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.
92U235 + n1→ 56Ba144 + 36Kr89 + 30n1 + 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.
1H2 + 1H2→ 2He4 + energy
b) Let the initial activity be Ao.
Let the time taken for the activity to reduce = t.
Half live is given T1/2 = 10years
If we take Ao = 100, then activity after time t will be
At = 3.125 (given)
We know the law of radioactivity decay is given as
At = Ao 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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