# 12th Class Physics Chapter 10 Nuclear Physics Short Questions Answer

## 12th Class Physics Chapter 10 Nuclear Physics Short Questions Answer

1.What are isotopes? What do you they have in common and what are their differences?
Isotopes are the nucleus of an element that have the charge number ’Z’ but have different mass number ‘A’. In these nuclei, number of protons is the same but number of neutrons is different.
2. Why are heavy nuclei unstable?
Heavy nuclei are unstable because their binding energy per nucleon is less than the lighter nuclei or those laying at the medical protein of the periodic table. Also neutrons are not so rightly bound with each other. As they are unstable, so less energy is required to split (break) heavy nuclei.
3. If a nucleus has half life of one year does this mean that it will completely decay after two years?
No, not at all. If the half life of nucleus is 1 year, then it does not mean that this element will completely decay after an infinite time. For example, Let N0 be the number of atoms of a radioactive element, then after one half – life, No/2 atoms will be left.
After second half – life. ½. No/2 = No/4 atoms will be left behind. It means that 75% of the total atoms have decayed and only 25% of the total atoms will be left behind after two years. Thus a nucleus will not be completely decayed after two years.
4. What fraction of a radioactive sample decays after two half lives have elapsed?
Let N0 be the original number of radioactive atom at any instant then.
Number of atoms decayed after first half life T = T1/2 = 1/2N0
Number of atoms decayed after 2nd half life = 2T1/2 = ½(1/2No = lN0)
Total number of atoms decayed after two half – lives = ½ No + ¼ N0 = 3/4N0
Fraction of sample decayed = 3/4No/No*100   = ¾ *100
= 75% Ans.
Note:- (25% will be left undecayed).
5. The radioactive elements: Ra has a half – life of 1.6 *103 years. Since the Earth is about 5 billion years old, how can you explain why we still can find this element in nature?
Half-life of 88Ra = T1/2 = 1.6 103 years
Age of Earth = 5 billion years
As we know that an infinite time is required for all atoms of radioactive element to complete decay (disappearance). But earth’s life is not infinite it is still 5 billion years. So we can still find the existence of 88Ra element in nature.
6. Give a brief account of interaction of various types of radiation with matter?
7. Explain how σ – and β – particles may ionize an atom without directly hitting the electrons? What is the difference in the action of the two particles for producing ionization?
As we know that σ – and β – particles are electrically charged, so they can causes ionization the electrons of the target atom. Since σ – particle is attracting positively charged particle; therefore it ionizes the atoms through electrostatic force of attraction. Similarly, β – particles is a negatively charged particle. So it ionizes the atom through electrostatic force of repulsion. Difference in their action is σ – particle pulls an electron out of the atom while β – particle repels an electron out of the atom in ionizing the atom.
8. A particle which produces more ionization is less penetrating. Why?
As energy is required to ionize an atom, so σ or β particle loses its energy during each ionization. A particle which produces more ionization is less penetrating because it loses all of its energy rapidlyin ionizing the atoms. Thus, it fails to penetrate to greater depth. For example, an σ – particle is more ionizing but less penetrating while β – particle is less ionizing but more penetrating.
9. What information is revealed by the length and shape of the tracks of an incident particle in Wilson cloud chamber?
In a Wilson cloud chamber.
(i). σ – particle:-  The straight, thicker and continuous tracks represent σ – particles because it has larger mass and greater ionizing power.
(ii) β – Particles:-
The tracks of β – particles are thinner, short and discontinuous. These particles have less mass and less value of ionizing power.
(i). λ – rays:-
λ –rays leave behind no definite tracks along their because of high penetrating power and very small ionizing power. It should be noted that length of the tracks shows the penetrating power and thickness shows the ionizing power of incident particle.
10. Why must a Geiger Muller tube for detecting σ – particle have a very thin window? Why does a Geiger Muller tube for detecting λ – rays not need a window at all?
A Geiger Muller tube has a very thin end window for detecting σ – particles because these particles process less penetrating power and very small range as compared to β and λ – rays. Therefore a thin window allows their entry into the Geiger Muller tube easily. For detecting λ – rays window becomes useless because of high penetrating power of λ – rays. Therefore the G. M Tubes does not need a window for detection of λ- rays.
11. Describe the principle of operation of a solid state detector of ionizing radiation in terms of generation and detection of change carriers?
Principle;-    The working  principle of solid state detector is based upon the reverse bias. The applied reverse –bias enlarges the charge – free region in a p- n junction. In other words, when radiation is allowed to enter the depletion, electron – hole pairs are produced by the incident radiation.
12. What do you mean by the term critical mass?
Such a mass of uranium in which neutron out of all neutrons produced in one fission reaction produces further fission is called critical mass.
13. Discuss the advantages and disadvantages of nuclear power compared to the use of fossil fuel generated power?

Nuclear power
1. It produces a large amount of energy.
2. 1 kg of uranium produces 2*107 kwh of energy during fission process.
3. It does not produce smoke. So atmosphere is free of population.
4. The cost of electricity produced by nuclear plant power is cheap.
5. Waste product can be reprocessed.
6. It is permanent for a given time.
Fossil Fuel Generated Power.
1. It produce Generated power of energy.
2. The same amount of energy is produced by using 3 million of coal.
3. It produces smoke and causes pollution in air
4. The cost of electricity produced by it is high.
5. Wast products cannot be used.
6. It is not permanent for a given period of time.

Nuclear power:-
1.Its  fuel not easily available in the market.
2. Its handeling is dangerous.
3. Its waste products are dangerous to be handled.
Fossil Fuel Generated power
1. Its fuel is easily available.
2. It is not dangerous.
3. Its waste products are not dangerous.

14. What factors makes a fusion reaction difficult to achieve?
1.The fusion reaction requires a very large amount of energy to bring two positively charged nuclei closer together against the electrostatic force of repulsion between them. For this purpose, the nuclei should move to words each other with very high speed. But this speed can only be produced by an accelerator. As this can be used on small scale and is not so easy. 2. The fusion reaction takes place at tremendously high temperature that is about 10 million degree Celsius. Such a high temperature cannot be obtained easily. It means fusion reaction requires highest temperature and high energy but both of them are difficult to achieve.
15. Discuss the advantages and disadvantages of fusion power from the point of view safety, pollution and resources?
Advantages:- 1. Sine the fusion reaction is free from radioactive products, so it will be safe, exempted from pollution and resourceful. 2. The fusion reaction produces more energy per nucleon as compared to nuclear fission reaction. 3. He energy produced by this process will be cheaper than that of fission process. Disadvantages:- 1. It is more difficult to start the fusion reaction. 2. Fusion reaction has not been brought under control like fusion reaction. Thus, it cannot be used to produce electricity.
16. What doo you understand by background radiation? State two sources of this radiation?
The radiation present in the space near a radioactive radiation detector such as Geiger tube are called back ground radiations. Following are the two sources of this radiation. (i). The cosmic rays entering the earth from the upper atmosphere along with the sunlight. (ii). the presence of radioactive materials in the atmosphere or the presence of radioactive wastes of nuclear reactors etc.
17. If you swallowed an - source and β – source, with would be the more dangerous to you? Explain why?
As  – particle have greater energy and greater ionizing power than β- particles, so  particle is more dangerous than β – particles. Therefore, – particle can cause much damage to issue (i.e body).
18. Which radiation does would deposit more energy to your body? (a) 10 m Gy to your hand or 1 m Gy does to your entire body?
Absorbed does is defined as the energy ‘E’ absorbed from ionizing radiation per unit mass’ m’ of the absorbing body ,i.e
Absorbed does = D*m ………….(1)
(a). For hand
Suppose mass of hand = ml
Does = D = 10 m Gy
By using equation (1) energy given to hand id found as
El = 10 mGy*ml                   (Gy  = J kg-1)
Or  El = 10 mj . Kg-1* ml  ………………………………..(2)
(b). For entire body
Suppose mass of body = m2
Does  = D = 1 m Gy
= 1 m j Kg-1
Energy given to entire body is found as
E2 = 1 mGy * m2 = 1 mj. Kj- 1* m2
Or E2 = 1 mj. Kg-1*m2……………………………………(3)
Compared equ. (2) and equ (3) We see that mass ‘m2’ of the whole body is for greater than mass ‘ml’ of the hand, therefore, in second cause (b) 1 mGy does gives more energy to the entire body.
19. (a). What is radiation tracer? (b). Describe one application each in medicine agriculture and industry?
Location and concentration of radioactive isotope can be determined by the strength of radiation it emits. A radioactive isotope behaves in the same way as normal isotope inside a living organism. The radiation isotope acts as as indicator or tracer that makes it possible to follow the course of a chemical or biological process.
(B). for answer see question No 48.
20. How can radioactive help in the treatment of cancer?
Cancerous cells which  increase rapidly absorb more radioactive and are more easily destroyed than normal cells by ionizing radiations. Radioactive with λ – rays from cobalt – 60 is often used in the treatment of cancer. The λ – rays are carefully focused on to the malignant tissue. Strict safety precautions are necessary for both patient and attendant medical staff.

Radioactive iodine – 131 is used to cure cancer of the thyroid gland. Since iodine tends to collect in the thyroid gland, radioactive isotopes lodge where they can destroy the malignant cells.
Skin cancer:-
For skin cancers, phosphours – 32 or strontium – 90 may be used. The produce β – radiations.
Encapsulated seeds:-
In some cases encapsulated seeds are implanted in the malignant tissue for local and short ranged treatment.
21. After four half – lives what percent of sample remains?
Suppose No is the total number of atoms at = t = 0
Number of atoms left after first half – life = T1/2 = 1/2N0
No of atoms left after second half- life = 2T1/2= ¼ N0
No of atoms left after third half- life = 3T1/2 = (1/8)N0
No of atoms left after fourth half- life = 4T1/2=(1/16)N0
Percentage of the No, of atoms left after 4 half – lives =    N0/16  * 100
N0
= 1/16*100= 6.25%
22. A slow neturon will cause fission in 92U23592U238?
A slow neutron will cause fission in 92U only.
23. Natural uranium is a low grade fuel. It is due to;
(a). High % of 92U234
(b) Hight % If 92U235
(c).High % of 92U238
Natural uranium is low grade fuel due to high percentage of 92U238.
24. It is said that nuclear energy is raw heat energy. How heat is produced in a nuclear reactor?
The fact that the fusion process release several neutrons makes possible a series of chain self – sustained induced fission. One neutron from fission event triggers the fusion another nucleus. The neutrons released from the fission process are fast and must be slowed down to produce further fission. As the neutrons released are very fast, they undergo several successive collisions with the material used as moderator. As these neutrons are slowed down, their kinetic energy is converted into heat energy, thereby heating the reactor. This heat can be used for boiling water and produces the steam. The steam is used for running the generator.
25. Nuclear energy released under fusion reaction is greater than what is obtained under fission reaction, although release the nuclear energy per fission is 200 Mev as compared to 25 Mev per fusion reaction. Comment?
When two protons and two neutrons are combined to from  helium nucleus energy of about 25 Mev is released. Thus the energy released per nucleon is 25/4 = 6.25 Mev.
But when 92U is broken in a fission process, about 200 Mev energy is released so energy liberated per nucleon is 200/235 = 0.85. Thus energy released per nucleon is fusion is greater than the energy released per nucleon in fission.
26. What does the name LMFBR stand for?
The name LMFBR stand for Liquid Metal Fast – Breeder Reactor.
27. What is the principle of Nuclear Reactor?
The environment of fissioning nuclei is controlled in such a way that only one neutron out of 2.5 neutrons released on the average is used to induce fission in another atom. In this way, the rate of energy generation is maintained at a constant level. This, controlled chain reaction in the principle of nuclear reactor.
28. What are the uses of Nuclear Reactor?
Uses:- Nuclear reactor for the following purposes. 1. Research 2. Production of plutonium which is used in atomic bombs as a fuel. 3. For obtaining useful energy for producing electrical and mechanical energy. 4. It is used for the production of atomic energy for industrial and peaceful purposes.
29. What are the Atomic Number of He3 and He4?
As  2He3 and  2He4 are the isotopes of helium.
(i). The atomic number of He3 is given by = Z = 2 Ans
(ii). Atomic number of He4 is given by Z = 2 Ans
30. How many photons, neutrons and electrons are there in nucleus of 86Rn222?
No of photons = Z = 86
No of electrons = Z = 86
Mass number = A = 222
No. of neutrons = N- = A –Z
Or   N = 222 – 86 = 136
Therefore,
No, of photons  = 86 Ans.
No of electrons = 86 Ans.
No of neutrons = 136 Ans.
31. A nuclide 86Rn222 decays to a new nuclide ‘X’ by two - emission and two β – emission. What is the new nuclide?
Since in one – emission charged No Z decreases ‘X’ by  2 and Mass No. A decreases by 4, so after two  – emission the nuclide becomes 82Y214.
Similarly, after one β – emission charge No Z increases by 1 and mass No. A remains the same so after two β = emission nuclide 82Y214 becomes 84Y214
New nuclide= 84X214.
32. How does cl36 differ from cl37? Why are they considered to be chemically identical even through their nuclei are not the same?
Difference between Cl35 and Cl37:-
The atomic number (Z) of chlorine is 17, so the chlorine muscles must contain 17 protons.
Therefore the number of neutron in cl35 = 35 – 17 =18
and the number of neutron in cl37 = 37 – 17 = 20.
These two types of chlorine nuclei have the same number of protons but different number of neutrons. These are called the isotopes of chlorine. The chemical protons of all the isotopes of the same element will be similar because the chemical properties of an element depends only upon atomic number (Z) i.e on valence electrons.
33. Do , β and λ rays comes from the same element? Why do we find all three in many radioactive elements?
Emission of , β, and λ rays:-  , β and λ rays many come from the same element but a radioactive element cannot emit  and β rays simultaneously because by the emission of  and β rays parent nuclei change and mass number of λ rays can be emitted along with  rays or β rays. When an atom of a radioactive element disintegrates by emitting  or β particles, the daughter atom is still radioactive and may sooner or later emits another particle ( or β) to become a still different atom. The process continues through a series of elements till a stable atom is formed. That is why we find all three rays in many radioactive elements.
34. It is more difficult to start a fusion reaction than a fission reaction. Why?
Fission and Fusion Reaction:- The fusion reaction releases much more energy per unit mass than the fission reaction. But it is very difficult to produce fusion reaction. This difficulty is due to the fact that when two positively charged nuclei are brought closer and closer and the fused together, work has to be done against the electrostatic force of repulsion. This will require a great deal of energy. Whereas it is energy to produce fission reaction because neutron is uncharged particle and it can wonder about the nucleus, without experiencing any electrostatic force of repulsion, before it is captured by the nucleus.
35. Is it possible that fusion of two small nuciel may accur without collision at extremely high enrgy?
Fusion requires high =energy:- It is possible that fusion of two small nuclei may accur without collision at extremely high energy. When the two positively charged nuclei are brought closer and closer, the electrostatic repulsive force between them becomes very high and thus for this collision extremely high energy is necessary to overcome the said repulsive force.
36. Explain how a nuclear reactor produces heat as a result of fission?
Heat produces by an a nuclear reactor. The fission of 92U235 can be represented by the equation given below
92U235 + 0n1 = 56Ba141 + 36Kr92 31 0n + Q
This equation shows that with the fission of every atom, Q energy is released which is nearly equal to 200 Mev. This energy is mostly appeared in the form of heat. Moreover, this supply of heat energy contains due to the chain reaction produced by three liberated neutrons.
37. Find the number of neutrons and protons in 92U238?
In 92U238
A = 238 and Z = 92
N = ? No of photons= ?
No .of neutrons = N = A-Z = 238- 92= 146
Or N = 146    Ans.
And No. of protons = Z =92
Or Z= 92   Ans.
38. The nucleus of nitrogen contain 9 neutrons. Find its charge number and its number is 16?

Number of neutrons = N = 9
Mass number = A = 16
Change number = z = ?

As      N = A – Z
Or  9 + 16 – Z
9 +Z  = 16
Z = 16 – 9 = 7
Or 16 – 7 = 7
Charge number  = Z – 7

39. 92U238 emits an alpha particle what still be the new element formed?
When uranium – 238 emits  – particle, following nuclear reaction takes place.
92U23892th234 + 2He4                     (- particle)
It means that thorium – 234 is formed when  U – 238 emits  particles.
40. Radium has a half life of 1600 years. How much of 60 gm Radium would be left after 4800 years
After one half (1600 years)  60 gm of radium is left = 30 gm.
After two half lives (3200 years) the amount of radium left = 15 gm
After three  half lives (4800 years) the amount of radium left 7.5 gm.
radium left = 7.5 gm     Ans
41. How do λ rays differ from X- rays?
(1). X –rays are produced by stopping high energy electrons on heavy metals such as tungsten λ – rays are products of radioactive decay of nuclei.
(2). Spectrum of X –rays is continuous for a certain range of wavelength depending upon the voltage of X –rays tube.
Spectrum of λ – rays is directed/line spectrum with wavelength depending upon the nature of radioactive nucleide.
42. Shows that 1 amu = 931 Mev?
We know that m = 1 amu = 1.66 * 10-27 E = mc2 = 1.66 *10-27 * (3*108)2 E = 14.94 * 10-11 j = Or E = 931.48 *106 eV = 931. 48 Mev E = 931Mev
43. Water is the batter shield against neutrons than lead? Explain. Why?
When neutrons collide with a heavy nuclei of lead, they will be back almost with the same velocity while lead atoms having more mass remain practically at rest. In the case of water, they can be slowed neutrons collide collisions with light hydrogen nuclei of their own mass. Such that proton is at rest while neutron is moving. Therefore, the water which has a large number of light hydrogen nuclei is a better shield against neutrons than lead.
44. For what purpose alcohol or bromine is mixed with principle gas in Geiger tube?
During the process if ionizing the heavy and slowly moving positive ions may release electrons on reaching the cathode. These electrons could them travel to the anode and may upset the recording of the incident particles. These electrons are prevented from reaching the anode by the molecules of alcohol or bromine mixed with the principle gas. Alcohol or bromine is called quenching agent, by which the gas becomes non- conducting automatically.
45. What is meant by the nuclear exposure? What are the major factors contributing towards nuclear exposure?

Nuclear Exposure  Radio – active radiations emitted from the nucleus of radioactive elements is called nuclear exposure.
(ii). Radioactive random gas is also present in the air in small quantity. It is deposited usually on dust particles which reach up to our lungs when we breath.
(iii). Tobacco leaves also collect radioactivity while drying. A cigarette smokes then inside his body.
(iv). Naturally according radio nuclides libe potassium – 40 and carbon – 14 enter the body through food chain. They are accumulated in the diet and make their way to the body where they emit β – particles.

46. What are the uses of nuclear radiation in medical diagnostics and treatments?
(i). Radio isotope are uses to cure cancer, leukemia and for the treatment of heart diseases. (ii). They are also used to diagnose tumors in brain other parts of the human body.
Uses:- (i). High energy radiations like X – rays and λ – rays can penetrate into human body. They can be focused on the cancerous tumors to destroy them. (ii). The tumors which are not effectively attacked by λ rays are cancer. This isotopes emit β –particles and high energy λ –rays. (iii). Iodine – 131 is taken inside the body to treat thyroid cancer. The radiation emitted by it destroys the cancerous cell. (V). Radioactive random gas n small gold capsule, known as random seeds, is employed to destroy the cancerous cell.
48. Write an application of tracer technique each in medicine, agriculture and industry?
Medicine:- radioactive tracers are used to follow the path that various chemical or food constituents take in the human body, animals and plants. Many other chemicals such as iodine tends to concentrate in the thyroid, phosphorus, and strontium in bond and cob late in liver as tracers, small quantity of radioactive substance is supplied through injection to the patient and location in diseased tissue may be checked by means of radiation detector.
Industry:- Radio isotopes can be used to trace underground pipe leakage or the position of buried pipes. The radioisotopes is added into the water and soil close to the leak becomes radioactive. The passage of water is followed by a radioactive detector above the ground and thus leakage is detected. Radio isotopes of short half – life such as na -24 or iodine – 131 are used as tracer to avoid prolonged contamination of the soil or water.
Agriculture:-  Radioactive carbon (C- 14) is one of the useful  tracer due to the presence of carbon in all organic molecules. It has made it possible to understand the complex process of photosynthesis in detail . This technique has given the detailed information on the series of chemical reaction that leads from carbondioxide to carbohydrate in the plants.
49. Which of ∝, β and λ – radiation would you advise and why for?(i). Treatment of skin of a patient. (ii). Treatment of flesh just under skin. (iii). Treatment of infection in the interior of the body.
∝ particles should be advised to treat the skin due to their very small penetrating power. (ii). β – particles should be advised for treatment of flesh penetration power than ∝- particles. (iii). Λ – rays should be advised for the treatment of an infection in the interior of the body because they have the largest penetration power.
50. How a radioisotopes be used the determine the effectiveness of a fertilizer?
Radioisotopes phosphorus of nitrogen used as a tracer in agriculture has provided information about the best fertilizer to supply to a particular crop and soil. Due to their use, verities of crop such as rice, wheat and cotton have improved. Moreover, plants have shown more resistance to dieses and give better yield and grain quality.

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