Radioactive sub stances emit very small particles (called alpha and beta particles) and 'gamma' radiation (like X-rays). As this happens the radioactive substance itself decays away; the time taken for half of the substance to decay away is called its half-life, and this can range from only seconds to thousands of years. For example if a radioactive substance is likened to 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