Radioactivity is different from radiation. Things, which emit radiation, e.g., alpha particles, electrons, and gamma rays, are called radioactive substances. While emitting radiation, radioactive substances change in form. A substance that has the ability to emit radiation is said to be radioactive. Radioactivity levels decrease over time. The time required for the radioactivity of a substance to be reduced to half its initial value is called its half-life. For example, if you think of radioactive substances represented in terms of a fireworks sparkler... Radioactivity: Radiation: Half-Life: Radioactivity Level: Effect on the Body: ability of the sparkler to emit sparkles sparkles emitted by the sparkler time it takes for the sparkler to decrease to half its length brightness of the sparkles (bequerels; Bq) how hot the sparkles are when they hit the body (seiverts; Sv) Among radiation, there are various types including alpha particles, beta particles, gamma rays, and neutrons. X-rays are one kind of radiation. Depending upon the type of radiation, the ability for radiation to pass through materials varies. Back