Figure 1. Schematic explanation of the relation of radioactivity on the ground to the brightness measured by ETCC.

(a) Schematic explanation of the correlation between emissivity (brightness) (Σ) on the ground and the measured gammas (dB) within a unit solid angle in the FoV of the ETCC, where A1 and A2 indicate the areas on the ground and ETCC, respectively, and D denotes the distance between the ground and the ETCC. (b) Schematic view of the positional relation of the ground, dosimeter (Horiba PA-1100) and tungsten rubber for our 1-cm dose measurement. The top and four sides of the dosimeter are covered by tungsten rubber to shield it from the downward gamma-ray radiation.