Free Nuclear Physics MCQs with Answers

1. Isotopes of an element have the same

• A. number of neutrons but different numbers of protons
• B. number of protons but different numbers of neutrons
• C. mass number but different atomic numbers
• D. number of protons and neutrons

Explanation: Atomic number fixes the element, so isotopes share chemical behaviour but differ in mass, as carbon 12 and carbon 14 do. Different proton numbers would make them different elements entirely. The difference in mass is what allows isotopes to be separated in a mass spectrometer.

2. The number of neutrons in a nucleus of uranium 235, atomic number 92, is

• A. 92
• B. 143
• C. 235
• D. 327

Explanation: Neutron number is mass number minus atomic number, so 235 minus 92 gives 143. The mass number counts protons and neutrons together, which are collectively called nucleons. Adding the two numbers, as in the 327 distractor, has no physical meaning.

3. The force that holds protons and neutrons together in a nucleus is

• A. the electrostatic force
• B. the gravitational force
• C. the strong nuclear force, which is short ranged and attractive
• D. the magnetic force

Explanation: The strong force acts over only a few femtometres but is far stronger than the electrostatic repulsion between protons at that range, which is what keeps the nucleus bound. Because it is so short ranged, very large nuclei become unstable as the repulsion, which has unlimited range, begins to win. Gravity is utterly negligible at nuclear scales.

4. An alpha particle is identical to

• A. a helium nucleus, two protons and two neutrons
• B. a fast electron
• C. a photon
• D. a proton

Explanation: Alpha emission therefore reduces the mass number by 4 and the atomic number by 2, moving the element two places back in the periodic table. Being heavy and doubly charged, alpha particles ionise strongly but are stopped by a sheet of paper or a few centimetres of air. A fast electron is a beta particle.

5. When a nucleus emits a beta minus particle, its atomic number

• A. decreases by 1
• B. increases by 1, while the mass number is unchanged
• C. decreases by 2
• D. is unchanged

Explanation: A neutron converts into a proton, an electron and an antineutrino, so the nucleus gains a proton while the total nucleon count stays the same, moving the element one place forward in the periodic table. The emitted electron comes from the nucleus, not from the electron shells. Gamma emission, by contrast, changes neither number.

6. Which type of radiation has the greatest penetrating power?

• A. Alpha
• B. Beta
• C. Gamma
• D. All are equal

Explanation: Gamma rays are uncharged electromagnetic radiation and need several centimetres of lead or a metre of concrete to attenuate them appreciably, whereas beta particles are stopped by a few millimetres of aluminium and alpha by paper. Penetrating power and ionising power run in opposite directions, so alpha ionises most heavily but travels least far. That is why an alpha emitter is most dangerous when it is inhaled or swallowed.

7. Gamma emission from a nucleus results in

• A. a change in mass number only
• B. a change in atomic number only
• C. no change in either mass number or atomic number
• D. a change in both

Explanation: A gamma ray carries away energy from an excited nucleus without removing any nucleons, so the nuclide is the same afterwards, simply in a lower energy state. Gamma emission usually follows alpha or beta decay, which often leaves the daughter nucleus excited. It is the nuclear analogue of an atom emitting a photon of light.

8. The half life of a radioactive sample is the time in which

• A. the whole sample decays
• B. half the undecayed nuclei present decay
• C. the activity becomes zero
• D. the mass of the sample halves

Explanation: Decay is a random process with a constant probability per nucleus per unit time, so a fixed fraction rather than a fixed number decays in each interval, which is what makes the half life constant. After two half lives one quarter remains, after three one eighth, and the count never quite reaches zero. The mass of the sample barely changes, since the daughter nuclei are almost as heavy.

9. A radioactive sample has a half life of 8 days. The fraction remaining undecayed after 24 days is

• A. one half
• B. one quarter
• C. one eighth
• D. one sixteenth

Explanation: Twenty four days is three half lives, so the sample is halved three times, leaving one half multiplied by one half multiplied by one half, which is one eighth. A common error is to divide 24 by 8 and answer one third, which misunderstands the exponential nature of decay. Iodine 131, used in thyroid treatment, has almost exactly this half life.

10. The activity of a radioactive source is measured in

• A. becquerel
• B. gray
• C. sievert
• D. coulomb

Explanation: One becquerel is one disintegration per second, so it measures how fast the source is decaying. The gray measures absorbed dose in joules per kilogram and the sievert measures the biologically weighted equivalent dose, which is what radiation protection limits are quoted in. Keeping these three units apart is a common examination point.