# 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?

##### 1 Answer

May 21, 2017

#### Answer:

32 days.

#### Explanation:

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: