Computational Assessment of Naturally Occurring Background Radiation Produced by Extraterrestrial Sources

摘要

This assessment of the variation in extraterrestrial-induced neutron background on the surface of Earth concludes that it would at best be difficult, and maybe impossible, to create a tool that can model background in real time for operational purposes because such a tool would need to account for many different measurement environments. Several aspects of the measurement environment can be accounted for more easily than others: geographic location, elevation, atmospheric pressure, humidity, solar cycle, and detector type. However, this assessment has shown that outside of elevation, the largest challenge for a tool to model background for operational purposes is Earth's surface composition, which varies dramatically in different locations and can vary locally based on recent weather patterns. Since MCNP6 was able to accurately model the extraterrestrial-induced neutron background, it seems more likely that a tool to model background could be created for measurements performed in a fixed location rather than measurements with dynamic location because there is less variability in the measurement environment with a fixed location. In reality, surface composition is only the first indicator of a more complex problem. Not only does one need to account for the surface composition, but most other massive features in the measurement environment (e.g., man-made structures) need to be considered. To better understand this, consider the data plotted in Fig. 5. The curve "base case - total" is the same as "base case" in Fig. 4, i.e. this is the neutron background flux 1 m above Earth's surface for the base computational model defined in Table I. The curve in Fig. 5 "base case - down" is the background neutron flux 1 m above Earth's surface that was created in the atmosphere without any interactions with the Earth or other objects on Earth's surface (i.e., man-made structures). These data were calculated by giving the cells in the MCNP6 geometry that model Earth zero importance. Fig. 5 shows that about 80 percent of the extraterrestrial-induced neutron background flux is dependent on Earth's surface composition or other objects on Earth's surface, particularly for neutrons below 100 MeV. To accurately account for all the possible variations of Earth's surface would be nearly impossible. These types of surface variations need to be modeled explicitly in a radiation transport calculation. MCNP6 can be used to model these variations and accurately calculate the extraterrestrial-induced neutron background, but not in real time with today's computers.