Pearson Edexcel GCSE (9-1) Physics revision notes
Radioactivity Revision Notes
Review revision notes for Radioactivity in Edexcel Physics.
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Radioactivity
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Understanding Background Radiation Revision Notes
Sources of Background Radiation
Background radiation originates from both natural and artificial sources. Natural sources contribute the majority, including cosmic rays from space, terrestrial radiation from rocks and soil (e.g., uranium, thorium), and radioactive gases like radon. Artificial sources include medical uses (X-rays, scans), nuclear power, and fallout from nuclear weapon testing.
Composition and Detection
Background radiation is composed of various types of ionising radiation, such as alpha, beta, gamma, and X-rays. Its intensity varies with location, for example, higher in areas with granite rock or at high altitudes due to increased cosmic ray exposure. It is detected using instruments like Geiger-Müller tubes, which can measure the count rate of ionising events.
Significance and Impact
While typically low-level, continuous exposure to background radiation can contribute to a small increase in cancer risk over a lifetime. Understanding and monitoring background radiation is crucial for evaluating risks from additional radiation exposure, such as from medical imaging or industrial activities. It forms the baseline against which other radiation exposures are measured.
Measuring and Detecting Radioactivity Revision Notes
Photographic Film for Detecting Radioactivity
Photographic film, similar to that used in traditional cameras, darkens upon exposure to ionising radiation. This darkening occurs due to the ionisation of silver halide crystals within the film's emulsion by radiation. The greater the exposure to radiation, the more silver halide crystals are ionised and converted into visible silver, resulting in a darker film. This method is often used in personal dosimeters worn by personnel in radiation environments to monitor their cumulative exposure over time. It provides a permanent record of exposure but does not offer real-time detection.
Geiger-Müller Tube for Measuring Radioactivity
A Geiger-Müller (GM) tube is an instrument designed for detecting and measuring ionising radiation. It comprises a gas-filled tube with a central electrode (anode) and an outer casing (cathode), usually under a high voltage. When radiation enters the tube, it ionises gas atoms, producing electrons and positive ions. These charged particles accelerate, causing further ionisation and an 'avalanche' effect. This generates a detectable electrical pulse, often heard as an audible 'click' or registered as a count on a display. The rate of these pulses is directly proportional to the intensity of the radiation, providing a real-time measurement of the count rate.
Comparison of Methods
Photographic film offers a cumulative, non-real-time measurement, providing a record of total radiation dose over an extended period. It is suitable for personal dosimetry. In contrast, a Geiger-Müller tube provides real-time detection and measurement of radiation intensity (count rate). It is excellent for immediate detection of radiation sources and monitoring changes in radiation levels. Both methods are effective but serve different purposes in radiation detection and measurement.
Understanding Radioactive Emissions Revision Notes
Alpha Particles (α)
An alpha particle is equivalent to a helium nucleus. This means it consists of two protons and two neutrons, giving it a positive charge (+2e) and a relative atomic mass of 4. Due to their relatively large size and charge, alpha particles have low penetrating power but are highly ionising.
Beta Particles (β)
A beta particle is an electron emitted from the nucleus of an atom. During beta decay, a neutron within the nucleus transforms into a proton, emitting an electron (beta particle) and an antineutrino. Beta particles have a negative charge (-1e) and a very small mass. They have moderate penetrating power, being stopped by a few millimetres of aluminium, and are moderately ionising.
Gamma Rays (γ)
A gamma ray is a form of electromagnetic radiation. Unlike alpha and beta particles, gamma rays are not particles of matter; they are high-energy photons, meaning they have no mass and no charge. They are emitted from an excited nucleus following alpha or beta decay. Gamma rays have very high penetrating power, requiring thick lead or concrete to stop them, but are weakly ionising compared to alpha or beta particles.
Origins of Background Radiation Revision Notes
Terrestrial Sources of Background Radiation
Background radiation from Earth, or terrestrial radiation, primarily originates from radioactive isotopes found in the Earth's crust. Key examples include uranium, thorium, and potassium-40. These elements are present in varying concentrations in rocks, soil, and water. A significant contributor is radon gas, which is a decay product of uranium and thorium. Radon can accumulate in enclosed spaces like homes, particularly in areas with underlying granite, which is rich in these radioactive elements. Other materials used in construction, such as bricks, concrete, and plaster, also contain trace amounts of these naturally occurring radioactive substances, adding to the terrestrial background radiation.
Cosmic Sources of Background Radiation
Radiation from space, known as cosmic radiation, is composed of high-energy particles from the Sun and distant stars. These primary cosmic rays interact with atoms and molecules in Earth's atmosphere, leading to the creation of secondary radiation, including muons, electrons, and photons. The Earth's atmosphere acts as a shield, reducing the intensity of this radiation at sea level. However, the dose of cosmic radiation increases significantly with altitude (e.g., during air travel) because there is less atmospheric shielding. The Earth's magnetic field also plays a role, deflecting some of these charged particles, with stronger deflection near the equator compared to the poles, meaning cosmic radiation levels vary with latitude.
Overall Contributions to Background Radiation
Natural sources account for about 85% of an average person's annual radiation dose. Terrestrial and cosmic radiation are the main natural components. The remaining percentage comes from human-made sources like medical X-rays, nuclear power, and other industrial applications. The exact proportion of terrestrial versus cosmic radiation varies geographically; for instance, mountainous regions experience higher cosmic radiation, while areas with high uranium concentrations in the ground experience higher terrestrial radiation.
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