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Question 1. Which of the following correctly defines the atomic number (Z)? A) Number of neutrons B) Number of protons C) Total number of nucleons D) Number of electrons in a neutral atom Answer: B Explanation: The atomic number Z is the number of protons in the nucleus; in a neutral atom it also equals the number of electrons. Question 2. The mass number (A) of a nucleus is equal to: A) Number of protons only B) Number of neutrons only C) Number of protons plus neutrons D) Number of electrons plus neutrons Answer: C Explanation: Mass number A = Z (protons) + N (neutrons). Question 3. An isotope is best described as: A) Nuclei with the same Z but different A B) Nuclei with the same A but different Z C) Nuclei with the same N but different Z D) Nuclei with identical proton and neutron numbers Answer: A
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Explanation: Isotopes share the same atomic number (same element) but differ in neutron count, giving different mass numbers. Question 4. The notation (_{Z}^{A}\text{X}) represents: A) A molecule of element X B) An ion of element X C) A nuclide of element X with Z protons and A nucleons D) The electron configuration of element X Answer: C Explanation: The superscript A is the mass number, the subscript Z is the atomic number, and X is the chemical symbol. Question 5. Two nuclides that have the same mass number (A) but different atomic numbers (Z) are called: A) Isotopes B) Isobars C) Isotones D) Isomers Answer: B Explanation: Isobars are nuclides with equal mass numbers but different proton numbers. Question 6. Nuclei that have the same neutron number (N) but different proton numbers are known as: A) Isotopes B) Isobars
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A) Long‑range and repulsive B) Short‑range and charge‑independent C) Acts only between protons D) Weak compared with gravity Answer: B Explanation: The strong force acts over ~1–2 fm, binds nucleons regardless of charge, and is much stronger than electromagnetic repulsion. Question 10. The saturation property of the strong force means: A) Each nucleon interacts with all others in the nucleus B) Each nucleon interacts only with its nearest neighbors C) The force strength increases with nuclear size indefinitely D) The force is independent of distance Answer: B Explanation: Saturation indicates a nucleon feels the strong force only from a limited number of nearby nucleons. Question 11. The mass defect (Δm) of a nucleus is defined as: A) Mass of the nucleus minus sum of constituent nucleon masses B) Sum of constituent nucleon masses minus mass of the nucleus C) Difference between neutron and proton masses D) Mass of the electron cloud Answer: B Explanation: Δm = (Z m_p + N m_n) – m_nucleus; it represents the mass converted to binding energy.
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Question 12. Binding energy (BE) of a nucleus can be calculated using: A) (E = mc^{2}) with the mass defect B) (E = \frac{1}{2}mv^{2}) C) (E = hf) where h is Planck’s constant D) (E = kT) where k is Boltzmann’s constant Answer: A Explanation: BE = Δm c² converts the mass defect to energy. Question 13. The binding energy per nucleon curve shows a maximum near which mass number? A) A ≈ 4 B) A ≈ 56 C) A ≈ 100 D) A ≈ 208 Answer: B Explanation: Nuclei around iron‑56 have the highest BE per nucleon, indicating greatest stability. Question 14. Which of the following nuclei is most stable according to the BE/A curve? A) (^4)He B) (^56)Fe C) (^238)U D) (^2)H Answer: B
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Answer: B Explanation: Activity A = λN, measured in becquerels (Bq = 1 decay s⁻¹). Question 18. One curie (Ci) equals how many becquerels? A) 3.7 × 10⁴ Bq B) 3.7 × 10⁶ Bq C) 3.7 × 10⁸ Bq D) 3.7 × 10¹⁰ Bq Answer: C Explanation: 1 Ci = 3.7 × 10¹⁰ Bq; the correct exponent is 10, not 8. (Thus answer C is a trap; correct answer is D.) Correct Answer: D Explanation: Historically, 1 Ci = 3.7 × 10¹⁰ decays s⁻¹. Question 19. The mean life (τ) of a radionuclide is related to the half‑life by: A) τ = T₁/₂ / ln 2 B) τ = T₁/₂ × ln 2 C) τ = T₁/₂ / 2 D) τ = 2 T₁/₂ Answer: A Explanation: τ = 1/λ and λ = ln 2 / T₁/₂, so τ = T₁/₂ / ln 2. Question 20. In a parent‑daughter decay chain where the daughter’s half‑life is much shorter than the parent’s, the system reaches:
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A) Secular equilibrium B) Transient equilibrium C) No equilibrium D) Chemical equilibrium Answer: A Explanation: When λ_d >> λp, the daughter activity quickly matches the parent’s, giving secular equilibrium. Question 21. Which decay mode involves the emission of a helium‑4 nucleus? A) β⁻ decay B) α decay C) γ decay D) Electron capture Answer: B Explanation: Alpha decay emits a ^4He nucleus (2 protons, 2 neutrons). Question 22. The Q‑value of an α‑decay is the: A) Kinetic energy of the emitted α particle only B) Energy released due to mass difference between parent and products C) Binding energy of the daughter nucleus D) Energy required to overcome the Coulomb barrier Answer: B Explanation: Q = (mass_parent – mass_daughter – massα) c²; it is the total energy released.
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Question 26. Electron capture (EC) differs from β⁺ decay because: A) EC emits a γ‑ray instead of a neutrino B) EC captures an inner‑shell electron, no positron is emitted C) EC occurs only in heavy nuclei D) EC produces two neutrons Answer: B Explanation: In EC, a proton captures an orbital electron, converting to a neutron and emitting a neutrino; no positron is released. Question 27. The primary purpose of γ‑ray emission following a nuclear transition is to: A) Change the atomic number B) Remove excess charge C) De‑excite the nucleus without changing Z or A D) Increase the mass number Answer: C Explanation: Gamma decay removes excess nuclear energy while leaving Z and A unchanged. Question 28. Which conservation law is NOT violated in any nuclear decay process? A) Charge conservation B) Lepton number conservation C) Baryon number conservation D) All of the above are always conserved Answer: D
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Explanation: All listed quantities are conserved in nuclear reactions and decays. Question 29. The Q‑value for a β⁻ decay is calculated as: A) Difference in atomic masses between parent and daughter B) Difference in nuclear masses between parent and daughter C) Difference in binding energies of parent and daughter D) Difference in electron masses only Answer: A Explanation: Since atomic electrons are present on both sides, using atomic masses automatically includes the emitted electron. Question 30. The threshold energy for an endothermic nuclear reaction is: A) Zero B) Equal to the Q‑value (negative) C) Greater than zero, equal to – Q plus recoil corrections D) Always larger than 10 MeV Answer: C Explanation: For reactions with negative Q, the projectile must supply at least – Q (plus small kinetic energy needed for momentum conservation). Question 31. In the liquid‑drop model of fission, the term that accounts for the surface energy opposes: A) Coulomb repulsion B) Neutron‑proton attraction C) Weak interaction
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A) Absorb neutrons completely B) Slow down fast neutrons to thermal energies C) Increase the kinetic energy of neutrons D) Provide cooling for the core Answer: B Explanation: Moderators (e.g., water, graphite) reduce neutron velocities, enhancing the probability of fission in fissile isotopes. Question 35. The predominant fusion reaction in the Sun’s core is: A) Deuterium‑tritium (D‑T) B) Proton‑proton (p‑p) chain C) Carbon‑nitrogen‑oxygen (CNO) cycle D) Helium‑helium (α‑α) fusion Answer: B Explanation: The Sun mainly fuses hydrogen via the p‑p chain. Question 36. The CNO cycle becomes the dominant energy source in stars with core temperatures above: A) 4 × 10⁶ K B) 1 × 10⁷ K C) 2 × 10⁷ K D) 5 × 10⁷ K Answer: C Explanation: At ≈ 2 × 10⁷ K, the CNO cycle overtakes the p‑p chain in massive stars.
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Question 37. The Bethe‑Bloch formula describes: A) The probability of photon absorption B) Energy loss per unit path length of charged particles in matter C) The rate of neutron capture D) The half‑life of unstable nuclei Answer: B Explanation: It gives the stopping power (dE/dx) for heavy charged particles traversing a medium. Question 38. Which interaction dominates for low‑energy (≤ 100 keV) photons in high‑Z materials? A) Photoelectric effect B) Compton scattering C) Pair production D) Rayleigh scattering Answer: A Explanation: The photoelectric cross‑section scales roughly as Z⁴–Z⁵ and dominates at low photon energies. Question 39. Pair production becomes possible when the photon energy exceeds: A) 511 keV B) 1.022 MeV C) 2.044 MeV D) 10 MeV Answer: B
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Answer: B Explanation: LNT posits that any non‑zero dose carries some risk, increasing proportionally. Question 43. In a Geiger‑Müller (GM) tube, the gas is typically: A) At atmospheric pressure and inert B) At high pressure and electronegative C) At low pressure and filled with a noble gas plus quench gas D) Liquid‑filled for better detection Answer: C Explanation: GM tubes use low‑pressure noble gases (e.g., argon) with a halogen quench to limit discharge. Question 44. The proportional counter differs from a GM tube mainly because: A) It operates at a higher voltage, giving pulse height proportional to energy deposited B) It cannot detect gamma rays C) It uses a solid‑state detector D) It has no dead time Answer: A Explanation: In proportional mode, the output pulse size is proportional to the ionization produced. Question 45. Scintillation detectors convert radiation into: A) Electrical current directly B) Heat C) Visible light photons, which are then amplified by a photomultiplier tube (PMT)
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D) Sound waves Answer: C Explanation: Scintillators emit photons that are detected and amplified by a PMT. Question 46. High‑purity germanium (HPGe) detectors are prized for: A) Their low cost B) Excellent energy resolution for gamma spectroscopy C) Ability to detect neutrons directly D) Insensitivity to temperature Answer: B Explanation: HPGe provides superior resolution (≈ 0.2 % at 1 MeV) due to its semiconductor properties. Question 47. The dead time of a radiation detector refers to: A) The time required to calibrate the instrument B) The period after each event during which the detector cannot record another event C) The half‑life of the source being measured D) The time needed for the detector to warm up Answer: B Explanation: During dead time, pulses are ignored, leading to count loss at high rates. Question 48. To correct for dead‑time losses in a non‑paralyzable detector, one uses: A) (R = r/(1 - r\tau)) where r is observed count rate and τ is dead time B) (R = r(1 - r\tau))
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B) (^241)Am C) (^60)Co D) (^90)Sr Answer: B Explanation: Americium‑241 emits α particles to ionize air in detectors. Question 52. The most abundant natural source of β⁻ radiation is: A) (^14)C B) (^3)H C) (^40)K D) (^222)Rn Answer: C Explanation: Potassium‑40 decays by β⁻ emission (≈ 89 % of its decay). Question 53. The primary cause of radiation damage to DNA from γ rays is: A) Direct ionization of the DNA backbone B) Production of free radicals via water radiolysis C) Thermal heating of cells D) Magnetic field disruption Answer: B Explanation: γ photons ionize water, creating OH· radicals that damage DNA. Question 54. In a neutron activation analysis, the target nuclei become radioactive by:
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A) Capturing an electron B) Capturing a neutron and then β⁻ decaying C) Emitting an α particle D) Fissioning spontaneously Answer: B Explanation: Neutron capture produces a heavier isotope that often β⁻ decays, emitting characteristic γ rays. Question 55. The term “isomeric transition” refers to: A) A change in isotopic composition of a sample B) Decay from a nuclear excited state to a lower state via γ emission C) The conversion of a neutron to a proton D) The process of electron capture Answer: B Explanation: An isomeric transition is a γ decay between nuclear energy levels. Question 56. The recoil energy of a daughter nucleus after α emission is generally: A) Negligible compared to the α kinetic energy B) Equal to the α kinetic energy C) Larger than the α kinetic energy D) Zero, because momentum is not conserved Answer: A Explanation: Conservation of momentum gives the daughter a much smaller kinetic energy due to its larger mass.
