64
90
120
108
Given : T1/2 = 18 days
Let the permissible activity after time t be Ro
∴ Initial activity of the radiation, R = 64Ro
Using : 0.693/T1/2 * t = 2.303 log10(R/Ro)
∴ 0.693/18 * t = 2.303 log10(64)
0.693/18 * t = 2.303 × 1.806
∴ t = 108 days