Answers to Review Questions: Ch # 19 Atomic Nucleus

XII BSEK ( Karachi Board)

Q.2 Do α, β and γ rays came from the same element .Why do we find all three in many radioactive elements?

Ans: A radioactive element either emits α-particles or β-Particles ,but never both. Gamma radiations are the companion of alpha or beta. So the radioactive sample is the mixture of radioactive elements so we can have all three in many.

Q.3 It is more difficult to start a fusion reaction that fission. Why?

Ans:In fission there in no extra work done to overcome the repulsive force between two protons as in the nuclear fusion so that is why it is difficult to produce he nuclear fusion

Q.4 Is it possible that fusion of two small nuclei may occur without collision at extremely high energy?

Ans:No .Two nuclei must collide with sufficient kinetic energy to penetrate their mutual columb barrir and coming with in the range of nuclear forces.

 Q.6 What are benefits and risks of nuclear reactors? Which reactor is relatively better from the point of safety?

 Ans:

Benefits	Risk
Produce Electricity	Leakage of radioisotopes
Nuclear fuels	Hazards for workers
Produce radioisotopes	Damages the environment
Clean burning	Limited supply of raw materials

Q.7 Both fission and fusion apparently produce energy. How can you reconcile this with the law of conservation of energy?

Ans: In fission of U-235 with thermal neutrons, the loss of mass (0.2153u) is converted into energy producing about 200 Mev per fission

            In Fusion, four protons may be combined to produce one helium nucleus and two positrons. here the loss of mass(0.027 u)is converted into energy producing about 26Mev.Thus in both the cases the mass-energy remains conserved

Q.8 When a photon disappears in producing an electron and positron, is the energy of a photon equivalent to that of particles produced? Explain.

Ans:No,the energy of photon is always grater than the rest mass energy of electron –positron pair(1.02Mev) The remaining energy is carried by the particles as their K.E.

                                                                                                                 

Q.10 Why do most moderators used in nuclear reactors are light atoms like ₁H¹, ₁H² or ₆C¹² to slow down the neutrons instead of using heavier atoms?

Ans:Fast moving neutrons can be stopped when they make elastic collision with stationary particles of the same mass. Since mass of protons, deuteron and graphite nucleus is comparable with the mass of neutrons, hence they slowed down.

Q.11 Can conventional fission reactors ever explode like a bomb does? Why?

Ans:In nuclear reactor ,a fission explosion is not possible, because the amount of fuel is of sub critical mass and it can shut off control rods in emergency. Also liquid poison can be inserted directly in to moderator if other safety devices fail.

Q.12 In a LMFBR why would you expect the radioactivity of sodium coolant to increase with the life time of the reactor.

Ans:Yes,Because sodium can captures neutrons: ₁₁Na²³+ ₀n^{1     ----------->}    ₁₁Na²⁴ + γ

            Here Na 24 is radioactive with half life of 15H