρ. (authors)