IGCSE Physics 0625

Topic: 5.2.1 Detection of Radioactivity

This section focuses on how we can detect the presence of ionising nuclear radiation. Radioactive materials emit different types of radiation, and specialized detectors are used to measure their intensity.

Ionising Nuclear Radiation

Ionising nuclear radiation is emitted from unstable atomic nuclei. It has enough energy to remove electrons from atoms, creating ions. The three main types of ionising radiation are:

• Alpha (α) particles: Consisting of two protons and two neutrons. They are relatively heavy and have a positive charge.
• Beta (β) particles: High-energy electrons or positrons. They are lighter than alpha particles and have a negative (electrons) or positive (positrons) charge.
• Gamma (γ) rays: High-energy electromagnetic radiation (like X-rays). They are uncharged and have no mass.

Detectors and Counters

Ionising radiation can be detected using various types of detectors. These detectors are connected to a counter, which measures the number of radiation events occurring over a period of time. The counter provides a reading that is proportional to the intensity of the radiation.

Types of Detectors

Different detectors are suitable for detecting different types of radiation. Here's a summary:

Detector Type	Radiation Detected	Principle of Operation	Example
Geiger-Muller (GM) tube	Alpha, Beta, Gamma	Ionising radiation ionises gas inside the tube, creating an electrical pulse.	Commonly used for detecting alpha and beta particles and gamma rays.
Scintillation Detector	Alpha, Beta, Gamma, X-rays	Radiation causes a scintillator material to emit light. The light is detected by a photomultiplier tube.	Used for detecting gamma rays and X-rays.
Photomultiplier Tube (PMT)	Gamma, X-rays	A highly sensitive light detector that converts light into an electrical signal.	Often used in conjunction with scintillator materials.

How a Geiger-Muller Tube Works

A Geiger-Muller (GM) tube is a common and relatively inexpensive detector. It consists of a metal tube filled with an inert gas (like argon or neon) at low pressure. A high voltage is applied between the metal tube and the central wire. When ionising radiation enters the tube, it ionises the gas atoms. The resulting ions and electrons are accelerated towards the central wire, creating a cascade of ionisations – a Geiger-Muller avalanche. This avalanche produces a measurable electrical pulse, which is counted by the counter.

The Radiation Counter

The radiation counter is the device that measures the number of radiation events. It receives the electrical pulses from the detector and displays the count rate. The count rate is usually expressed in counts per minute (CPM) or counts per second (CPS). A higher count rate indicates a higher intensity of radiation.

$$\text{Count Rate} = \frac{\text{Number of Events}}{\text{Time Interval}}$$

Applications of Radiation Detection

Radiation detectors and counters are used in a wide range of applications, including:

• Medical imaging: X-rays and gamma rays are used to create images of the inside of the body.
• Industrial gauging: Radiation is used to measure the thickness of materials or the level of liquids in tanks.
• Environmental monitoring: Radiation detectors are used to monitor levels of radioactive contamination in the environment.
• Scientific research: Radiation detectors are used in a variety of scientific experiments.