Background radiation equivalent time BRET value corresponding to a dose of radiation is the number of days of average background dose it is equivalent to. It is calculated from the equivalent dose in sieverts by dividing by the average annual background radiation dose in Sv, and multiplying by 365: The definition of the BRET unit is apparently unstandardized, and depends on what value is used for the average annual background radiation dose, which varies greatly across time and location. The 2000 UNSCEAR estimate for worldwide average natural background radiation dose is 2.4 mSv (240 mrem), with a range from 1 to 13 mSv. A
Ionizing radiation ionizing, but moves at about 5% c, and an electron with 33 eV (enough to ionize) moves at about 1% c. The first two ionizing sources to be recognized were given special names used today: Helium nuclei ejected from atomic nuclei are called alpha particles, and electrons ejected usually (but not always) at relativistic speeds, are called beta particles. Natural cosmic rays are made up primarily of relativistic protons but also include heavier atomic nuclei like helium ions and HZE ions. In the atmosphere such particles are often stopped by air molecules, and this produces short-lived charged pions, which soon