F.Sc ICS Notes: Physics XII: Chapter 21 Nuclear Physics Exercise Short Questions:

FSc ICS Notes: Physics XII: Chapter 21 Nuclear Physics Exercise Short Questions: 2nd Year Physics Notes Online Taleem Ilm Hub

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Question 21.1 What are isotopes? What do they have in common and what are their differences?
Answer 21.1 Nuclei of the same elements which have same charge number but different mass number are called isotopes of an element.
Isotopes of an element have:

• Same charge number or atomic number.
• Same chemical properties.
• Same number of protons.

Isotopes of an element have different:

• Number of neutrons.
• Different physical properties.
• Mass number or weight.

Question 21.2 Why are heavy nuclei unstable?

Answer 21.2 Light and stable nuclei have equal or almost equal number of protons and neutrons. However heavy nuclei such as 238U92 Uranium contains92 protons and 146 neutrons. Similarly 226ra88 Radium contains 88 protons and 138 neutrons. The wide difference between number of protons and neutrons makes the heavy nuclei unstable.

Question 21.3 If a nucleus has a half-life of 1 year, does this mean that it will be completely decayed after 2 years? Explain.

Answer 21.3 If a radioactive element has half-life as one year, then after one year, half of the total number of atoms are left behind UN-decayed with the passage of another year, half of the UN-decayed atoms decay leaving behind 1/4 th of the atoms UN-decayed. Thus after two year, the element under consideration will not completely vanish. But only 3/4 of the total number of atoms will decay.

Question21.4 What fraction of radioactive sample decays after two half-lives have elapsed?

Answer 21.4 Let N be the total number of atoms of the radioactive element present at the beginning of time. After one half life, N/2 atoms decay while N/4 atoms decay in the second half life. Total number of atoms decayed in two half lives is N/2 + N/4 = 3/4 th of the radioactive sample decays in two half lives.

Question 21.5 The radioactive element 88Ra226 has a half life of 1.6 x 103 years. Since the Earth is about 5 billion years old, how can you explain why we still find this element in nature?

Answer 21.5 As half life of 226Ra88 is 1.6 * 10(3) years, so after 1.6 * 10(3) years, half of 226Ra88 decays leaving behind half of 226Ra88. Similarly after every half life, half of the initial atoms are left behind and it takes infinite time for entire element to decay completely. So even after billion years we still have some UN-decayed atoms of 226Ra88.

Question 21.6 Describe a brief account of interaction of various types of radiations with matter.

Answer 21.6  Interaction of alpha particle with matter: Alpha particles can ionize and excite atoms of the element through which they pass. The ionization caused by alpha particles may be due to direct elastic collision with atoms or due to electrostatic attraction between them and electrons of the atoms. The property of ionization is used to detect and measure the energy of alpha particles. For an alpha particle creates about 10(4) ionization in 1mm of air.

Interaction of Beta particle with matter: Beta particles are negatively charged, fast moving electrons coming out of nuclei of radioactive elements. Beta particles are 7000 times lighter than alpha particles and their charge is half of the charge of alpha particle. Ionization caused by beta particle is due to electrostatic repulsion between them and electron of the atom. ionizing ability of beta particles is about 100 times less than that of alpha particle since its mass and charge are smaller than alpha particle. However the range of beta particle through gas is about 100 times more than alpha particle. Path of beta particle is not straight due to deflection caused by collision with atoms of the medium.

Interaction of gamma particle with matter:  Gamma rays are mass less charge less radiation coming out of the nuclei of radioactive elements. The ionization caused by gamma ray photons is almost negligible, gamma rays interact with matter in three different ways depending on their energy.

• At low energy less than 0.5 Mev, gamma rays interact with matter and produce photoelectric effect.
• At intermediate energy between 0.5 Mev - 1.0 Mev, gamma rays produce Comptons's effect.
• At  energy more than 1.02 Mev, gamma rays produce pair production.

Question 21.7 Explain how α and β -particles may ionize an atom without directly hitting the electrons? What is difference in action of the two particles for producing ionization?

Answer 21.7  We know that alpha particle are doubly positively charged and beta particles are negatively charged. Being charged particles they can ionize an atom without directly hitting the electrons. When an alpha particle passes close to an atom, it attracts its electrons and the atom is ionized. On the other hand when a beta particle passes close to an atom it repels the electron of the atom and knocks it out.

Question 21.8 A particle which produces more ionization is less penetrating. Why?

Answer 21.8 The particle having more ionizing power will lose whole of its energy in the short span. On the other hand, the particle with low ionizing power can travel greater distance in that medium because it will produce smaller number of ions.

Question 21.9 What information is revealed by the length and shape of the tracks of an incident particle in Wilson cloud chamber?

Answer 21.9 The thick straight and continuous tracks in Wilson cloud chamber are due to intense ionization produced by alpha particles.

The thin and discontinuous tracks extending in erratic manner shows the presence of beta particles. They show frequent deflections.

The radiations with negligible ionizing power gamma rays leave no definite track. The irregular tracks are those of photo-electrons produced by gamma rays.

Question 21.10 Why must Geiger Muller tube for detecting α -particles have a very thin end window? Why does a Geiger Muller tube for detecting γ-rays not need a window at all?

Answer 21.10 Since  α particle posses less penetrating power than gamma rays, therefore, a thin window allows their entry into Geiger tube easily. For detecting gamma rays, window become useless because of their high penetrating power. Therefore, G.M tube does not need a window for detection of gamma rays.

Question 21.11 Describe the principle of operation of a solid state detector of ionizing radiation in terms of generation and detection of charge carriers.

Answer 21.11 A solid-state detector is a specially designed p-n junction, operating under a reverse bias in which electron hole pairs are produced by the incident radiation to cause a current pulse to flow through the external circuit. Then the electrical pulse is amplified and recorded.

Question 21.12 What do you mean by the term critical mass?

Answer 21.12 The mass of uranium in which one neutron, out of all the neutrons produced in one fission reaction, produce further fission is called critical mass. The minimum mass of a material that can sustain a nuclear chain reaction. It is the quantity of such mass, which is enough to absorb most of neutrons produced in fission chain reaction and to produce large amount of energy.

Question 21.13 Discuss the advantages and disadvantages of nuclear power compared to the use of fossil fuel generated power.
Answer 21.13  Advantages:

• The fuel has small cost.
• It does not produce smoke.
• Waste products can be reprocessed.
• Cost of electricity is cheap.

            Disadvantages:

• It has radiation effect.
• Its waste products are strongly radioactive and dangerous and cannot be easily dumped.

Question 21.14 What factors make a fusion reaction difficult to achieve?

Answer 21.14 When two light nuclei are brought together, a strong force of repulsion comes into play. This force of repulsion prevents the nuclei to fuse together. To overcome this difficulty the nuclei should be very high velocity, which needs temperature of the order  10(7)C. Such a high temperature cannot be produced on earth by any other means.

Question 21.15 Discuss the advantages and disadvantages of fusion power from the point of safety, pollution and resources.
Answer 21.15 Advantages:

• Since the fusion reaction is free from radioactive products, so it will be safe and free from pollution and resources.
• The fusion reaction produces more energy per nucleon.
• The energy produced by fusion is cheaper than fission energy.

             Disadvantages:

• It is more difficult to start fusion reaction.
• It cannot be controlled like fission reaction.

Question 21.16 What do you understand by “background radiation”? State two sources of this radiation.

Answer 21.16 G.M tube record radiation even if no radioactive source is close to it. Thus record is due to radiation called background radiation. Background radiation is partly due to cosmic radiation which comes to us from some outer space and partly due to naturally occurring radioactive substances in earth's crust.

Question 21.17 If you swallowed an α -source and a β –source, which would be the more dangerous to you? Explain why?

Answer 21.17  α - particle have more ionizing power while β - particle are less ionizing. Thus α - particle can cause more damage to the tissue of the body.

Question 21.18 Which radiation dose would deposit more energy to your body (a) 10 mGy to your hand, or (b) 1 mGy dose to your entire body.
Answer 21.18 As we know that:

D = absorbed dose = Energy / Mass

E = D * Mass

Since the mass of the whole body is far greater than mass of hand, therefore according to above equation the case (b) 1mGy dose given to the entire body deposits more energy.  

Question 21.19 What is a radioactive tracer? Describe one application each in medicine, agriculture and industry.

Answer 21.19  Radioactive tracer: Radio-isotopes is used to trace the path or position of an element through a biological, chemical, or mechanical system.

Medicine: Diagnosis: by taking radioactive iodine with food, position of iodine can be followed by G.M. counter. So detector tells the position of the food in the digestive system.

Agriculture: Productivity of food grains: Labeled fertilizer of radio phosphorous ( P32 ) is placed at several depths and distances from plant. The relationship between the root growth and taking of phosphorous from the soil determine percentage productivity of food grains.

Industrial: Labeling the elements: labeled radioactive carbon (C14) mixed in certain compound provide a simple test of leaks in pipes and the flow of rates of liquid without effecting the actual flow.

Question 21.20 How can radioactivity help in the treatment of cancer?

Answer 21.20 Radiotherapy kills cancer cells by damaging their DNA. Radiotherapy uses high energy radiation to destroy damaged cells. Radiotherapy with gamma rays form cobalt-60 is used in the treatment of cancer. Radioactive Iodine-131 is used for treatments of thyroid gland cancer. For skin cancer phosphorous- 32 or strontium-90 may be used.

Written By: Asad Hussain.