Iodine-125 (^125I) is used to treat, among other things, brain tumors and prosta

Question

Iodine-125 (^125I) is used to treat, among other things, brain tumors and prostate cancer. It decays by gamma decay with a half-life of 54.90 days. Patients who fly soon after receiving^125l implants are given medical statements from the hospital verifying such treatment because their radiation could set off radiation detectors at airports. If the initial decay rate (or activity) was 537 mu Ci, what will the rate be after 397.0 days? How many months after the treatment will the decay rate be reduced to 11.5% of the original value? Assume months of 30 days.

Explanation / Answer

I-125 inital activity   = 537 mic. Cu.

half life T1/2 = 54.90 dys

acitivity after 397 days   = A0 *exp(-0.693t/T1/2)

                                        = 537*(exp(-0.693*537/54.9) = 0.611 mic.Cu

decya rate will be reduced to 11.5 % after :

0.115A0 = A0*(exp(-0.693*t/54.9)

t = - ln(0.115) *54.9/0.693

   = 171.34 days

   = 5.71 months

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