Terrestrial Thermal Neutrons

The first NIST-traceable measurements of environmental thermal neutrons have been completed. The average sea level flux of thermal neutrons is 4 cm 2/hr. This flux changes less than two times over wide varieties of terrain, in all weather conditions and over the solar year. The neutron flux is normalized to 39N-76.5W. This flux scales with geomagnetic Lat/Long and with altitude in a manner similar to the flux of incident energetic cosmic rays. The shielded flux has been measured under 450 g/cm2, with a residual flux of 0.1 times the sea-level value. Preliminary shielding studies inside buildings and under water shows significant variability in the thermal neutron mean attenuation coefficient.