\(_{92}^{238}U\) is known to undergo radioactive decay to form \(_{82}^{206}Pb\) by emitting alpha and beta particles. A rock initially contained 68 × 10–6 g of \(_{92}^{238}U\) . If the number of alpha particles that it would emit during its radioactive decay of \(_{92}^{238}U\) to \(_{82}^{206}Pb\) in three half-lives is Z × 1018 , then what is the value of Z ?