Question

Strontium 90 (sr-90, a radioactive isotope of strontium, is present in the fallout resulting from nuclear explosions. it is especially hazardous to animal life, including humans, because, upon ingestion of contaminated food, it is absorbed into the bone structure. its half-life is 27 years. if the amount of sr-90 in a certain area is found to be four times the "safe" level, find how much time must elapse before the safe level is reached.

Answer 1

This is a relatively easy half-life problem.
Whenever a half-life has expired, one half of the radioactive isotope will no longer be present.
When 2 half-lives have occurred, then one quarter of the isotope remains.
Therefore, the strontium - 90 will be at a safe level in 54 years.