Understanding Physics: Radioactivity and particles: Edexcel IGCSE Science Double Award Explained

Radioactivity and particles takes you inside the atom itself. This section of the Edexcel IGCSE Science Double Award covers the nature of radioactive decay, the properties of different types of radiation, and the processes of nuclear fission and fusion.

The edexcel igcse science double award physics: radioactivity and particles section of the 4SD0 specification addresses three topics: units, radioactivity, and fission and fusion. The material is precise and terminology-heavy, but the underlying principles are coherent: atoms can be unstable, unstable atoms emit radiation to become more stable, and nuclear processes release far more energy than chemical processes. These edexcel igcse science double award revision notes present each topic with the scholarly rigour the examination demands.

Units

Activity is the number of nuclear decays per second. One becquerel equals one decay per second. Half-life may be expressed in seconds, minutes, hours, days or years depending on the isotope. Some isotopes have half-lives of fractions of a second; others have half-lives of billions of years.

Radioactivity

Atomic structure: An atom consists of a central nucleus containing protons and neutrons, surrounded by orbiting electrons. The atomic number (Z) is the number of protons. The mass number (A) is the total number of protons and neutrons. Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Some isotopes are unstable and undergo radioactive decay, emitting radiation to become more stable.

There are three types of nuclear radiation:

When an atom undergoes alpha decay, it emits an alpha particle. The mass number decreases by 4 and the atomic number decreases by 2, producing a new element. For example, uranium-238 decays to thorium-234:

²³⁸₉₂U -> ²³⁴₉₀Th + ⁴₂He

In beta decay, a neutron in the nucleus converts into a proton and an electron. The electron is emitted as a beta particle. The mass number stays the same, but the atomic number increases by 1. For example, carbon-14 decays to nitrogen-14:

¹⁴₆C -> ¹⁴₇N + ⁰_-1e

Gamma emission often accompanies alpha or beta decay. It does not change the mass number or atomic number because a gamma ray is pure energy, not a particle. The edexcel igcse science double award specification requires you to write and balance nuclear equations for both alpha and beta decay.

Half-life: The half-life of a radioactive isotope is the time taken for half of the radioactive nuclei in a sample to decay, or equivalently, the time for the activity (count rate) to fall to half its initial value. After one half-life, 50% of the original atoms remain. After two half-lives, 25% remain. After three half-lives, 12.5% remain, and so on.

Radioactive decay is a random process. You cannot predict which individual nucleus will decay next, only the probability that a certain proportion of a large sample will decay in a given time. This randomness means that measurements of count rate fluctuate, and the igcse 4sd0 physics: radioactivity and particles specification expects you to understand that background radiation (from natural and man-made sources) must be subtracted from measured count rates to obtain the corrected count rate.

Uses of radioactivity: Different types of radiation are suited to different applications based on their penetrating and ionising properties:

• Alpha sources: Used in smoke detectors. Alpha particles ionise the air between two plates, creating a small current. Smoke particles absorb the alpha radiation, reducing the current and triggering the alarm.
• Beta sources: Used to monitor the thickness of materials in manufacturing (e.g. paper or metal foil). If the material becomes too thick, fewer beta particles pass through, and the detector signals for adjustment.
• Gamma sources: Used in medical imaging (tracers), sterilising medical equipment, and treating cancer (radiotherapy). Their high penetrating power allows them to pass through the body or packaging.

Background radiation: We are all exposed to low levels of radiation from natural and man-made sources. Natural sources include radon gas (from rocks, the largest contributor), cosmic rays from space, and radioactive isotopes in food and the ground. Man-made sources include medical X-rays, nuclear power and nuclear weapons testing fallout. When measuring the activity of a radioactive source in an experiment, you must subtract the background count rate from the measured count rate to obtain the corrected count rate due to the source alone.

Carbon dating: Carbon-14 is a radioactive isotope of carbon with a half-life of about 5730 years. Living organisms absorb carbon-14 from the atmosphere, maintaining a constant ratio of carbon-14 to carbon-12. When the organism dies, it stops absorbing carbon-14, and the existing carbon-14 decays. By measuring the remaining proportion of carbon-14 in a sample, scientists can estimate how long ago the organism died. This technique is effective for dating objects up to about 50,000 years old.

Hazards and safety: All ionising radiation can damage living cells. Alpha radiation is the most dangerous if ingested or inhaled (high ionising power in close contact with tissue), while gamma radiation is the most dangerous from external sources (it penetrates the body). Safety precautions include minimising exposure time, maximising distance from the source, and using appropriate shielding (lead aprons, lead-lined containers, tongs to handle sources).

Fission and fusion

Nuclear fission is the splitting of a large, unstable nucleus into two smaller nuclei, releasing energy and additional neutrons. Fission occurs when a neutron is absorbed by a heavy nucleus such as uranium-235 or plutonium-239. The nucleus becomes unstable and splits, releasing two or three neutrons, two daughter nuclei and a large amount of energy.

The released neutrons can go on to cause further fission reactions in nearby nuclei, creating a chain reaction. In a nuclear power station, the chain reaction is controlled: control rods (made of boron or cadmium) absorb excess neutrons to maintain a steady rate of fission. In a nuclear weapon, the chain reaction is uncontrolled, releasing enormous energy in a very short time.

The energy released by fission is used to heat water, producing steam that drives turbines connected to generators. The edexcel igcse science double award explained specification requires you to describe this sequence and to explain how control rods regulate the reaction.

Nuclear fusion is the joining of two small, light nuclei to form a larger nucleus, releasing energy. Fusion is the process that powers stars, including the Sun. Hydrogen nuclei fuse to form helium, releasing vast amounts of energy. Fusion releases more energy per kilogram than fission, but it requires extremely high temperatures (millions of degrees) to overcome the electrostatic repulsion between positively charged nuclei. Achieving and sustaining these conditions on Earth remains one of the greatest engineering challenges in physics.

Exam preparation

The physics: radioactivity and particles edexcel igcse practice questions in the edexcel igcse science double award test nuclear equations, half-life calculations, descriptions of radiation types, and explanations of uses and safety. The exam rewards precise terminology: do not say "radioactive waves" when you mean "gamma rays," and do not describe a neutron as "a neutral atom" when it is a subatomic particle.

Study these edexcel igcse science double award notes until you can write nuclear equations, calculate half-lives, and describe fission and fusion without hesitation. Use the Green Bridge CBT platform for edexcel igcse science double award practice questions and edexcel igcse science double award revision notes and past questions. Precision and logical structure are the hallmarks of strong answers in this section, and consistent practice builds both.