In the 1986 disaster at the Chernobyl reactor in the Soviet Union (now Ukraine), about 18 of the 137Cs present in the reactor was released. The isotope 137Cs has a half-life for β decay of 30.07 y and decays with the emission of a total of 1.17 MeV of energy per decay. Of this, 0.51 MeV goes to the emitted electron and the remaining 0.66 MeV to a γ ray. The radioactive 137Cs is absorbed by plants, which are eaten by livestock and humans. How many 137CS atoms would need to be present in each kilogram of body tissue if an equivalent dose for one week is 3.5 Sv? Assume that all of the energy from the decay is deposited in that 1.0 kg of tissue and that the RBE of the electrons is 1.5.