Which of the following is used as a moderator in nuclear reactors? 

1. Plutonium 

2. Cadmium 

3. Heavy water

4. Uranium 

The mass density of a nucleus varies with mass number A as 

1. $A^2$

2. A 

3. Constant 

4. $\frac{1}{A}$

Energy released in the fission of a single ${}_{92}U_{235}$ nucleus is 200 MeV. The fission rate of a ${}_{92}U_{235}$ filled reactor operating at a power level of 5 W is 

1. $1.56\times {10}^{-10}<mspace\; width="1em"></mspace>s^{-1}$ 

2. $1.56\times {10}^{11}<mspace\; width="1em"></mspace>s^{-1}$

3. $1.56\times {10}^{-16}<mspace\; width="1em"></mspace>s^{-1}$   

4. $1.56\times {10}^{-17}<mspace\; width="1em"></mspace>s^{-1}$

Heavy water is used as a moderator in a nuclear reactor. The function of the moderator is 

1. to control energy released in the reactor

2. to absorb neutrons and stop the chain reaction

3. to cool the reactor

4. to slow down the neutrons to thermal energies

A nuclear decay is expressed as:
\(_{6}^{11}\mathrm{C}\rightarrow _{5}^{11}\mathrm{B}+\beta^{+}+\mathrm{X}\)
Then the unknown particle \(X\) is:
1. neutron 
2. antineutrino
3. proton 
4. neutrino