Free Radioactive Decay MCQs with Answers

1. 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.

2. 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.

3. 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.

4. 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.