EXERCISE SHORT QUESTIONS
CHAPTER # 21: NUCLEAR PHYSICS
Q # 1: What are isotopes? What do they have in common and what are their differences?
Ans. Isotopes are those nuclei, which have same atomic number but have different mass number. The isotopes have the same number of protons and have different number of neutrons.
Q # 2: Why are heavy nuclei unstable?
Ans. The heavy nuclei have very small value of their binding energy per nucleon. So they are unstable, and less energy is required to split it.
Q # 3: If a nucleus has a half life of 1 year, does this mean that it will be completely decayed after 2 years? Explain.
Ans. No. As decay rate decreases with the decrease of number of radioactive atoms, so total life is much greater than twice of half life. Total decay is possible after infinite years.
Q # 4: What fraction of a radioactive sample decays after two half lives have elapsed?
Ans. The total un-decayed atoms of an radioactive element N is described by the formula:
\[N = N_{0}\left( \frac{1}{2} \right)^{n}\]
where\(N_{0}\) is the total number of atoms of radioactive element and \(n\) is total number of half lives. So,
Number of un-decayed atoms after two half lives
\(= N_{0}\left( \frac{1}{2} \right)^{2} = \frac{N_{0}}{4} = 25\ \%\)
Number of decayed atoms after two half lives \(= 75\ \%\)
Q # 5: The radioactive element}
\(_{\mathbf{88}}^{\mathbf{226}}\mathbf{Ra}\) \textbf{has a half-life of}
\(\mathbf{1}\mathbf{.}\mathbf{6}\mathbf{\times}\mathbf{10}^{\mathbf{3}}\)
\textbf{years. Since the earth is about 5 billion years old, how can you explain why we still can find this element in nature?
Ans. The half life of \(_{88}^{226}{Ra}\) is \(1.6 \times 10^{3}\) years but its total life is equal to infinity. This is common property of all radioactive elements. That's why \(_{88}^{226}{Ra}\) still found on earth while earth's life is 5 billion years.
Q # 6: Describe a brief account of interaction of various types of radiations with matter.}
Ans. Electromagnetic radiation interact with matter in three different ways mainly depending upon their energies. These three processes are:
- Photoelectric effect
- Compton effect
- Pair production
Q # 7: Explain how \(\mathbf{\alpha}\) \textbf{and} \(\mathbf{\beta}\mathbf{-}\)\textbf{particles may ionize an atom without directly hitting the electrons? What is the difference in the action of two particles for producing ionization?
Ans. An\(\alpha -\)particle is nucleus of helium, it requires electrons. So an energetic \(\alpha -\)particle, while passing through matter, ionizes thousands of atoms by attracting their electrons.
But the energetic \(\beta -\)particles ionize the atoms by ejecting their electrons by the force of repulsion.
Q # 8: A particle which produces more ionization is less penetrating. Why?
Ans. A particle with greater ionizing power will lose its whole of energy in a short distance inside a medium. So, its range in that medium is very small.
Q # 9: What information is revealed by the length and shape of the tracks of an incident particle in Wilson cloud chamber?
Ans. In Wilson cloud chamber, the length and shape of the tracks gives the following information.
The tracks of \(\alpha -\)particles are straight, continuous and thicker because these particles have greater mass as well as greater ionizing power.
The tracks of \(\beta -\)particles are thinner, short and discontinuous tracks because these particles has less mass and less value of ionizing power as compared to \(\alpha -\)particles.
\(\gamma -\)rays have no definite tracks bacuse of high penetrating power and less ionizing power.
Q # 10: Why must a Geiger Muller tube for detecting \(\mathbf{\alpha}\mathbf{-}\)\textbf{particle have a very thin end window? Why does a Geiger Muller tube for detecting \(\mathbf{\gamma}\mathbf{-}\) \textbf{rays not need a window at all?
The GM tube has a very thin end window for detecting \(\alpha -\)particles because this window provides easy way for these low penetrating particles, to enter into the tube.
For detecting \(\gamma -\)rays, there is no need of such a window because \(\gamma -\)rays are highly penetrating.
Q # 11: Describe the principle of operation of a solid state detector of ionizing radiation in terms of generation and detection of charge carriers.
Ans. Its principle based upon the production of electron-hole pair by getting energy from incident radiation. These generated carriers cause current pulse, which is used for detection purposes.
Q # 12: What do we mean by term critical mass?
Ans. It is the quantity of mass of nuclear fuel, which is enough to absorb most of neutrons for self sustained fission chain reaction.
Q # 13: Discuss the advantages and disadvantages of nuclear power compared to the use of fossil fuel generated power.
Advantages
It is cheaper for electricity
It is permanent for a given period of time
It does not produce smoke
It is of large amount & It is not cheaper
It is not permanent and not for long period of time
It produces smoke
It is not of large amount
Disadvantages
Nuclear power has radiation effects which makes it dangerous.
Q # 14: What factors make a fusion reaction difficult to achieve?
Ans. The fusion reaction requires temperature up to million degree centigrade and high energy. These requirements are very difficult to achieve.
Q # 15: Discuss the advantage and disadvantages of fusion power from the point of safety, pollution and resources.
Ans.
Disadvantage: The fusion reaction requires temperature up to million degree centigrade and high energy. These requirements are very difficult to achieve.
Q # 16: What do you understand by ``background radiations''? State the two sources of this radiation.
Ans. The radiation present due to cosmic rays and due to presence of radioactive materials under crest of earth, are called background radiations.
Q # 17: If you swallowed a} \(\mathbf{\alpha -}\)\textbf{source and \(\mathbf{\beta}\mathbf{-}\)\textbf{source, which would be more dangerous to you? Explain why?
Ans. If someone swallowed \(\alpha -\)source, then it will damage more blood cells due to its high ionizing power as compare to \(\beta -\)source.
Q # 18: Which radiation does would deposit more energy to your body (a) 10 mGy to your hands or (b) 1 mGy does to your entire body.
Ans. We know that
\[(AbsorbedEnergy)\ = (AbsorbedDose) \times (Mass)\]
As the mass of body is much greater than hand so in second case, more energy will be absorbed.
Q # 19: What is radioactive tracer? Describe on application in medicine, agriculture and industry.
The use of phosphorous or nitrogen as a tracer has helped to adopt a better mode of fertilizer supply to plants.
Radioactive iodine can be used to check that a person's thyroid gland is working properly or not. A similar method can be used to study the circulation of blood using sodium-24.
Q # 20: How can radioactivity help in treatment of cancer?
Ans. High energy radiation can penetrate deep into the body and can be used to intentional selective destruction of tissues, such as cancer tumor.
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