After nuclear power plant disaster, the following data (in counts-per-second, cps, vs. time in days) were collected with a Geiger counter. The data show a bi-exponential decay resulting from radioactive decay of ¹³¹I (half-life: 8.02 days) and an unknown radioactive substance. The initial count attributable to ¹³¹I is 453.7 cps. Determine the half-life of the unknown substance in days. You may use a spreadsheet program to solve this problem.

 

time [day] cps 366.7 6. 231.86 12 150.07 20 88.04 36 36.98 60 15.94 100 6.49 150 2.41 220 0.61