Methods of Measuring Radioactivity

Radioactivity

• Alpha, beta and gamma are the three main types of radiation. Alpha particles consist of two neutrons and two protons. Protons and neutrons are particles found in the nucleus of an atom. Beta particles are electrons. Gamma particles are photons, which transmit light. During radioactive decay, the nucleus of an unstable radioactive atom emits an alpha, beta or gamma particle. This changes the composition of the nucleus of the atom and makes it more stable.

Units of Measurement

• Three units are used to measure radioactivity: becquerel (Bq), gray (Gy) and sievert (Sv). Becquerels measure the number of radioactive decays per second and have the unit inverse seconds. Gray is the absorbed dose of radiation. It is defined as the amount of energy deposited in unit mass of some material and has units of joules per kilogram. Sievert is the biological dose of radiation and measures the amount of harm done to biological tissue by radiation.

Ionization Detectors

• Ionization occurs when radiation, such as an alpha, beta or gamma particle, passes through the air and causes electrons to detach from atomic nuclei. The most common way to measure radioactivity consists of a gas-filled chamber with a voltage across it, which is then placed in front of a source of radioactivity. The radiation ionizes the gas in the chamber. The detached, negatively charged electrons then move to the positively charged side of the chamber, while the positively charged atomic nuclei move toward the negatively charged side of the chamber. This creates a current which you can measure. The higher the current, the more radioactivity. A Geiger-Muller detector is an example of this method of measuring radioactivity.

Scintillation Detectors

• Some materials produce light when radiation passes through them. This process is known as scintillation and can be exploited to measure radioactivity. Scintillation detectors comprise a material that undergoes scintillation, most commonly a salt called sodium-iodide, and a photomultiplier tube. Light created by scintillation is detected by the photomultiplier tube. Photomultiplier tubes convert the light into electrons. The electrons generate a current which can measured to determine the amount of radioactivity.