Bulk material interrogation experimental results and validation with Geant4 for replacement of dangerous radiological sources in oil-well logging industries.

The Kansas State University Materials Interrogation (KSUMI) test facility was set up to enable bulk-material irradiation experiments that replicate similar oil-well logging scenarios, with an aim to address the problem of replacement of conventional radioisotope sources commonly used in oil-well logging industries. An exploration tool similar to an oil-well logging tool was used to conduct experiments with water and sand as testing materials. The facility includes a 2500-gallon concrete test chamber with an aluminum pipe going horizontally through it. A machine-based 14.1 MeV deuterium-tritium neutron source as an alternative to conventional neutron sources was used. High energy neutrons assist in the investigation of a larger volume of material and also generate high energy inelastic scatter gamma rays, which provide useful information on composition. Experiments were performed with tap water and sand as a bulk testing material. Irradiation was done for one hour and results were obtained from a He neutron sensor, a BF neutron sensor, and two NaI gamma sensors placed at different locations within the exploration tool. Geant4, a Monte-Carlo based toolkit, was deployed on a high-performance computing system to simulate the entire experiment in order to benchmark the experimental responses obtained from the photon and neutron sensors. The facility was modeled in detail with accurate dimensions and material compositions. Materials such as tap water, high-density polyethylene, and aluminum metal were modeled with thermal neutron scattering cross-sections. The reference physics list QGSP_BIC_HP along with G4NDL and S(α,β) cross-sections were found to be appropriate for simulation of neutron interrogation experiments with neutron energies lower than 20 MeV. The experimental results obtained were successful in characterizing the bulk testing materials, and results obtained from Geant4 were found to be in good agreement with the experimental results in most cases.