12th Class Physics Nuclear Physics And Radioactivity Question Bank

done Nuclei Conceptual Problems Total Questions - 97

• question_answer1)
  What will be the ratio of the radii of two nuclei of mass numbers \[{{A}_{1}}\] and \[{{A}_{2}}\]?
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• question_answer2)
  What is the ratio of the nuclear densities of two nuclei having mass numbers in the ratio 1:4?
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• question_answer3)
  What does exactly make heavy nuclei unstable?
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• question_answer4)
  Why is the number of neutrons in heavier nuclei more than the number of protons?
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• question_answer5)
  Why is the density of a nucleus much more than that of the atoms?
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• question_answer6)
  The value of one unified atomic mass unit is\[1.66\times {{10}^{-27}}kg\]. Calculate the mass of one atom of \[^{12}C\] in kilogram.
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• question_answer7)
  What holds nucleons together in a nucleus?                              
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• question_answer8)
  Why is the mass of a nucleus always less than the sum of the masses of its  constituents, neutrons and protons?                 
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• question_answer9)
  Define mass defect of a nucleus. How is it related to the binding energy of the nucleus?
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• question_answer10)
  The binding energies of deuteron \[(_{1}^{2}H)\] and \[\alpha \]-particle \[(\frac{4}{2}He)\] are 1.25 and 7.2 \[MeV\]/nucleon. Which nucleus is more stable?         
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• question_answer11)
  Electrons cannot be a part of a nucleus but protons can be a constituent part of it. Why?
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• question_answer12)
  Name two radioactive elements which are not found in observable quantities in nature. Why it is so?                                               
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• question_answer13)
  Why is it said that nuclear forces are saturated forces?                       
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• question_answer14)
  What do you mean by the charge independent character of nuclear forces?
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• question_answer15)
  Is free neutron a stable particle? If not, what is its mode of decay?   
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• question_answer16)
  In the nuclear decay reaction \[_{1}^{1}H\to _{0}^{1}n+\frac{P}{Q}X\] find \[\mathbf{P},\text{ }\mathbf{Q}\] and hence identify \[X\].
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• question_answer17)
  Can radioactivity be controlled?
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• question_answer18)
  Natural radioactive nuclei are the nuclei of high mass number. Why?
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• question_answer19)
  Why do alpha particles have a high ionising power?
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• question_answer20)
  What is the difference between an electron and a \[\beta \] -particle?                 
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• question_answer21)
  A nucleus contains no electrons, yet it ejects them. How? Or How are \[\beta \] -rays emitted from a nucleus, when it does not contain electrons?                                    
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• question_answer22)
  Why is the energy distribution of beta rays continuous?                                Or Why do all the electrons emitted during beta decay not have the same energy?           
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• question_answer23)
  When does a nucleus emit a \[\gamma \] -ray photon?
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• question_answer24)
  Why are \[\gamma \] -rays also called electro- magnetic waves?
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• question_answer25)
  Why are \[\gamma \]-rays not deflected by electric and magnetic fields?
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• question_answer26)
  Why are \[\alpha \]-particles emitted rather than protons or \[_{2}^{3}He\] nuclei?
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• question_answer27)
  Why it is not possible to define total life of a radioactive substance?
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• question_answer28)
  A radioactive sample having N nuclei has activity R. Write down an expression for its half-life in terms of R and N.          
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• question_answer29)
  The radioactivity of the sample is \[{{R}_{1}}\] at time \[{{t}_{1}}\] and \[{{R}_{2}}\] at time \[{{t}_{2}}\]. The mean life of the sample is \[\tau \]. What is the number of nuclei that have disintegrated in the time interval \[({{t}_{1}}-{{t}_{2}})\]?
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• question_answer30)
  Show that the decay rate \['R'\] of a sample of a radionuclide is related to the number of radioactive nuclei \['N'\] at the same instant by the expression \[R=\lambda N\]  
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• question_answer31)
  Tritium has a half-life of 12.5 years against beta decay. What fraction of a sample of pure tritium will remain undecayed after 25 years?                         
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• question_answer32)
  What percentage of a given mass of a radioactive substance will be left undecayed after five half-life periods?            
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• question_answer33)
  A radioactive substance decays to \[1/32\]th of its initial activity in 25 days. Calculate its half life.                                    
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• question_answer34)
  Plutonium decays with a half-life of 24,000 years. If plutonium is stored for 72,000 years, what fraction of its remains?   
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• question_answer35)
  In a radioactive decay, a nucleus emits an \[\alpha \]-particle and two \[\beta \]-particles successively. Show that the final nucleus is an isotope of the original nucleus.
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• question_answer36)
  Why is neutron so effective as bombarding particle?
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• question_answer37)
  Why is \[_{92}^{238}\text{U}\] not suitable for chain reaction?                         
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• question_answer38)
  Why are the control rods made of cadmium?
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• question_answer39)
  What is meant by multiplication factor \[(k)\] of a fissionable material? For what value of k, a chain reaction will grow?
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• question_answer40)
  Why is nuclear fusion difficult to carry out?                           Or Why is nuclear fusion not possible in a laboratory?                
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• question_answer41)
  Why do lighter nuclei tend to fuse together?
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• question_answer42)
  Why are fusion reactions also known as thermonuclear reactions?
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• question_answer43)
  Why should nuclear fission precede a nuclear fusion?
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• question_answer44)
  A fusion reaction is more energetic than a fission reaction. Comment.     
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• question_answer45)
  The sun is constantly losing mass due to thermonuclear fusion. Comment.         
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• question_answer46)
  Some scientists have predicted that a global nuclear war on earth would be followed by 'nuclear winter'. What would cause 'nuclear winter'?                                   
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• question_answer47)
  In the nuclear reaction \[_{1}^{1}H\to _{0}^{1}n+_{Q}^{P}X\] Find \[\mathbf{P},\text{ }\mathbf{Q}\] and hence identify \[\mathbf{X}\].   
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• question_answer48)
  When \[_{3}^{7}\] \[Li\] is bombarded with ascertain particle, two alpha particles are produced. Identify the bombarding particle.                      
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• question_answer49)
  State the reason, why heavy water is generally used as a moderator in a nuclear reactor.                                             
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• question_answer50)
  Calculate the energy equivalent (in \[\mathbf{Me V}\]) of one twelfth of the mass of one atom of carbon 12.                                                
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• question_answer51)
  The atomic mass of an element is the weighted average of the atomic masses of different isotopes of the element. This explains, why atomic masses of many elements show large departures from integer values. However, even if we consider masses of individual isotopes, they are not strictly integer multiples of the mass of a hydrogen atom. How do you account for this fact?
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• question_answer52)
  The isotopes \[_{8}^{16}O\] has 8 protons, 8 neutrons and 8 electrons, while \[_{4}^{8}\] Be has 4 protons, 4 neutrons and 4 electrons. Yet the ratio of their atomic masses is not exactly 2. Why?
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• question_answer53)
  You are given two nuclides \[_{3}^{7}X\] and  \[_{3}^{4}\Upsilon \] (i) Are they the isotopes of the same element   Why? (ii) Which one of the two is likely to be more stable? Give reason.
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• question_answer54)
  Group the following six nuclides into three pairs of (i) isotones, (ii) isotopes and (iii) isobars; \[_{6}^{12}C,_{2}^{3}He,_{80}^{198}Hg,_{1}^{3}H,_{79}^{197}Au,_{6}^{14}C\] How does the size of a nucleus depend on its mass number? Hence explain why the density of nuclear matter should be independent of the size of the nucleus.                
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• question_answer55)
  What is meant by binding energy per nucleon? The binding energies of deuteron \[(_{1}^{2}H)\] and \[\alpha \] -particle (\[\frac{4}{2}\] He) are 1.25 and \[\mathbf{7}.\mathbf{2}\text{ }\mathbf{MeV}/\]nucleon respectively. Which nucleus is more stable?     
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• question_answer56)
  A heavy nucleus \[\mathbf{X}\] of mass number \[\mathbf{A}=\mathbf{240}\]and binding energy per nucleon \[\mathbf{7}.\mathbf{6}\text{ }\mathbf{MeV}\] is split into two nearly equal fragments \[\Upsilon \] and \[\mathbf{Z}\] of mass numbers \[{{A}_{1}}=110\] and \[{{A}_{2}}=130\]. The binding energy of each one of these nuclei is \[\mathbf{8}.\mathbf{5}\text{MeV}\] per nucleon. Calculate the total binding energy of each of the nuclei \[\mathbf{X},\text{ }\mathbf{Y}\] and \[\mathbf{Z}\] and hence the energy Q released per fission in \[\mathbf{MeV}\].                      
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• question_answer57)
  Write down the value of charge and mass of an \[\alpha \] -particle. Express charge in terms of electronic charge and the mass in terms of the mass of a proton. Can an \[\alpha \] -particle be compared with a helium atom?
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• question_answer58)
  Give the nature of  \[\alpha -,\beta -\]and \[\gamma \] radiations.
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• question_answer59)
  The half life period of a radioactive element A is the same as the mean life of another radioactive element B. Initially both of them have the same number of atoms. The radioactive element B decays faster than A. Explain, why.                     
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• question_answer60)
  Explain how radioactive nuclei can emit \[\beta \] -particles even though atomic nuclei do not contain these particles. Hence explain why the mass number of a radioactive nuclide does not change during \[\beta \] -decay.
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• question_answer61)
  A nucleus \[_{n}^{m}X\] emits one alpha particle and one beta particle. Find the mass number and atomic number of the product nucleus.         
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• question_answer62)
  With the help of an example, explain, how the neutron to proton ratio changes during the alpha decay of a nucleus.  
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• question_answer63)
  Explain with an example, whether the neutron-proton ratio in a nucleus increases or decreases due to beta (\[\beta \]) decay.
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• question_answer64)
  Explain why the \[\beta \] -decay of a free proton is not possible but that of a proton bound in the nucleus is possible?                 
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• question_answer65)
  Explain what is meant by radioactive decay. A radioactive nucleus is represented by the symbol \[_{b}^{a}V\]. How is the new nucleus represented after the emission of (i) an alpha particle, (ii) a beta particle and (iii) a gamma ray? The activity of a source under- going a single type of decay is \[{{R}_{0}}\] at time \[t=0\]. Obtain an expression in terms of the half-life. \[{{T}_{1/2}}\] for the activity R at any subsequent time t.                                             
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• question_answer66)
  Define decay constant of a radioactive sample. Which of the following radiations, \[\alpha \]-rays, \[\beta \]-rays, \[\gamma \]-rays (i) are similar to \[\mathbf{X}\] -rays ? (ii) are easily absorbed by matter? (iii) travel with greatest speed? (iv) are similar in nature to cathode rays ?
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• question_answer67)
  Write the nuclear reactions for the following: (i) \[\alpha \]-decay of \[_{84}^{204}Po\]             (ii) \[\beta -\]decay of \[_{15}^{32}P\] (iii) \[{{\beta }^{+}}-\]decay of \[_{6}^{11}C\]                
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• question_answer68)
  A radioactive nucleus 'A' decays as given below : \[A\xrightarrow{_{-1}^{0}e}{{A}_{1}}\xrightarrow{\alpha }{{A}_{2}}\] If the mass number of atomic number of \[{{A}_{1}}\] are 180 and 73 respectively, find the mass number and atomic number of A and \[{{A}_{2}}\]                               
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• question_answer69)
  The sequence of stepwise decays of a radioactive nucleus is \[D\xrightarrow{\alpha }{{D}_{1}}\xrightarrow{{{\beta }^{-}}}{{D}_{2}}\xrightarrow{\alpha }{{D}_{3}}\] If the nucleon number and atomic number of \[{{D}_{2}}\] are 176 and 71 respectively, what are the corresponding values of D and \[{{D}_{3}}\]? Justify your answer in each case.     
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• question_answer70)
  (a) If the \[\alpha \]-decay of \[^{238}U\]is energetically allowed (i.e., the decay products have a total mass less than the mass of \[^{238}U\]), what prevents \[^{238}U\] from decaying all at once ? Why is its half life so large? (b) The \[\alpha \]-particle faces a Coulomb barrier. A neutron being uncharged faces no such barrier. Why does the nucleus \[_{92}^{238}U\] not decay spontaneously by emitting a neutron?                                               
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• question_answer71)
  (a) The observed decay products of a free neutron are a proton and an electron. The emitted electrons are found to have a continuous distribution of kinetic energy with a maximum of \[({{m}_{n}}-{{m}_{p}}-{{m}_{e}}){{c}^{2}}\]. Explain clearly why the presence of a continuous distribution of energy is a pointer to the existence of other unobserved product(s) in the decay. (b) If a neutron is unstable with a half life of about 1000 s, why don't all the neutrons of a nucleus decay eventually into protons? How can a nucleus of \[\mathbf{Z}\] protons and (A-Z) neutrons ever remain stable if the neutrons themselves are unstable?
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• question_answer72)
  (a) Why is the mass of a nucleus always less than the sum of the masses of its constituents, neutrons and protons? (b) If the total number of neutrons and protons in a nuclear reaction is conserved, how then is the energy absorbed or evolved in the reaction? Explain.    
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• question_answer73)
  Give the mass number and atomic number of elements on the right-hand side of the decay process. \[_{86}^{220}RU\to Po+He\] The graph shows how the activity of a sample of radon-220 changes with time. Use the graph to determine its half-life. Calculate the value of decay constant of radon-220.
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• question_answer74)
  What are alpha particles? In the reaction: \[_{Z}^{A}X\to \alpha +\Upsilon ,\] give the atomic number and mass number of \[\Upsilon \].                
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• question_answer75)
  The isotope of uranium \[_{92}^{238}\text{U}\] decays successively to form \[_{90}^{234}Th,_{91}^{234}Pa,_{92}^{234}\]\[U,_{90}^{230}\]\[Th,_{88}^{226}Ra,\] and \[_{86}^{222}Rn\]. What are the radiations emitted in each decay process?                     
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• question_answer76)
  A neutron strikes a \[_{5}^{10}\]B nucleus with the subsequent emission of an alpha particle. Write the, corresponding nuclear reaction. Find the atomic number, mass number and the chemical name of the remaining nucleus.                 
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• question_answer77)
  What is the difference between a photon and a neutrino?
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• question_answer78)
  What is the role of control rods in a nuclear reactor? Why are they made of cadmium?
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• question_answer79)
  What are delayed neutrons? Explain the role of delayed neutrons in a nuclear reactor.
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• question_answer80)
  What is a moderator? Briefly explain its action in slowing down the neutrons.
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• question_answer81)
  In a controlled thermal fusion reactor, what is the function of (i) the moderator, (ii) the control rods, (iii) the coolant, and (iv)the heavy water ? \[_{92}^{236}X\to _{a}^{141}\Upsilon +_{36}^{b}Z+3_{0}^{1}n\] What are the values of the numbers a and b? Calculate the total energy released per nuclear fission in \[\mathbf{MeV}\] units when the masses in \[\mathbf{amu}\] units are of neutron \[=\mathbf{1}.\mathbf{009}\] of \[\mathbf{X}\] - nucleus\[=\text{ }\mathbf{235}.\mathbf{891}\] of \[\mathbf{Y}\] -nucleus\[=\text{ }\mathbf{140}.\mathbf{673}\] of \[\mathbf{Z}\] -nucleus = 91.791        
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• question_answer82)
  Neutrons produced in fission can be slowed down even by using ordinary water. Then, why is heavy water used for this purpose?   
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• question_answer83)
  A chain reaction dies out sometimes. Why?
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• question_answer84)
  Give one similarity and one dissimilarity between nuclear fission and nuclear fusion.
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• question_answer85)
  Give two points of difference between nuclear fission and nuclear fusion.    
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• question_answer86)
  Safety of nuclear reactors is an important issue that has attracted much attention recently. Guess some of the safety problems that a nuclear engineer must cope with in reactor design.
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• question_answer87)
  What is nuclear fallout? How can it be reduced?
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• question_answer88)
  Name the reaction which takes place when a slow neutron beam strikes \[_{92}^{235}U\] nuclei. Write the nuclear reaction involved.               
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• question_answer89)
  Calculate the binding energy per nucleon (in \[\mathbf{MeV}\]) for \[_{2}^{4}\] He and \[_{2}^{3}\] He. Comment on the difference of these binding energies and its significance in relation to \[\alpha \] -decay of the nuclei. [Given : mass of \[_{1}^{1}\] \[H=\mathbf{1}.\mathbf{00783}\text{ }\mathbf{u},\] mass of \[_{0}^{1}n=1.00867u,\] mass of \[_{2}^{3}\] He  \[=3.01664u,\] mass of \[_{2}^{4}\] He \[=4.00387u]\]
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• question_answer90)
  A nucleus makes a transition from one permitted energy level to another level to lower energy. Name the region of the electromagnetic spectrum to which the emitted photon belongs. What is the order of its energy in electron volts? Write four characteristics of nuclear force.                
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• question_answer91)
  Define mass number (A) of an atomic nucleus. Assuming the nucleus to be spherical, give the relation between mass number (A) and the radius (R) of the nucleus.                                   Calculate the density of nuclear matter. Radius of nucleus of \[_{1}^{1}H=1.1\times {{10}^{-15}}\overset{\text{o}}{\mathop{\text{A}}}\,\] . What is the ratio of the order of magnitude of density of nuclear matter and density of ordinary matter?       
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• question_answer92)
  Define the term 'Activity' of a radio- active substance. State its \[SI\] unit. Two different radioactive elements with half lives \[{{T}_{1}}\] and \[{{T}_{2}}\] have \[{{N}_{1}}\] and \[{{N}_{2}}\] (undecayed) atoms respectively present at a given instant. Determine the ratio of their activities at this instant.                  
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• question_answer93)
  Define the activity of a radionuclide. Write its SI unit. Give a plot of the activity of a radioactive species versus time. How long will a radioactive iostope, whose half life is T years, take for its activity to reduce to \[1/8\text{th}\] of its initial value?                                
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• question_answer94)
  Prove that the instantaneous rate of change of the activity of a radioactive substance is inversely proportional to the square of its half life.       
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• question_answer95)
  The potential energy (V), of a pair of nucleons varies with separation (r) between them, in the manner shown in Fig. Use this graph to explain why the force between the nucleons must be regarded as (i) strongly repulsive for separation values less than \[{{r}_{0}}\]. (ii) attractive nuclear force \[(r>{{r}_{0}})\]
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• question_answer96)
  Why is it necessary to slow down the neutrons, produced through the fission of \[_{92}^{235}\text{U}\] nuclei (by neutrons), to sustain a chain reaction? What type of nuclei are  (preferably) needed for slowing down neutrons?                                  
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• question_answer97)
  Give reasons for (a) Lighter elements are better moderators  for a nuclear reactor than heavier elements. (b) Cadmium rods are provided in a reactor. (c) Very high temperatures as those obtained in the interior of the sun are required for fusion reaction to take place.
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