1. Radioactivity was discovered by:
a) Marie Curie
b) Henri Becquerel
c) Rutherford
d) J.J. Thomson
Answer: b) Henri Becquerel
Explanation: In 1896, Henri Becquerel discovered radioactivity while working with uranium salts.
2. Marie Curie discovered which radioactive elements?
a) Uranium and Thorium
b) Polonium and Radium
c) Uranium and Radium
d) Thorium and Radon
Answer: b) Polonium and Radium
Explanation: Marie and Pierre Curie discovered polonium and radium from pitchblende ore.
3. Radioactivity is a:
a) Physical property
b) Chemical property
c) Nuclear property
d) Mechanical property
Answer: c) Nuclear property
Explanation: Radioactivity originates from the instability of atomic nuclei.
4. The particles emitted in α-decay are:
a) Electrons
b) Helium nuclei
c) Protons
d) Neutrons
Answer: b) Helium nuclei
Explanation: Alpha particles = 2 protons + 2 neutrons (⁴₂He²⁺).
5. β-particles are:
a) Protons
b) Neutrons
c) Electrons
d) Positrons
Answer: c) Electrons
Explanation: Beta particles are fast-moving electrons emitted from the nucleus.
6. γ-rays are:
a) Particles
b) Electrons
c) Electromagnetic waves
d) Neutrons
Answer: c) Electromagnetic waves
Explanation: Gamma rays are high-energy photons with no mass or charge.
7. Which radiation has the highest penetrating power?
a) α-rays
b) β-rays
c) γ-rays
d) Neutrons
Answer: c) γ-rays
Explanation: γ-rays can penetrate thick lead sheets, unlike α and β.
8. Which radiation is stopped by a sheet of paper?
a) α-particles
b) β-particles
c) γ-rays
d) Neutrons
Answer: a) α-particles
Explanation: Alpha particles have very low penetration and are stopped by paper/skin.
9. The charge on α-particle is:
a) +1
b) +2
c) -1
d) 0
Answer: b) +2
Explanation: Alpha particle = 2 protons + 2 neutrons → net charge +2.
10. Which radiation has the least ionizing power?
a) α-rays
b) β-rays
c) γ-rays
d) Neutrons
Answer: c) γ-rays
Explanation: γ-rays are least ionizing, while α-rays are most ionizing.
11. The speed of β-particles is nearly:
a) Speed of sound
b) Speed of light
c) Zero
d) Half of light
Answer: b) Speed of light
Explanation: β-particles (electrons) move with speed close to that of light.
12. Which of the following is not a radioactive element?
a) Uranium
b) Radium
c) Thorium
d) Aluminium
Answer: d) Aluminium
Explanation: Aluminium is stable, not radioactive.
13. The SI unit of radioactivity is:
a) Curie
b) Becquerel
c) Rutherford
d) Roentgen
Answer: b) Becquerel
Explanation: 1 Bq = 1 disintegration per second (SI unit).
14. 1 Curie (Ci) is equal to:
a) 3.7 × 10¹⁰ disintegrations/sec
b) 1 disintegration/sec
c) 100 disintegrations/sec
d) 10⁶ disintegrations/sec
Answer: a) 3.7 × 10¹⁰ disintegrations/sec
Explanation: 1 Ci = activity of 1 g radium = 3.7 × 10¹⁰ Bq.
15. Which radiation is used for sterilization of food?
a) α-rays
b) β-rays
c) γ-rays
d) Neutrons
Answer: c) γ-rays
Explanation: γ-rays kill bacteria and preserve food.
16. In β⁻-decay, a neutron is converted into:
a) Proton + electron + antineutrino
b) Proton + positron + neutrino
c) Proton only
d) Electron only
Answer: a) Proton + electron + antineutrino
Explanation: n → p + e⁻ + ῡₑ.
17. In β⁺-decay, a proton changes into:
a) Neutron + positron + neutrino
b) Neutron + electron + antineutrino
c) Electron + proton
d) Neutron only
Answer: a) Neutron + positron + neutrino
Explanation: p → n + e⁺ + νₑ.
18. Which type of radiation does not change the mass number of the nucleus?
a) α-rays
b) β-rays
c) γ-rays
d) All of these
Answer: c) γ-rays
Explanation: γ emission only releases energy, no change in nucleons.
19. The radioactive isotope used in cancer treatment is:
a) C-14
b) Co-60
c) U-238
d) Th-232
Answer: b) Co-60
Explanation: Cobalt-60 emits γ-rays used in radiotherapy.
20. Half-life of a radioactive substance is:
a) Time to complete disintegration
b) Time to reduce activity to zero
c) Time for half the atoms to decay
d) Time for activity to double
Answer: c) Time for half the atoms to decay
Explanation: Half-life = time taken for 50% nuclei to disintegrate.
21. Radioactive decay is governed by:
a) Zero order kinetics
b) First order kinetics
c) Second order kinetics
d) Third order kinetics
Answer: b) First order kinetics
Explanation: Rate of decay ∝ number of nuclei → first order.
22. The nucleus of uranium-238 contains:
a) 92 protons and 146 neutrons
b) 238 protons
c) 146 protons and 92 neutrons
d) 119 protons and 119 neutrons
Answer: a) 92 protons and 146 neutrons
Explanation: Atomic number (Z) = 92, neutrons = 238 – 92 = 146.
23. Which isotope is used in radiocarbon dating?
a) C-12
b) C-13
c) C-14
d) O-16
Answer: c) C-14
Explanation: C-14 half-life = 5730 years → used in archaeological dating.
24. Which radioactive element is used in luminous paints?
a) Radium
b) Uranium
c) Thorium
d) Polonium
Answer: a) Radium
Explanation: Radium salts glow in the dark → used in luminous paints (historically).
25. The discovery of the nucleus by Rutherford was based on:
a) β-particle scattering
b) γ-ray diffraction
c) α-particle scattering
d) Neutron bombardment
Answer: c) α-particle scattering
Explanation: Rutherford’s gold foil experiment with α-particles revealed the nucleus.
26. The half-life of C-14 is about:
a) 100 years
b) 5730 years
c) 10,000 years
d) 1 million years
Answer: b) 5730 years
Explanation: C-14 has a half-life of 5730 years, useful in archaeology.
27. If a radioactive element has half-life of 10 days, after 30 days only:
a) 1/2 remains
b) 1/3 remains
c) 1/8 remains
d) 1/16 remains
Answer: c) 1/8 remains
Explanation: 30 days = 3 half-lives → (1/2)³ = 1/8.
28. The activity of a radioactive sample is directly proportional to:
a) Time
b) Number of undecayed nuclei
c) Mass of the container
d) Temperature
Answer: b) Number of undecayed nuclei
Explanation: A = λN, where A = activity, λ = decay constant.
29. Decay constant (λ) is related to half-life (T₁/₂) as:
a) λ = T₁/₂ × log 2
b) λ = ln 2 / T₁/₂
c) λ = T₁/₂ / ln 2
d) λ = 1 / (2T₁/₂)
Answer: b) λ = ln 2 / T₁/₂
Explanation: Radioactive decay follows first-order kinetics.
30. If the half-life of a substance is large, its decay constant will be:
a) Large
b) Small
c) Zero
d) Infinite
Answer: b) Small
Explanation: λ ∝ 1/T₁/₂ → large half-life means small decay constant.
31. Which equation describes radioactive decay?
a) N = N₀ e^(-λt)
b) N = N₀ e^(λt)
c) N = N₀ / (1+λt)
d) N = N₀ – λt
Answer: a) N = N₀ e^(-λt)
Explanation: Exponential decay law governs radioactivity.
32. After two half-lives, the fraction of undecayed atoms is:
a) 1/2
b) 1/3
c) 1/4
d) 1/8
Answer: c) 1/4
Explanation: (1/2)² = 1/4.
33. The unit of half-life is:
a) Becquerel
b) Curie
c) Seconds, minutes, years
d) Rutherford
Answer: c) Seconds, minutes, years
Explanation: Half-life is expressed in units of time.
34. Which isotopes are more stable?
a) Even Z – Even N
b) Odd Z – Odd N
c) Even Z – Odd N
d) None
Answer: a) Even Z – Even N
Explanation: Nuclei with even protons & neutrons are most stable.
35. Radioactive decay releases mainly:
a) Mechanical energy
b) Nuclear binding energy
c) Electrical energy
d) Magnetic energy
Answer: b) Nuclear binding energy
Explanation: Instability → loss of nuclear binding energy.
36. Uranium-238 decays finally to form:
a) Lead-206
b) Thorium-232
c) Radium-226
d) Bismuth-209
Answer: a) Lead-206
Explanation: U-238 undergoes a decay series ending in Pb-206.
37. The stable element formed after the decay series of Thorium-232 is:
a) Lead-208
b) Lead-206
c) Radium-226
d) Uranium-235
Answer: a) Lead-208
Explanation: Th-232 → Pb-208 (end product of thorium series).
38. The isotope used in treatment of thyroid diseases is:
a) I-127
b) I-131
c) C-14
d) Co-60
Answer: b) I-131
Explanation: I-131 accumulates in thyroid → used in diagnosis and treatment.
39. Which isotope is used in tracer studies of photosynthesis?
a) C-12
b) C-13
c) C-14
d) O-16
Answer: c) C-14
Explanation: Radioactive C-14 is used to trace carbon in plants.
40. Which isotope is used for radiometric dating of rocks?
a) C-14
b) U-238
c) K-40
d) Both b and c
Answer: d) Both b and c
Explanation: U-238 and K-40 are used to date rocks and minerals.
41. The half-life of radon-222 is about:
a) 1 year
b) 1600 years
c) 3.8 days
d) 30 years
Answer: c) 3.8 days
Explanation: Radon-222 has short half-life → radioactive gas.
42. The isotope used in smoke detectors is:
a) Am-241
b) U-235
c) Co-60
d) Th-232
Answer: a) Am-241
Explanation: Americium-241 emits α-particles → used in ionization-type smoke detectors.
43. The half-life of tritium (H-3) is:
a) 5730 years
b) 12.3 years
c) 100 years
d) 1 day
Answer: b) 12.3 years
Explanation: Tritium (³H) has a half-life of ~12.3 years.
44. Which element has the longest half-life?
a) U-235
b) Th-232
c) U-238
d) Pb-206
Answer: b) Th-232
Explanation: Thorium-232 has a half-life of ~1.4 × 10¹⁰ years.
45. The number of disintegrations per second is called:
a) Decay constant
b) Half-life
c) Activity
d) Power
Answer: c) Activity
Explanation: Activity A = λN, measured in Becquerel.
46. Which radioactive isotope is used in carbon dating?
a) C-12
b) C-13
c) C-14
d) O-16
Answer: c) C-14
Explanation: Archaeological dating uses C-14.
47. Which radioactive isotope is used to study red blood cell circulation?
a) Fe-59
b) Co-60
c) I-131
d) C-14
Answer: a) Fe-59
Explanation: Iron-59 is used to trace blood circulation.
48. Which of the following isotopes is used in agriculture to study phosphorus uptake?
a) P-32
b) S-35
c) C-14
d) I-131
Answer: a) P-32
Explanation: Phosphorus-32 is used in tracer studies in plants.
49. Which is used in sterilization of medical instruments?
a) β-rays
b) γ-rays (Co-60)
c) α-rays
d) Neutrons
Answer: b) γ-rays (Co-60)
Explanation: Gamma irradiation sterilizes surgical instruments.
50. The main reason for nuclear stability is:
a) Balance of protons and neutrons
b) Electrons in orbit
c) High atomic weight
d) High density
Answer: a) Balance of protons and neutrons
Explanation: Stability depends on neutron-to-proton (N/Z) ratio.
51. A nuclear reaction is a change that occurs in:
a) Electrons
b) Atomic nucleus
c) Orbitals
d) Molecules
Answer: b) Atomic nucleus
Explanation: Nuclear reactions involve changes in the composition of nuclei.
52. In a nuclear fission reaction, a heavy nucleus splits into:
a) Lighter nuclei
b) Neutrons only
c) Protons only
d) Electrons
Answer: a) Lighter nuclei
Explanation: Fission splits a heavy nucleus (like U-235) into two medium nuclei plus neutrons and energy.
53. Which of the following is used as fuel in nuclear reactors?
a) U-235
b) Th-232
c) Pu-239
d) All of these
Answer: d) All of these
Explanation: Uranium-235, Thorium-232, and Plutonium-239 are nuclear fuels.
54. Which particle initiates nuclear fission?
a) Proton
b) Neutron
c) α-particle
d) Electron
Answer: b) Neutron
Explanation: A neutron is used because it has no charge and can easily enter the nucleus.
55. Which isotope is most commonly used in nuclear power plants?
a) U-235
b) U-238
c) Th-232
d) C-14
Answer: a) U-235
Explanation: Uranium-235 undergoes controlled fission in nuclear reactors.
56. In a nuclear chain reaction, neutrons released cause:
a) Cooling of reactor
b) Further fission reactions
c) Reduction in energy
d) Production of γ-rays only
Answer: b) Further fission reactions
Explanation: Each fission releases neutrons → sustains chain reaction.
57. The moderator used in nuclear reactors is:
a) Graphite or Heavy water
b) Lead
c) Mercury
d) Sodium
Answer: a) Graphite or Heavy water
Explanation: Moderators slow down fast neutrons for sustained fission.
58. Which of the following is used as a coolant in nuclear reactors?
a) Graphite
b) Sodium or Heavy water
c) Zinc
d) Lead oxide
Answer: b) Sodium or Heavy water
Explanation: Liquid sodium or heavy water removes heat from the reactor.
59. Control rods in nuclear reactors are made of:
a) Aluminium
b) Cadmium or Boron
c) Iron
d) Lead
Answer: b) Cadmium or Boron
Explanation: These absorb excess neutrons to control the chain reaction.
60. Nuclear fusion is the process where:
a) Heavy nuclei split
b) Light nuclei combine
c) Neutrons are absorbed
d) Electrons are emitted
Answer: b) Light nuclei combine
Explanation: Fusion combines light nuclei (like H isotopes) into heavier nuclei with huge energy release.
61. Energy produced in stars, including the Sun, comes from:
a) Fission
b) Fusion of hydrogen nuclei
c) Radioactive decay
d) Combustion
Answer: b) Fusion of hydrogen nuclei
Explanation: Stars fuse hydrogen → helium, releasing massive energy.
62. Which isotopes of hydrogen are used in nuclear fusion?
a) Protium and Deuterium
b) Deuterium and Tritium
c) Protium and Tritium
d) Only Protium
Answer: b) Deuterium and Tritium
Explanation: D + T → He + neutron + huge energy.
63. Which reaction has higher energy output?
a) Nuclear fission
b) Nuclear fusion
c) Chemical combustion
d) Electrolysis
Answer: b) Nuclear fusion
Explanation: Fusion releases more energy per nucleon than fission.
64. The hydrogen bomb is based on:
a) Nuclear fission
b) Nuclear fusion
c) Both fission and fusion
d) Chemical explosion
Answer: c) Both fission and fusion
Explanation: Fission triggers the extremely high temperature needed for fusion.
65. The atomic bomb dropped on Hiroshima (1945) used:
a) U-235
b) Pu-239
c) Th-232
d) C-14
Answer: a) U-235
Explanation: “Little Boy” bomb used U-235.
66. The atomic bomb dropped on Nagasaki (1945) used:
a) U-235
b) Pu-239
c) Th-232
d) Co-60
Answer: b) Pu-239
Explanation: “Fat Man” bomb used Pu-239.
67. The peaceful use of nuclear energy is in:
a) Nuclear weapons
b) Nuclear power plants
c) Bomb testing
d) Radiation hazards
Answer: b) Nuclear power plants
Explanation: Controlled fission generates electricity.
68. Which of the following is a fusion reaction?
a) ²H + ³H → ⁴He + n
b) ²³⁵U + n → fission products
c) ²³⁸U → ²³⁴Th + α
d) ⁶⁰Co → ⁶⁰Ni + γ
Answer: a) ²H + ³H → ⁴He + n
Explanation: Deuterium + Tritium fusion reaction.
69. Which radioactive isotope is used as a fuel in breeder reactors?
a) U-235
b) U-238
c) Th-232
d) Both b and c
Answer: d) Both b and c
Explanation: U-238 and Th-232 convert into fissile Pu-239 and U-233 in breeder reactors.
70. The first nuclear reactor in India was:
a) CIRUS
b) Dhruva
c) Apsara
d) Kamini
Answer: c) Apsara
Explanation: Apsara, set up in 1956 at Trombay, was India’s first reactor.
71. Which Indian nuclear power station is the largest?
a) Tarapur
b) Kaiga
c) Kudankulam
d) Kalpakkam
Answer: c) Kudankulam
Explanation: Kudankulam (Tamil Nadu) is India’s largest nuclear power station.
72. The energy produced in nuclear reactions comes from:
a) Conversion of electrons
b) Mass-energy equivalence (E=mc²)
c) Movement of neutrons
d) Nuclear force only
Answer: b) Mass-energy equivalence (E=mc²)
Explanation: Loss of mass in reactions converts to energy.
73. Which radioactive isotope is used in pacemakers?
a) U-235
b) Pu-239
c) Co-60
d) Pu-238
Answer: d) Pu-238
Explanation: Plutonium-238 provides long-lasting power in heart pacemakers.
74. The critical mass in a fission reaction is:
a) Minimum mass of fuel required to sustain chain reaction
b) Maximum fuel mass
c) Zero
d) Amount of fuel that stops reaction
Answer: a) Minimum mass of fuel required to sustain chain reaction
Explanation: Critical mass ensures enough neutrons for continuous fission.
75. The coolant used in fast breeder reactors is:
a) Water
b) Sodium metal
c) Mercury
d) Heavy water
Answer: b) Sodium metal
Explanation: Fast breeder reactors use liquid sodium as coolant due to excellent heat transfer.
76. The harmful effect of α-particles is mainly due to:
a) High penetration
b) High ionization power
c) High speed
d) Low energy
Answer: b) High ionization power
Explanation: α-particles have low penetration but strong ionization, damaging tissues if ingested or inhaled.
77. β-particles cause damage because they:
a) Are heavy particles
b) Have moderate penetration and ionizing power
c) Have no charge
d) Do not interact with matter
Answer: b) Have moderate penetration and ionizing power
Explanation: β-rays penetrate skin and damage living tissues.
78. γ-rays are dangerous because:
a) They are positively charged
b) They have very high penetration power
c) They are heavy particles
d) They do not interact with tissues
Answer: b) They have very high penetration power
Explanation: γ-rays penetrate deeply, causing severe internal damage.
79. Which unit measures the biological effect of radiation?
a) Becquerel
b) Curie
c) Sievert
d) Hertz
Answer: c) Sievert
Explanation: Sievert (Sv) measures dose equivalent (biological effect).
80. The device used to detect and measure radiation is:
a) Ammeter
b) Voltmeter
c) Geiger-Müller counter
d) Galvanometer
Answer: c) Geiger-Müller counter
Explanation: GM counter detects α, β, γ radiation by ionization of gas.
81. Which device uses photographic film to measure radiation exposure?
a) Scintillation counter
b) Dosimeter
c) GM counter
d) Potentiometer
Answer: b) Dosimeter
Explanation: Film badges (dosimeters) record accumulated radiation exposure of workers.
82. Which radiation detector uses flashes of light?
a) Scintillation counter
b) GM counter
c) Dosimeter
d) Electroscope
Answer: a) Scintillation counter
Explanation: Scintillations (flashes) occur when radiation hits a fluorescent screen.
83. The harmful effects of radiation depend on:
a) Type of radiation
b) Dose received
c) Duration of exposure
d) All of the above
Answer: d) All of the above
Explanation: All factors contribute to radiation damage.
84. Acute radiation syndrome occurs due to:
a) Small exposure
b) High radiation dose in short time
c) Long exposure to low dose
d) Only α-rays
Answer: b) High radiation dose in short time
Explanation: Acute high exposure damages tissues severely.
85. Which cells are most sensitive to radiation?
a) Brain cells
b) Nerve cells
c) Reproductive cells and bone marrow
d) Muscle cells
Answer: c) Reproductive cells and bone marrow
Explanation: Rapidly dividing cells are most radiation-sensitive.
86. Which radioactive gas causes lung cancer?
a) Radon
b) Argon
c) Krypton
d) Neon
Answer: a) Radon
Explanation: Radon gas accumulates in basements → lung cancer risk.
87. Which of the following is a man-made radiation hazard?
a) Cosmic rays
b) Radon gas
c) X-rays
d) Solar radiation
Answer: c) X-rays
Explanation: X-rays are artificially produced ionizing radiation.
88. The safe limit of radiation for humans per year is about:
a) 1 Sievert
b) 100 Sieverts
c) 1 milliSievert
d) 20 milliSieverts
Answer: d) 20 milliSieverts
Explanation: Recommended occupational dose limit ≈ 20 mSv/year.
89. The effect of long-term low radiation exposure is called:
a) Acute radiation syndrome
b) Chronic radiation effect
c) Radiation shock
d) None
Answer: b) Chronic radiation effect
Explanation: Long-term exposure → cancer, genetic effects.
90. The isotope used in radiotherapy for cancer is:
a) Co-60
b) C-14
c) I-131
d) U-235
Answer: a) Co-60
Explanation: Cobalt-60 emits γ-rays used in cancer treatment.
91. The isotope used in PET (Positron Emission Tomography) scans is:
a) C-14
b) F-18
c) Co-60
d) U-235
Answer: b) F-18
Explanation: Fluorine-18 is used in PET imaging.
92. Which isotope is used to detect blood clots?
a) I-131
b) Na-24
c) Xe-133
d) Co-60
Answer: b) Na-24
Explanation: Sodium-24 is used as a tracer in blood circulation studies.
93. Radiation used to sterilize medical instruments:
a) α-rays
b) β-rays
c) γ-rays
d) Neutrons
Answer: c) γ-rays
Explanation: Gamma irradiation sterilizes without heat.
94. Which law states that radioactivity decreases exponentially with time?
a) Newton’s law
b) First order kinetics law
c) Radioactive decay law
d) Avogadro’s law
Answer: c) Radioactive decay law
Explanation: N = N₀e^(-λt).
95. The binding energy of a nucleus explains:
a) Nuclear stability
b) Atomic size
c) Electron affinity
d) Ionization energy
Answer: a) Nuclear stability
Explanation: Higher binding energy per nucleon = more stable nucleus.
96. Radiation causes genetic damage by:
a) Ionization of DNA molecules
b) Heating the body
c) Increasing cell growth
d) Strengthening tissues
Answer: a) Ionization of DNA molecules
Explanation: Ionizing radiation alters DNA → mutations.
97. Which radiation is deflected by electric and magnetic fields?
a) α and β
b) γ only
c) Neutrons
d) All of the above
Answer: a) α and β
Explanation: α (+ charge) and β (– charge) are deflected; γ and neutrons are neutral.
98. Which nuclear radiation is most suitable for thickness measurement in industries?
a) α-rays
b) β-rays
c) γ-rays
d) X-rays
Answer: b) β-rays
Explanation: Beta radiation is used to monitor thickness of sheets in industries.
99. The term “radioisotope” refers to:
a) A stable isotope
b) A radioactive isotope
c) A mixture of isotopes
d) A tracer only
Answer: b) A radioactive isotope
Explanation: Radioisotopes are unstable isotopes emitting radiation.
100. Which principle of Einstein explains nuclear energy?
a) E = mc²
b) E = hf
c) PV = nRT
d) F = ma
Answer: a) E = mc²
Explanation: Mass defect converts into energy during nuclear reactions.