Question

Let T be the mean life of a radioactive sample. 75% of the active nuclei present in the sample initially will decay in time 

(a) 2T 

(b) 1/2 (In 2)T 

(c) 4T 

(d) 2(In 2)T 

Answer 1

Correct Answer is: (d) 2(In 2)T 

When 75 % decays, 25 % is left undecayed. This requires a time

t = 2T_1/2 , where T_1/2 = half-life = In 2/λ. Also, T₁/λ.

∴ t = 2 (In2/λ) = 2(In 2)T.