Spatial Distribution of Terrestrial Neutron Flux in a Five-story Concrete Building

The terrestrial neutron flux and its energy distribution were systematically measured at multiple locations within a five-story concrete building to investigate their spatial variation. The measurements showed that the neutron flux gradually decreases from the fifth floor (top floor) to the first floor (ground level) due to attenuation in the concrete structure, with an overall reduction of about 62% for neutrons above 1 MeV. A lateral gradient was also observed: the neutron flux decreased from the window side toward the aisle side on each floor, showing an attenuation of about 44% for neutrons above 1 MeV on the first floor. Neutron transport simulations using the PHITS code were performed with the PARMA model as the source term, including albedo neutrons from the ground. The simulated neutron fluxes and energy spectra agreed well with the measurements, successfully reproducing both the attenuation trends and the absolute flux values within 10% for the integrated neutron flux from thermal energies up to 10 GeV.