There are many different ways of measuring the intensity of nuclear radiation. Some of the units mentioned below are now obsolete, but they are included because they may still be found in earlier publications.
The roentgen is a measure of the ionisation produced in a tissue, and is such that one roentgen produces two billion ion pairs in a cubic centimetre of standard air. The number of atoms in a cubic centimetre of air is so large that a roentgen ionises only about one atom in ten billion. This unit was originally defined for X-rays and gamma rays, and later a similar unit, the rad, was defined for any ionising radiation. The rad corresponds to the absorption of a hundred ergs of energy per gram. Since a roentgen delivers about 84 ergs per gram the two units are very roughly the same. More recently, a new unit, the gray, has been introduced. This is defined as the radiation corresponding to an absorption of 1 joule per kilogram. Thus 1 gray is equivalent to 100 rad.
Another important unit is the curie, defined as the radioactivity of 1 gram of radium. This may be extended to other radioactive substances by defining the curie as the radioactivity of an amount of that substance that has the same number of distintegrations per second, thirty-seven billion, as a gram of radium. For must purposes this is an inconveniently large unit, so the millicurie and the microcurie, respectively one thousandth and one millionth of a curie, are often used instead.
Since some types of radiation cause more damage than others, a unit has been defined to provide a standard of comparison between them. This is the relative biological effectiveness (RBE), defined as the dose from 220 KeV X-rays causing a specific effect divided by the does from the radiation causing the same effect. To make it possible to define the relative effects of various radiations on man an effective quality factor Q may be used. This has the value unity for X-rays, gamma rays and beta rays, ten for neutrons and protons and twenty for alpha particles and other multiply-charged particles.
When considering the effects of nuclear radiations on man, it is also necessary to include the different sensitivities of the different organs of the body. This is done by defining a unit, the rem, that is the product of the absorbed dose in rads, the effective quality factor and any other modifying factor. The rem can also be defined as the dose given by gamma radiation that transfers 100 ergs of energy to each gram of biological tissue; for any other type of radiation it is the amount that does the same amount of biological damage. A more recent unit, the sievert, is defined as 100 rem. A millisevert is thus 0.1 rem.
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