Shielding Materials for Low Radioactive Background Projects - 21330

The design of shielding materials from high energy cosmic-ray particles is crucial for the reduction of radioactive background in radiation detection and measurements. The main concern is from the interaction of high energy particles such as protons, neutrons and muons with the shielding materials leading to production of secondary high energy neutrons that can contribute to the signal as radioactive background. The contemporary scientific literature, both experimental and computational, has scarce data on the induced leakage of the secondary neutrons from the shielding materials in the energy range of 0-20 MeV. In this work, an assessment of the various shielding materials was performed using the GEANT4.10.06 Monte Carlo (MC) simulation toolkit, and the Cosmic-Ray Shower Library (CRY, LLNL). The investigated materials suitable for shielding such as iron, copper, concrete, lead, polyethylene (PE), and water were selected in the studies to determine the most efficient material or a composite of the materials that will help substantially reduce the radioactive background due to secondary neutrons. All materials and their composites were simulated according to their dimensions considering different thicknesses. The simulations have shown that iron, copper and lead seem to be very efficient shields, but the cost of the ultimate shield made from them will increase with increasing dimensions and thicknesses. Neutron stopping is a two-step effect: slowing down of fast neutrons and absorption of thermal ones. The secondary neutron flux formed from primary protons and neutrons escaping from the iron, copper or lead decreases in the energy range of 0-20 MeV with increasing thickness of these materials which is caused by large scattering cross-sections in the high-density materials leading to the large energy loss for each scattering effect. It was shown, based on these studies, that the optimal and cost-effective shielding is a composite of iron backed by concrete. When the iron is backed with concrete, secondary high energy neutrons attributed to primary protons and neutrons scatter off of the iron nuclei via elastic and inelastic processes, and they are captured in the concrete when the energy of the neutrons reduces to thermal energies. Overall, the neutron fluxes in the energy range of 0-20 MeV substantially decrease. This article demonstrates pros and cons of the investigated shielding materials using detailed MC simulations. (authors)