Edexcel GCSE Physics (1PH0) — Radioactivity
Every time a patient undergoes a medical tracer scan, doctors must select a substance that will safely disappear from the body within hours, rather than lingering to cause damage for decades. This decision relies on understanding the relationship between half-life and activity. Activity is the rate at which unstable nuclei decay, measured in Becquerels (Bq).
Activity is inversely proportional to half-life. Isotopes with a short half-life decay rapidly, resulting in a very high initial activity and an intense burst of ionising radiation. However, because the nuclei decay so fast, this high hazard level drops quickly.
Conversely, isotopes with a long half-life have a lower decay rate and lower initial activity. While they emit radiation less intensely at any given moment, they remain a persistent and dangerous radiation hazard for generations. Scientists generally classify a radioactive sample as safe once its activity falls below 1% of its starting value, which takes roughly seven half-lives.
The fraction of a sample that has decayed over a specific period can be calculated using the net decline formula:
Where represents the number of half-lives that have passed.
It is a common misconception that exposing food to nuclear radiation makes the food itself radioactive. Exposing an object to external nuclear radiation is called irradiation. Crucially, an irradiated object does not become radioactive because the radioactive source never physically touches it.
In contrast, contamination is the unwanted presence of radioactive atoms on or inside an object or person. A contaminated object becomes radioactive and will continuously emit radiation until the source decays completely or is physically removed.
The specific dangers of these two processes depend entirely on the type of radiation involved:
Understanding how radiation spreads allows scientists to design strict safety protocols based on three key factors: time, distance, and shielding. Because exposure is directly proportional to time, workers must minimise the duration a source is kept outside its storage container and maintain detailed usage logs. To prevent contamination, workers wear airtight suits, masks, and gloves to avoid inhaling or ingesting radioactive dust.
Distance reduces radiation exposure according to the Inverse Square Law, meaning that doubling your distance from a source reduces the radiation intensity to one-quarter. Scientists frequently use long tongs or robotic arms to safely maximize their distance from hazardous materials.
A radiographer records a radiation count rate of while standing away from a gamma source. Calculate the expected count rate if they use a robotic system to move the source away.
Step 1: Identify the change in distance.
Step 2: Apply the Inverse Square Law principle.
Step 3: Calculate the new count rate.
If radioactive materials cannot be kept at a safe distance, their emissions must be blocked using appropriate shielding. Alpha particles are stopped by simple paper, while beta particles require a few millimetres of aluminium. Highly penetrating gamma rays require several centimetres of lead or thick concrete to significantly reduce their intensity.
Workers frequently exposed to radiation wear dosemeters, which are radiation badges containing photographic film that fogs up upon exposure to track cumulative doses over time.
When managing long-lived radioactive waste, such as Uranium-235, the materials must be encased in thick lead-lined or concrete containers. These containers are then buried deep underground in geologically stable locations to prevent highly active waste from ever leaking into the water table. All studies regarding these safety measures and the biological effects of radiation must be peer-reviewed to ensure they are validated by the broader scientific community.
Students often state that irradiation makes objects 'glow' or become radioactive. Edexcel examiners will penalise this; you must specify that only contamination leaves a radioactive source on or inside an object.
When explaining hazards in a 6-mark question, always build a clear causal chain: state that a short half-life means many nuclei decay per second, leading to high activity and a high immediate dose of radiation.
Remember that lead-lined aprons protect a worker against irradiation, but they do not prevent contamination. Gloves and masks are required to stop radioactive particles from getting onto or inside the body.
When comparing the dangers of alpha and gamma radiation, always separate them by location: alpha is the most dangerous inside the body (contamination), but gamma is the most dangerous outside the body (irradiation).
Half-life
The time required for the activity or the number of unstable nuclei in a radioactive isotope to decrease by half.
Activity
The rate at which a source of unstable nuclei decays, measured in decays per second.
Becquerels (Bq)
The standard unit of radioactive activity, where 1 Bq equals one decay per second.
Ionising radiation
Radiation with enough energy to remove electrons from atoms or molecules, creating ions and potentially causing cellular damage.
Irradiation
The process of exposing an object to ionising radiation from an external source without the object itself becoming radioactive.
Contamination
The unwanted presence of radioactive atoms on materials or inside the human body, causing the object to become radioactive.
Inverse Square Law
A physical principle stating that the intensity of radiation is inversely proportional to the square of the distance from the source.
Dosemeter
A badge or device worn by workers that monitors and records their cumulative exposure to ionising radiation.
Lead
A dense metal commonly used as shielding to absorb and block highly penetrating gamma radiation.
Concrete
A construction material used in thick layers to shield against gamma radiation and other high-energy emissions.
Photographic film
A material used in radiation dosemeters that darkens upon exposure to radiation, indicating the level of exposure.
Peer-reviewed
The evaluation of scientific work by others in the same field to ensure its accuracy, validity, and reliability.
Put your knowledge into practice — try past paper questions for Physics
Half-life
The time required for the activity or the number of unstable nuclei in a radioactive isotope to decrease by half.
Activity
The rate at which a source of unstable nuclei decays, measured in decays per second.
Becquerels (Bq)
The standard unit of radioactive activity, where 1 Bq equals one decay per second.
Ionising radiation
Radiation with enough energy to remove electrons from atoms or molecules, creating ions and potentially causing cellular damage.
Irradiation
The process of exposing an object to ionising radiation from an external source without the object itself becoming radioactive.
Contamination
The unwanted presence of radioactive atoms on materials or inside the human body, causing the object to become radioactive.
Inverse Square Law
A physical principle stating that the intensity of radiation is inversely proportional to the square of the distance from the source.
Dosemeter
A badge or device worn by workers that monitors and records their cumulative exposure to ionising radiation.
Lead
A dense metal commonly used as shielding to absorb and block highly penetrating gamma radiation.
Concrete
A construction material used in thick layers to shield against gamma radiation and other high-energy emissions.
Photographic film
A material used in radiation dosemeters that darkens upon exposure to radiation, indicating the level of exposure.
Peer-reviewed
The evaluation of scientific work by others in the same field to ensure its accuracy, validity, and reliability.
