Home » Handbook of O Level Physics » UNIT 25: RADIOACTIVITY AND THE NUCLEAR ATOM


  • Characteristics of nuclear radiations:

Alpha (α)

Beta (β)

Gamma (γ)


Helium nucleus

High speed electron

Electromagnetic wave

Ionizing effect






(may be blocked by
paper or few cm of air)


(may be blocked by
0.5 cm of aluminum)


(may be blocked by
2 cm of lead)

Deflection in magnetic field


(use Left-hand-rule)


(use Left-hand-rule)


Deflection in electric field




  • Radioactive decay: It is the break down of unstable nuclei in order to become more stable. In the process α, β or γ radiations are emitted.

    • Types:

      • Asterisk (*) indicates that the nucleus is excited. Gamma rays are usually emitted at the same moment as either an alpha or beta particle.

  • Radioactivity is a phenomenon that is random in terms of:

    • time (You can’t exactly predict when a nucleus will decay.)

    • space (You can’t predict which nucleus will decay next.)

    • direction of emission

  • Methods of detecting radioactivity

    • Photographic plates are fogged.

    • Charged electroscope is neutralized because some ions made in air are attracted to it.

    • Diffusion cloud chamber shows tracks formed by alpha, beta or gamma radiations because alcohol vapour condenses on the ions formed.

    • GM-tube connected to a ratemeter or scaler.

      • Construction: See Fig 25.8 on page 398.

      • Working: When radiation ionizes argon gas inside GM-tube, an electrical pulse is produced.

  • Background radiation: It is the radiation in our surroundings due to:

    • cosmic rays from stars

    • underground radioactive rocks

  • Half-life: It is the time taken for half of the unstable nuclei to decay.

  • Uses of radioactive materials

    • tracers

    • penetrating radiation for thickness control or to reveal faults in weldings

    • nuclear fuel (e.g., Uranium-235)

    • treating cancer (by using gamma radiations)

    • archaeological dating (also known as carbon dating)

  • Hazards of radiations: Overexposure may lead to radiation burns, eye-cataracts, leukaemia (blood cancer), genetic mutations or even death.

  • Precautions

    • Keep distance by using forceps

    • Wear lead lined suits

    • Avoid eating and drinking

  • Conclusions from Geiger Marsden experiment (alpha scattering by gold foil):

    • Most alpha particles passed straight through. Therefore, most of the space in an atom is empty.

    • Few alpha particles were deflected by huge angles. Therefore, there must be a place (nucleus) where positive charges (protons) are concentrated.

  • Nucleon number (mass number) (A): It is the sum of the number of protons and the number of neutrons in an atomic nucleus.

  • Proton number (atomic number) (Z): It is the number of protons in an atomic nucleus.

  • Isotopes: Isotopes of an element are atoms which have the same proton number but different nucleon numbers.

  • Energy and mass can be inter-converted according to the formula:

    E = m c²

    where c is the speed of light in vacuum.

  • Nuclear fission: It is the process in which heavy unstable nuclides break up to produce energy.

    • Some related equations:

      Note: If the 3 neutrons are made to collide with other Uranium-235 atoms, a chain-reaction starts.

  • Nuclear fusion: It is the process in which lighter nuclides fuse together to form a heavier nucleus with the release of energy.

  • Star formation: Theories SUGGEST that a star is born within a giant cloud of dust (called nebula).
  1. Turbulence deep within these clouds gives rise to knots with sufficient mass that the gas and dust can begin to collapse under its own gravitational attraction.

  2. Friction raises temperature.

  3. When the temperature is high enough, nuclear fusion of hydrogen starts (to form Helium) and a star is born.

Go back to table of contents

Have questions? See our compilation of questions and answers.