Short answers, pulled from the story.
The gray is an SI derived unit that measures the absorbed dose of ionizing radiation. One gray equals one joule of energy deposited into one kilogram of matter. This unit determines the physical reality of radiation exposure in any material it strikes.
Louis Harold Gray was a British physicist who died in 1965 and pioneered the study of radiation effects on living tissue. The International Commission on Radiation Units and Measurements named the new SI unit the gray in 1975, five years after his death. His work shifted the focus from the radiation source to the material receiving the energy.
The gray unit was adopted in 1975 to replace the older rad unit and provide a coherent universal standard for absorbed dose. This transition culminated from the development of the International System of Units in the 1950s. The gray emerged from measurement chaos before 1975 to become the standard for absorbed dose.
Curative doses for solid epithelial tumors typically range from 60 to 80 grays. Lymphomas require between 20 and 40 grays to be treated effectively. These treatments are delivered in fractions often between 1.8 to 2 grays per session to allow healthy tissue to recover.
The LD50 dose lethal to 50 percent of the population is 5 grays. This dose represents 375 joules for a 75-kilogram adult and usually leads to death within 14 days. A whole-body acute exposure of 5 grays or more is fatal to half the population.
Scientists use the sievert to account for biological damage by applying weighting factors to the gray. For X-rays and gamma rays, one gray is numerically equivalent to one sievert. For alpha particles, one gray equals 20 sieverts due to varying degrees of biological damage.