A lecture theatre whose walls, floor and ceiling are made of concrete (10 x 10 x 4 m) has not been aired for several days, a specific activity A from 222Rn of 100 Bq/m is measured. (b) Calculate the activity of 222Rn as a function of the life times of the parent and daughter nuclei. How high is the concentration of 2U in the concrete if the effective thickness from which the 222Rn decay product can diffuse is 1.5 cm?