What special name is given to J/kg?

The unit J/kg is called a Gray (Gy), which is the SI unit used to measure the absorbed dose of ionizing radiation. When one joule of energy is deposited in one kilogram of matter, it equals one Gray. The Gray quantifies the amount of radiation energy absorbed by tissues, which is essential in dosimetry for evaluating tissue damage and treatment effectiveness in radiation therapy.

Other units such as rads, rem, and sieverts are related to radiation measurements but serve different purposes. Rads, for example, is an older unit of radiation dose, where one rad is equivalent to 0.01 Gray. Rem and sievert are units that account for the biological effect of radiation, with the rem being the traditional unit and the sievert being its SI equivalent, often used to evaluate the risk associated with exposure to radiation. However, when specifically referring to the measure of absorbed dose expressed in J/kg, Gray is the correct term.