Question: Q. 9. (i) Derive the mathematical expression for the radioactive decay for a sample of a radioactive nucleus.

(ii) How is the mean life of a given radioactive nucleus related to the decay constant? U [OD North 2016]

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Solution:

Ans. (i) Let there be N0 radioactive nuclei at t=0

If N is the number of nuclei left over at t=t, we get

or

dNdtN dNdt=λN

(λ= decay constant )12

(ii) The half life of ${ }{92}^{238} \mathrm{U}undergoing\alphadecayis4.5 \times10^{9}years.Determinetheactivityof10 \mathrm{~g}sampleof{ }{92}^{238} \mathrm{U}.Giventhat1 \mathrm{~g}of{ }_{92}^{238} \mathrm{U}contains25.3 \times 10^{20}$ atóms.

R A [OD Comptt. I, II, III 2014]

(i) Try yourself, Similar to Q. 7, Short Answer Type Questions-II of this topic.

The S.I. unit of ‘activity’ is becquerel.

(ii) Try yourself, Similar to Q. 8, Short Answer Type Questions-II

Commonly Made Errors

  • Few students don’t recall the correct formula.
  • A number of students do calculation mistake/put wrong values in the formula.


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