Vertical and horizontal distributions of Cs on paved surfaces affected by the Fukushima Dai-ichi Nuclear Power Plant accident.

Vertical and horizontal distributions are fundamental for sampling and in-situ gamma spectrum measurement strategies. The distributions of Cs were investigated for paved surfaces affected by the Fukusima Dai-ichi Nuclear Power Plant accident. Additionally, the effects of the distributions on the measurement uncertainties of in-situ spectrometry were evaluated. Relaxation mass depth, representing the depth profile of Cs, was estimated to be less than 0.23 g cm. Variation in the relaxation mass depth, of 0.1-0.23 g cm, led to a minor error (less than 5%) in the spectral analysis of theCs inventory (activity per unit area, kBq m). The Cs inventory, within a 20 × 20 m square of 400 cells each measuring 1 m, showed an uneven distribution with large variation; coefficient of variation ranged from 54 to 136% of geometric average inventory of 424 kBq m. Increasing the grid size decreased Cs inventory variation among cells, revealing the relationship between instrument field of view and the spatial uncertainty of the results of in-situ gamma spectrometry.