RADs

The rad is a non-SI unit of absorbed radiation dose that measures how much energy from ionizing radiation is deposited in a kilogram of matter. By definition, one rad corresponds to 0.01 gray, which is equivalent to 0.01 joule per kilogram. Because 1 gray equals 100 rad, the rad is a legacy unit, largely supplanted by the gray in scientific and regulatory contexts. In practical terms, 1 rad equals 10 milligrays (mGy).

Historically, the rad was widely used in radiology, nuclear medicine, and radiation protection to describe the amount of energy deposited in tissue or other materials. Over time, the gray was adopted as the SI unit of absorbed dose, leading to a gradual decline in new measurements reported in rads. Nonetheless, rad appearances persist in older literature, some equipment specifications, and certain regulatory documents.

The concept of dose in rads is separate from dose equivalent, which accounts for biological effect using the rem or the sievert. The rem is defined as rad times a quality factor that reflects radiation type. For many diagnostic X-rays and gamma rays, the quality factor is about 1, so 1 rad is roughly equivalent to 1 rem or 0.01 sievert. In modern practice, radiological doses are usually reported in grays or milligrays, and dose equivalents in sieverts or millisieverts.

In summary, the rad is a historical unit for absorbed dose, equal to 0.01 Gy (10 mGy), with 1 Gy equal to 100 rad. Its use has diminished in favor of SI units, though it remains encountered in older data and certain contexts.