# NCERT Physics Class 12 Exemplar Ch 13 Nuclei Part 3 (For CBSE, ICSE, IAS, NET, NRA 2022)

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## Long Answer

**21** Sometimes a radioactive nucleus decays into a nucleus which itself is radioactive. An example is:

Assume that we start with nuclei at time . The number of is of count zero at and will again be zero at . At what value of *t*, would the number of counts be a maximum?

Ans:

At time *t*, Let have active nuclei and have active nuclei.

**But**

**Multiplying by** **and rearranging**

Integrating both sides.

Since at

For maximum count,

On solving,

**22** Deuteron is a bound state of a neutron and a proton with a binding energy . A -ray of energy *E* is aimed at a deuteron nucleus to try to break it into a (neutron + proton) such that the *n* and *p* move in the direction of the incident -ray. If , show that this cannot happen. Hence calculate how much bigger than *B* E must be for such a process to happen.

Ans:

- From conservation of energy

′

- From conservation of momentum
- If , the first equation gives and hence the second equation cannot be satisfied, and the process cannot take place.
- For the process to take place, Let , where λ would be .
- Then: substituting for pn from Equation (2) into Equation (1) ,

- Since the determinant must be positive for to be real:

Or