Question

what is the half life of strontium -90

Answer 1

this belongs either in chemistry, or biology. people here won't have a clue.

Answer 2

i can try though.

Answer 3

oh my bad this is nuclear science. Well then, okay.
For each isotope, the amount is takes to decay into a stable one is different with each one. To determine its half life in theory would be relatively easy. Considering you would use proportions to find the answer by setting up one where the rate is over its half life and setting that equal to the "other half life" which is really the stopping point to where it ends up becoming an entirely stable isotope. the top value above that would be the value you would find.
First you need to tell me how -90 affects the isotope, and what it can tell us. Then from there you can go.