1)By mass spectral analysis, a sample of strontium is known to contain 2.64x1010 atoms of Sr-90 as the only radioactive element. The absolute disintegration rate of this sample is measured as 1238 disintegrations per minute. Calculate the half-life (in years) of Sr-90.
2)How long will it take (in years) for the disintegration rate of this sample to drop to 954 disintegrations per minute?