CBSE Class 12 Physics Nuclei MCQs Set 05

Question. Suppose we consider a large number of containers each containing initially 10000 atoms of a radioactive material with a half life of 1 year. After 1 year,
(a) all the containers will have 5000 atoms of the material.
(b) all the containers will contain the same number of atoms of the material but that number will only be approximately 5000.
(c) the containers will in general have different numbers of the atoms of the material but their average will be close to 5000.
(d) none of the containers can have more than 5000 atoms.

Answer: C

Question. During a mean life of a radioactive element the fraction that disintegrates is:
(a) e
(b) 1/e
(c) e – 1/e
(d) e/e – 1

Answer: C

Question. How much energy will approximately be released if all the atoms of 1 kg of deuterium could undergo fusion? [Assume energy released per deuterium nucleus is 2 MeV]
(a) 2 × 10⁷ kWh
(b) 9 × 10¹³ J
(c) 6 × 10²⁷ calorie
(d) 9 × 10¹³ MeV

Answer: B

Question. The binding energies per nucleon of deuteron (₁H²) and helium (₂He⁴) nuclei are 1.1 MeV and 7 MeV respectively. If two deuterons fuse together to form a helium nucleus, then energy produced is:
(a) 5.9 MeV
(b) 23.6 MeV
(c) 26.9 MeV
(d) 32.4 MeV

Answer: B

Question. If radius of the ₁₃²⁷Al nucleus is taken to be R_Al, then the radius of ¹²⁵₅₃Te nucleus is nearly
(a) (3/5)R_Al
(b) (13/53)^1/3 R_Al
(c) (53/13)^1/3 R_Al
(d) (5/3)R_Al

Answer: B

Question. The equation _ZX^A → _{Z + 1} Y^A +  _-1e⁰ + v represents
(a) β-decay
(b) γ-decay
(c) fusion
(d) fission

Answer: A

Question. When a nucleus in an atom undergoes a radioactive decay, the electronic energy levels of the atom
(a) do not change for any type of radioactivity.
(b) change for α and β radioactivity but not for γ-radioactivity.
(c) change for α-radioactivity but not for others.
(d) change for β-radioactivity but not for others.

Answer: B

Question. M_x and M_y denote the atomic masses of the parent and the daughter nuclei respectively in a radioactive decay. The Q-value for a β^– decay is Q₁ and that for a β⁺ decay is Q_2. If me denotes the mass of an electron, then which of the following statements is correct?
(a) Q₁ = (M_x – M_y) c₂ and Q₂ = (M_x – M_y – 2m_e) c₂
(b) Q₁ = (M_x – M_y) c₂ and Q₂ = (M_x – M_y ) c₂
(c) Q₁ = (M_x – M_y – 2 m_e) c₂ and Q₂ = (M_x – M_y + 2 m_e) c₂
(d) Q₁ = (M_x – M_y + 2 m_e) c₂ and Q₂ = (M_x – M_y + 2 m_e) c₂

Answer: A

Question. When boron (¹⁰₅B) is bombarded by neutron, alpha-particles is emitted. The resulting nucleus has the mass number
(a) 11
(b) 7
(c) 6
(d) 15

Answer: B

Question. Tritium is an isotope of hydrogen whose nucleus Triton contains 2 neutrons and 1 proton. Free neutrons decay into p + e + v . If one of the neutrons in Triton decays, it would transform into He³ nucleus. This does not happen. This is because
(a) Triton energy is less than that of a He³ nucleus.
(b) the electron created in the beta decay process cannot remain in the nucleus.
(c) both the neutrons in triton have to decay simultaneously resulting in a nucleus with 3 protons, which is not a He³ nucleus.
(d) because free neutrons decay due to external perturbations which is absent in a triton nucleus.

Answer: A

Question. Heavy stable nuclei have more neutrons than protons. This is because of the fact that
(a) neutrons are heavier than protons.
(b) electrostatic force between protons are repulsive.
(c) neutrons decay into protons through beta decay.
(d) nuclear forces between neutrons are weaker than that between protons.

Answer: B

Question. In a nuclear reactor, moderators slow down the neutrons which come out in a fission process. The moderator used have light nuclei. Heavy nuclei will not serve the purpose because
(a) they will break up.
(b) elastic collision of neutrons with heavy nuclei will not slow them down.
(c) the net weight of the reactor would be unbearably high.
(d) substances with heavy nuclei do not occur in liquid or gaseous state at room temperature.

Answer: B

Question. Samples of two radioactive nuclides A and B are taken. λ_A and λ_B are the disintegration constants of A and B respectively. In which of the following cases, the two samples can simultaneously have the same decay rate at any time?
(a) Initial rate of decay of A is twice the initial rate of decay of B and λ_A = λ_B.
(b) Initial rate of decay of A is twice the initial rate of decay of B and λ_A > λ_BB.
(c) Initial rate of decay of B is twice the initial rate of decay of A and λ_A > λ_B.
(d) Initial rate of decay of B is same as the rate of decay of A at t = 2h and λ_B < λ_A

Answer: B, D

Question. The half life of ²¹⁵At is 100 μs. The time taken for the activity of the sample of ²¹⁵At to decay to 1/16 th of its initial value is
(a) 400 μs
(b) 300 μs
(c) 40 μs
(d) 6.3 μs

Answer: A

Question. For a radioactive material, half-life is 10 minutes. If initially there are 600 number of nuclei, the time taken (in minutes) for the disintegration of 450 nuclei is
(a) 20
(b) 10
(c) 30
(d) 15

Answer: A

Question. When an a-particle of mass m moving with velocity v bombards on a heavy nucleus of charge Ze, its distance of closest approach from the nucleus depends on m as
(a) 1/m²
(b) m
(c) 1/m
(d) 1/√m

Answer: C

Question. The binding energy per nucleon in ₃⁷Li and ₂⁴He are 7.06 MeV and 5.60 MeV respectively, then in the reaction: p +₃⁷Li → 2(₂⁴He) the energy of proton must be: 
(a) 28.24 MeV
(b) 17.28 MeV
(c) 1.46 MeV
(d) 39.2 MeV

Answer: B

Fill in the Blanks

Question. The rest mass of a nucleus is _________________ than the sum of the rest masses of its constituent nucleons.
Answer: less

Question. A radioactive isotope of silver has half life of 20 minutes. The fraction of the original activity that remain after one hour is _________________.
Answer: 1/8

Question. One atomic mass unit is defined as _________________ of mass of an atom of ₆C¹².
Answer: 1/12th

Question. Isotopes of an element are the atoms of an element which have _________________ but different atomic weights.
Answer: same atomic number

Question. Isobars are the atoms of different element which have same _________________ but different atomic number.
Answer: atomic weights

Question. Isotones are the nuclides which contains _________________.
Answer: same number of neutrons

Question. The process responsible for energy production in the sun is _________________.
Answer: nuclear fusion

Question. In both the processes of nuclear fission an nuclear fusion, a certain mass disappears. This is called _________________.
Answer: mass defect

Question. The Apsara reactor at the Bhabha Atomic Research Centre (BARC), Mumbai, uses _________________ as moderator.
Answer: water