Radioactive Substances

The half-life differs for each radioactive substance.

Substances that emit radiation are called radioactive substances. The ability of radioactive substances to emit radiation is called radioactivity. The extent of this ability is shown in becquerels (Bq).
Generally, when radioactive substances emit radiation, their ability to emit radiation is lost, and they are no longer radioactive substances. The time until they stop being radioactive substances depends on the substance. The time required for 1/2 of a given radioactive substance to stop emitting radiation, in other words, the time until the amount of radiation from this substance falls by 1/2 is called the half-life. For example, the half-life of cesium 137 is 30 years.