試題詳解

23. A rock contains 0.688 mg of ²⁰⁶ Pb for every 1.000 mg of ²³⁸U present. Assuming that no lead was originally present, which all the ²⁰⁶Pb formed over the years has remained in the rock, and that the number of nuclides in intermediate stages of decay between ²³⁸U and ²⁰⁶Pb is negligible, the age of the rock is calculated to be _____ years. (For ²³⁸U, t_1/2 = 4.5 ×10⁹ years.)
(A) 3.8 × 10⁶
(B) 3.8 × 10⁷
(C) 3.8 × 10⁸
(D) 3.8 × 10⁹
(E) 3.8 × 10¹⁰