Properties of RadiationEdexcel IGCSE Physics: Revision notes
Section 1
Atomic structure and notation
An atom consists of a small, dense nucleus containing protons and neutrons, surrounded by electrons in shells. Protons carry a positive charge, electrons a negative charge, and neutrons no charge.
- The atomic (proton) number is the number of protons in the nucleus (also equal to the number of electrons in a neutral atom).
- The mass (nucleon) number is the total number of protons and neutrons in the nucleus.
- An isotope is a form of an element with the same number of protons but a different number of neutrons.
Nuclei are written using notation such as , where the top number is the mass number and the bottom number is the atomic number.
Section 2
Types of ionising radiation
Unstable nuclei randomly emit ionising radiation to become more stable. There are four main types:
| Type | Nature | Penetrating power | Ionising ability |
|---|---|---|---|
| Alpha (α) | 2 protons + 2 neutrons (helium nucleus) | Stopped by paper/a few cm of air | Strongly ionising |
| Beta (β⁻) | A fast-moving electron | Stopped by a few mm of aluminium | Moderately ionising |
| Gamma (γ) | Electromagnetic radiation | Reduced by several cm of lead / metres of concrete | Weakly ionising |
| Neutron | A neutron ejected from the nucleus | Stopped by thick concrete/water | Non-ionising directly |
The emission of radiation from unstable nuclei happens randomly — it cannot be predicted when any individual nucleus will decay.
There is an inverse relationship worth stating explicitly: the more strongly ionising a radiation type, the less penetrating it is, and vice versa.
Section 3
Practical: investigating penetrating power
The penetrating power of alpha, beta and gamma radiation can be investigated using a radioactive source (or a simulation) and a Geiger–Müller (GM) tube/detector, placing different absorbers (paper, aluminium sheets of increasing thickness, lead) between the source and detector and recording the count rate.
Photographic film or a Geiger–Müller detector can be used to detect ionising radiation — film darkens on exposure, and a GM tube gives an audible click or numerical count per unit time.
Background radiation must be measured and subtracted from readings in this practical, otherwise the count rate overstates the source's activity.
Section 4
Effects on the nucleus and balancing equations
Each type of decay changes the nucleus differently:
- Alpha decay: mass number decreases by 4, atomic number decreases by 2.
- Beta decay: mass number unchanged, atomic number increases by 1 (a neutron becomes a proton, emitting an electron).
- Gamma decay: no change to mass number or atomic number (the nucleus loses energy only).
- Neutron emission: mass number decreases by 1, atomic number unchanged.
Nuclear equations must balance: the sum of mass numbers on each side must be equal, and the sum of atomic numbers (charges) on each side must be equal.
Alpha decay of radium-226: . Mass numbers: 226 = 222 + 4. Atomic numbers: 88 = 86 + 2.
Must Know
- atomic (proton) number = number of protons; mass (nucleon) number = protons + neutrons; isotopes share proton number but differ in neutron number
- alpha (helium nucleus), beta (fast electron) and gamma (EM radiation) are ionising radiations emitted randomly from unstable nuclei
- alpha: most ionising, least penetrating (stopped by paper); beta: intermediate (stopped by aluminium); gamma: least ionising, most penetrating (reduced by thick lead)
- photographic film and Geiger–Müller detectors are used to detect ionising radiation
- alpha decay: mass number −4, atomic number −2; beta decay: mass number unchanged, atomic number +1; gamma decay: no change to either
- nuclear equations must balance mass number and atomic number (charge) on both sides
That's the notes covered.
Carry on to the next subtopic.