Radioactivity Revision Notes

What is Radioactive Activity?

Radioactive activity refers to the rate at which unstable atomic nuclei within a sample disintegrate. Each disintegration involves the emission of radiation (alpha, beta, or gamma). The higher the activity, the more decay events happen per unit of time, and consequently, the more radiation is being emitted.

The Becquerel (Bq): Unit of Activity

The standard international (SI) unit for radioactive activity is the Becquerel (Bq). One Becquerel is defined as one radioactive decay or disintegration per second. This means if a source has an activity of 100 Bq, it is undergoing 100 atomic decays every second. The unit is named after Antoine Henri Becquerel, who discovered radioactivity.

Significance in Applications

Understanding activity is crucial in various fields, including medicine (e.g., dosage in radiotherapy), industry (e.g., gauging thickness), and environmental monitoring. Measuring activity allows for the quantification of a source's strength, helping to ensure safety and effective use of radioactive isotopes. For example, a medical isotope used for imaging will have its activity carefully measured to minimise patient exposure while providing sufficient diagnostic information.

Random Nature of Decay

Radioactive decay is a spontaneous and random process. It is impossible to predict when an individual unstable nucleus will decay. Each nucleus decays independently of others and of external conditions like temperature or pressure.

Predictability with Half-Life

Although individual decays are random, the decay of a large number of identical unstable nuclei can be predicted statistically using half-life. Half-life is the time required for half of the radioactive nuclei in a sample to decay. This allows us to predict the activity of a large sample over time, even though we cannot predict specific decay events.

Understanding Half-Life

The half-life is defined as the period required for half of the radioactive nuclei in a substance to decay. This rate is unique to each isotope and is unaffected by external conditions like temperature or pressure. While it is not possible to predict when a particular atom will decay, the overall rate for a large number of atoms can be reliably described by the half-life.

Calculating Decay Using Half-Life and Graphs

To find the amount left after a certain time, divide total time by the half-life, then apply repeated halving for each interval. On a decay graph (activity or mass vs. time), the half-life is the width between points where the measurement halves vertically—a constant interval producing an exponential (not linear) decrease.

Smoke Alarms

Smoke alarms typically contain Americium-241, an alpha emitter. Alpha particles ionise the air between two electrodes, creating a small current. When smoke enters the chamber, it absorbs the alpha particles, reducing the ionisation and current flow, which triggers the alarm.

Food Irradiation and Equipment Sterilisation

Gamma radiation is used for both food irradiation and sterilisation of medical equipment. Gamma rays are highly penetrating and effectively kill bacteria, moulds, and insects on food products, extending their shelf life. For medical equipment, gamma radiation sterilises instruments by destroying microorganisms without using heat, making it suitable for heat-sensitive items.

Industrial Tracing and Gauging

Radioisotopes are used as tracers to detect leaks in pipes or monitor fluid flow in industrial systems. The radiation emitted can be detected externally. For thickness gauging, the amount of radiation passing through a material is measured. The thicker the material, the more radiation is absorbed, allowing for continuous, non-contact measurement of material thickness during manufacturing.

Medical Diagnosis and Treatment

In diagnosis, short-lived gamma-emitting radioisotopes are ingested or injected and their paths traced using detectors to image internal organs and functions. For cancer treatment, high-energy radiation (often gamma rays or particle beams) is directed at cancerous cells to destroy them, while minimising damage to surrounding healthy tissue.