Iodine-131, #t_(1/2)# = 8.0 days, is used in treatment of thyroid gland diseases. If a sample of iodine-131 initially emits #9.95 x 10^18 beta# particles per day, how long will it take for the activity to drop to #6.22 x 10^17 beta# particles per day?
The expression for 1st order decay is:
Taking natural logs of both sides:
The activity of a sample is proportional to the number of undecayed atoms so we can write: