Question

In a hypothetical radioactive material, the total number of active photons are 20000 and the decay constant is found out to be 4.916 X 10^-17 per second. How much of the material will be left in a 100 years? (1 year = 365 days. Leap year is not assumed in the calculations)

(a) 1589.99999

(b) 19,999.9999

(c) 19.999999

(d) 123.99999

This question was addressed to me during an online interview.

The question is from Diathermy, Measurement of Radiation, Dose and its Calculations topic in division Radiation Therapy of Clinical Science

Answer 1

The correct choice is (b) 19,999.9999

Explanation: N = N0 e ^-λt where N = particles left after decay, N0 is the original number of particles, t is the time duration and λ is the decay constant. Thus, putting the values in the formula, we get :

N = 20000 * e^(- (4.916* 10^-17) * 365*100*24*60*60)= 19999.99999.