Answer :

a) The plot of binding energy per nucleon (BE/A) versus the mass number A is given below:

From the plot we note that

__i) During nuclear fission__

A heavy nucleus in the larger mass region ( A>200) breaks into two middle level nuclei, resulting in an increase in B.E/ nucleon. This results in a release of energy.

Example:

_{92}U^{235} + n^{1}→ _{56}Ba^{144} + _{36}Kr^{89} + 3_{0}n^{1} + energy

__ii) During nuclear fusion__

Light nuclei in the lower mass region (A<20 ) fuse to form a nucleus having higher B.E / nucleon. Hence Energy gets released.

Example:

_{1}H^{2} + _{1}H^{2}→ _{2}He^{4} + energy

b) Let the initial activity be A_{o}.

Let the time taken for the activity to reduce = t.

Half live is given T_{1/2} = 10years

If we take A_{o} = 100, then activity after time t will be

A_{t} = 3.125 (given)

We know the law of radioactivity decay is given as

A_{t} = A_{o} e^{-λt} ….(i)

Where λ = decay constant.

Therefore, substituting the values of At Ao and λ in equation (i),

3.125 = 100e^{-0.0693t}

Taking natural logarithm on both sides of above equation

∴ the time taken for the activity to reduce to 3.125% is 50 years.

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