Monte Carlo simulation analysis of X-ray density logging shields

Abstract

BackgroundThe use of controlled X-ray sources instead of 137Cs radioactive sources in density logging has become a new trend. The intensity of the X-ray source is substantially influenced by the high voltage on the target substrate, and the density measurement uncertainty can be maintained at 0.01 g·cm-3 when the high voltage is 350 kV.PurposeThis study aims to analyze the parameters of the shielding material and thickness suitable for the 350 kV high-voltage X-ray density logging instrument.MethodsThe Monte Carlo method was used to analyze the energy spectrum and counting rate of X-rays passing through different materials and thicknesses. By comparing the correlation between the 0~0.15 MeV and 0.15~0.35 MeV energy windows, the reasons for the difference between the X-ray attenuation and detector count rate in different energy windows were determined. In addition, combined with the actual instrument model construction of the four-detector X-ray density logging instrument, the influence of the three parts of particles on the detector was primarily considered. The placement mode and optimal thickness of each part of the shield for detectors were analyzed and designed using Monte Carlo N-particle (MCNP) simulation.ResultsThe simulation results show that the attenuation of X-rays in high- and low-energy windows increases with increase of atomic number and thickness of shielding materials. When tungsten nickel iron alloy is selected as the shielding material for the four-detector X-ray density logging instrument model, the suitable thickness of the shield between the base and the near-source detector is 1.75 cm. Meanwhile, to maintain the high voltage of X-ray generator at 350 kV, a shield layer with a thickness of 0.2 cm is placed between each detector, and a shield layer with a thickness of 0.35 cm is added to the back of the detector.ConclusionsThis study provides the design theory and key parameters for shielding materials and structures in the development of X-ray density logging tool.

Keywords

• controllable source
• x-ray
• density logging
• monte carlo simulation
• shield