Understanding Radioactivity and particles: Edexcel IGCSE Physics Explained

Radioactivity and particles takes you inside the atom itself. This section of the Edexcel IGCSE Physics specification demands precise terminology, careful equation balancing, and a clear understanding of nuclear processes.

The radioactivity and particles edexcel igcse section covers three topics: units, radioactivity, and fission and fusion. It is one of the most regularly examined areas, with half-life calculations and nuclear equations appearing frequently on both papers. The edexcel igcse physics radioactivity and particles content is conceptually rich: you need to understand atomic structure, the nature of ionising radiation, the randomness of decay, and the energy released in nuclear reactions. Precise language is essential throughout.

These edexcel igcse physics revision notes provide a rigorous treatment of every concept in this section. Each definition, each equation and each worked example is crafted to match what the mark scheme expects. Master these revision notes, and the radioactivity questions on the exam will feel like familiar ground.

Units for radioactivity

Atomic structure

An atom consists of a central nucleus surrounded by orbiting electrons. The nucleus contains protons and neutrons. Protons carry a positive charge (+1), neutrons carry no charge (0), and electrons carry a negative charge (-1). In a neutral atom, the number of protons equals the number of electrons.

• Atomic number (Z): the number of protons in the nucleus. This defines the element.
• Mass number (A): the total number of protons and neutrons in the nucleus.
• Number of neutrons: A - Z.

Notation: a nucleus is written as ^A_ZX, where X is the element symbol. For example, carbon-14 is ¹⁴₆C (6 protons, 8 neutrons).

Isotopes

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have the same atomic number but different mass numbers. For example, carbon-12 (¹²₆C) and carbon-14 (¹⁴₆C) are both carbon but have 6 and 8 neutrons respectively.

Ionising radiation

Unstable nuclei emit ionising radiation in a random process called radioactive decay. There are three main types of radiation, plus neutron radiation:

Effects on atomic and mass numbers

• Alpha decay: mass number decreases by 4, atomic number decreases by 2. Example: ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He
• Beta decay: mass number stays the same, atomic number increases by 1 (a neutron converts to a proton and emits an electron). Example: ¹⁴₆C → ¹⁴₇N + ⁰_-1e
• Gamma emission: no change in mass number or atomic number. Gamma is emitted alongside alpha or beta to release excess energy from the nucleus.
• Neutron emission: mass number decreases by 1, atomic number stays the same.

Balancing nuclear equations

In any nuclear equation, the total mass numbers on both sides must be equal, and the total atomic numbers on both sides must be equal. This is how you determine the identity of an unknown product.

Detection

Ionising radiation can be detected by photographic film (it darkens when exposed) or by a Geiger-Muller (GM) detector connected to a counter or ratemeter.

Background radiation

Background radiation is always present. Sources include radon gas from the ground, cosmic rays from space, rocks and soil containing radioactive minerals, medical sources, and nuclear fallout. When measuring the activity of a source, you must subtract the background count rate to find the corrected count rate.

Half-life

The half-life of a radioactive isotope is the time it takes for half the unstable nuclei in a sample to decay, or equivalently, the time for the activity to halve. Different isotopes have different half-lives, ranging from fractions of a second to billions of years.

Half-life calculations can be done numerically or graphically. On a graph of activity vs time, the half-life is the time for the activity to drop from any value to half that value.

Uses of radioactivity

• Medical: Gamma rays are used to sterilise surgical instruments. Radioactive tracers (beta or gamma emitters with short half-lives) are used to track the flow of substances through the body.
• Industrial: Beta radiation is used to monitor the thickness of materials (paper, metal sheets). Gamma radiation is used to detect cracks in metal structures (radiography).
• Carbon dating: Carbon-14 (half-life about 5700 years) is used to estimate the age of organic remains.

Contamination vs irradiation

Contamination means radioactive material has been deposited on or inside an object or person. The source of radiation is in contact, and it continues to irradiate until removed. Irradiation means exposure to radiation from an external source. Once the source is removed or shielded, the exposure stops. The irradiated object does not become radioactive.

Dangers of ionising radiation

Ionising radiation can damage cells and DNA, causing mutations that may lead to cancer. The severity depends on the type of radiation, the dose, and the exposure time. Radioactive waste must be disposed of carefully: high-level waste is vitrified (turned into glass) and stored deep underground; low-level waste is sealed in containers and buried in lined landfills. Risk is reduced by minimising exposure time, maximising distance from sources, and using appropriate shielding.

Fission and fusion

Nuclear fission

Fission is the splitting of a heavy nucleus into two lighter daughter nuclei, releasing energy and additional neutrons. The most common fission fuel is uranium-235. When a neutron strikes a U-235 nucleus, it splits into two radioactive daughter nuclei, typically releases 2 or 3 neutrons, and releases energy as kinetic energy of the products.

If the neutrons released by one fission event strike other U-235 nuclei, they cause further fissions. This is a chain reaction. In a nuclear reactor, the chain reaction is controlled by:

• Control rods (boron or cadmium): absorb excess neutrons to control the rate of fission. Inserting them further slows the reaction; withdrawing them speeds it up.
• Moderator (graphite or water): slows down the fast neutrons to a speed at which they are more likely to cause fission of U-235.
• Shielding (thick concrete): absorbs radiation to protect workers and the environment.

Nuclear fusion

Fusion is the joining of two light nuclei to form a heavier nucleus, with a release of energy and a loss of mass. Fusion is the energy source for stars: hydrogen nuclei fuse to form helium under the extreme temperatures and pressures in a star's core.

Fusion does not occur at low temperatures and pressures because the positively charged nuclei repel each other (electrostatic repulsion). Extremely high temperatures (millions of degrees) are needed to give the nuclei enough kinetic energy to overcome this repulsion and get close enough for the strong nuclear force to bind them together.

Common mistakes in radioactivity and particles

1. Confusing atomic number and mass number. Atomic number (Z) is the number of protons. Mass number (A) is protons + neutrons. The mass number is always the larger of the two values written with the symbol.
2. Saying beta particles come from the electron shell. Beta particles are produced in the nucleus when a neutron converts to a proton and emits a high-speed electron. They do not come from the orbiting electrons.
3. Not subtracting background radiation. When calculating corrected activity or half-life from experimental data, you must subtract the background count rate first.
4. Stating that irradiated objects become radioactive. Irradiation is exposure to radiation from an external source. The object does not become radioactive itself (unless it undergoes neutron activation, which is beyond this specification).
5. Mixing up fission and fusion. Fission splits heavy nuclei. Fusion joins light nuclei. Both release energy. Fission is used in nuclear power stations; fusion is the process that powers stars.

Self-check: edexcel igcse physics practice questions

1. Thorium-232 (²³²₉₀Th) undergoes alpha decay. Write the balanced nuclear equation and identify the daughter element.
2. A radioactive source has an activity of 6400 Bq. After 20 minutes, the activity has fallen to 400 Bq. Calculate the half-life.
3. Explain why gamma radiation is more penetrating than alpha radiation.
4. Describe the role of the moderator in a nuclear fission reactor.
5. Explain why nuclear fusion requires extremely high temperatures.

Answers: (1) ²³²₉₀Th → ²²⁸₈₈Ra + ⁴₂He (radium-228); (2) 6400 → 3200 → 1600 → 800 → 400 = 4 half-lives in 20 min, half-life = 5 min; (3) Gamma rays are electromagnetic radiation with no charge and no mass, so they interact less strongly with matter and pass through more material before being absorbed, unlike alpha particles which are large, charged and interact strongly; (4) The moderator slows down fast neutrons to a speed at which they are more likely to cause fission of U-235; (5) Nuclei are positively charged and repel each other; extremely high temperatures give the nuclei enough kinetic energy to overcome this electrostatic repulsion and get close enough for the strong nuclear force to fuse them.

These edexcel igcse physics notes cover the complete igcse 4ph1 radioactivity and particles specification. The edexcel igcse physics explained treatment above gives you every definition, equation and process you need. Work through the examples systematically, practise the practice questions under timed conditions, and this section of the edexcel exam will become one of your strongest areas.