Radioactivity absorbed dose: gray

The two basic quantities in the assessment of radiation levels and effects are the activity of a radioactive material and the radiation dose. The activity of a radioactive material is the number of nuclear disintegrations per unit time, unit becquerel (Bq). One becquerel is one disintegration per second. The term radiation dose can mean several things (e.g. absorbed dose, dose equivalent or effective dose equivalent). The absorbed dose of radiation is the energy imparted per unit mass of the irradiated material. The unit of absorbed dose is joule/kg, for which the special name gray (Gy) is used ... 

However, scientists (and the public) are often more interested in the relative hazard posed by the dose of a radioactive substance than the absolute amount (in grams or curies). The dose of radiation received has commonly been expressed in a couple of ways. The dose can be expressed as a rad (radiation absorbed dose) that is equal to 0.01 J of energy absorbed per kg of absorber and it is not limited to the type of radiation. This is a reasonable dimension since the amount of energy absorbed is related to the subsequent damage. The rad has been replaced by the International System of Units (Systeme International [SI]) measurement unit called the gray that is equivalent to 100 rad. 

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