Radioactive Emissions (Leaving Cert Physics): Revision Notes

Radioactive Emissions

Detecting nuclear radiation

Understanding how to detect radioactive emissions is crucial for both scientific research and safety monitoring. There are several methods available, each with specific advantages for different situations.

The Geiger-Müller tube

The Geiger-Müller (GM) tube is one of the most common devices for detecting the presence of radioactivity through the ionisation it produces. This versatile instrument can also measure the activity of radioactive samples.

The GM tube operates using a simple but effective principle:

• Detection mechanism: Radiation passes through a thin mica window into argon gas at low pressure
• Ionisation process: The radiation ionises argon atoms, creating positive argon ions and negatively charged electrons
• Electrical response: When a high voltage is applied between the wire anode and cylindrical cathode, electrons move rapidly towards the anode
• Signal amplification: These electrons collide with other argon atoms, producing an avalanche of electrons that creates a measurable pulse of current
• Counting system: Each radiation event produces a distinct pulse that can be counted using electronic equipment like a ratemeter

The solid-state detector

The solid-state detector represents a more modern approach to radiation detection, using semiconductor technology rather than gas-filled tubes.

Key features of solid-state detectors include:

• Construction: Built around a reverse-biased p-n junction connected to counting equipment such as a ratemeter
• Detection process: When nuclear radiation strikes the depletion layer, it creates electron-hole pairs
• Signal generation: These charge carriers move under the influence of applied voltage, forming a pulse of current
• Amplification: The signal is amplified before being sent to a pulse counter for measurement

Artificial radioactivity

Scientists can create radioactive isotopes artificially through nuclear bombardment processes. This involves bombarding stable isotopes with neutrons, which are then captured by atomic nuclei. The process typically occurs in nuclear reactors and produces isotopes that are widely used in medicine and industry.

Many artificially produced isotopes have practical applications in medical diagnosis, treatment, and industrial processes, making this an important aspect of nuclear technology.

Safety implications of radioactive emissions

Understanding ionising radiation

Ionising radiation refers to any form of radiation capable of knocking outer electrons from atoms. This process creates ions and can cause significant biological damage when absorbed by human tissue.

The extent of radiation damage depends on several critical factors:

• Type of radiation (alpha, beta, gamma, X-rays, or neutrons)
• Activity of the radioactive source producing the radiation
• Duration of exposure to the radiation
• Type of body tissue being irradiated (some tissues are more sensitive than others)

Health effects of radiation exposure

Ionising radiation can cause both immediate and long-term health problems:

Short-term effects from high radiation doses:

• Radiation sickness symptoms appearing within hours
• Vomiting, diarrhoea, and fever
• Severe cases can be fatal
• These effects occur when radiation damages sufficient molecules in cells to kill them

Long-term effects from lower doses:

• Increased cancer risk, particularly leukaemia
• Genetic damage that may affect reproductive cells
• These effects may not appear for years after exposure
• Even low doses can cause DNA damage leading to genetic changes and various cancers

Precautions when using ionising radiations

Background radiation

We are constantly exposed to low levels of ionising radiation called background radiation. This radiation comes from both natural and human-made sources and represents the baseline level of radiation present everywhere on Earth.

Natural sources (accounting for about 87% of background radiation)

Outer space: High-energy cosmic radiation continuously reaches Earth from outer space. The atmosphere provides some protection, but the amount of cosmic radiation exposure increases with altitude. Air travellers and airline crew receive higher doses due to prolonged time at high altitudes.

Rocks in the Earth's crust: Naturally occurring radioactive elements like uranium and thorium exist in rocks and soil. These elements and their decay products contribute to background radiation levels. The concentration varies significantly by geographical location.

Food and drink: Natural radioactive substances present in food and water contribute to internal radiation exposure. The amounts are generally very small but represent a constant, low-level source.

Human-made sources

Human activities contribute additional radiation to the background level:

Medical sources: Diagnostic procedures and treatments expose people to ionising radiation. Common examples include dental X-rays, chest X-rays, mammography for breast cancer detection, CT scans, and angiocardiopathy for heart conditions. Medical radiation accounts for 10.4% of average exposure in Ireland, though the health benefits often far outweigh the risks.

Nuclear weapons testing: Past atmospheric nuclear weapons testing has contributed to background radiation, though this has decreased significantly since atmospheric testing was largely discontinued.

Nuclear accidents: Accidents at nuclear facilities can release radioactive materials into the environment, contributing to background radiation levels.

Nuclear power plants: Normal operation of nuclear power facilities contributes minimally to background radiation.

Demonstrating background radiation

Scientists can prove that background radiation exists through careful experimental measurement.