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
Rad (unit) in the latest NIST definition. Even where the standard medium is fully defined, the ionizing energy is often not precisely known. In 1940, British physicist Louis Harold Gray, who had been studying the effect of neutron damage on human tissue, together with William Valentine Mayneord and John Read published a paper in which a unit of measure, dubbed the ""gram roentgen"" (symbol: gr) defined as "that amount of neutron radiation which produces an increment in energy in unit volume of tissue equal to the increment of energy produced in unit volume of water by one roentgen of radiation" was proposed.