Cobalt - 61

The half-life of cobalt 61 is 100 minutes. How long does it take for the activity of the sample to fall to 80% of its initial value?

N_t = 0.8N₀

t = ?

half life = 100 minutes

l = ln2/(half life) = 0.693/100 minutes^-1= 0.00693 minutes^-1

0.8 = e^-^lt

Take logs

ln 0.8 = -lt

Substitute for l

ln 0.8 = - 0.00693 x t

Look up the log

-0.223 = - 0.00693 x t

t = 32 minutes