Strontium-90 is a beta emitter.  It is one of the radio-nuclides found in the fall out from an atomic bomb explosion.  It can be absorbed into the bone.  It emits beta particles and has a half life of 28 years.  What is the time needed for the activity to fall to 5 % of the original?                                                                                         

We need to know how many half lives gives 5 %

(0.5)^y = 0.05 (P)

y ´ log (0.5) = log (0.05)

y ´ -0.3010 = - 1.3010

y = 1.3010 ¸ 0.3010 = 4.32 half lives (P)

Time taken = 4.32 ´ 28 (P)

Time taken = 121 years (P)