# NCERT Physics Class 12 Exemplar Ch 13 Nuclei Part 5

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**25** The activity *R* of an unknown radioactive nuclide is measured at hourly intervals. The results found are tabulated as follows:

Plot the graph of

*R*versus*t*and calculate half-life from the graph.(ii) Plot the graph of versus

*t*and obtain the value of half-life from the graph.

Ans:

min (approx)

Slope of graph

So

So min or 40 min (approx)

26. Nuclei with magic no. of proton and magic no. of neutrons and 126 are found to be very stable. (i) Verify this by calculating the proton separation energy *Sp* for and. The proton separation energy for a nuclide is the minimum energy required to separate the least tightly bound proton from a nucleus of that nuclide. It is given by

.

Given, , .

What does the existence of magic number indicate?

Ans:

Since Sn nucleus is more stable than Sb nucleus.

(ii) It indicates shell structure of nucleus similar to the shell structure of an atom. This also explains the peaks in BE/ nucleon curve.