Short answers, pulled from the story.
The gray (symbol: Gy) is the SI unit of absorbed dose of ionizing radiation, defined as the absorption of one joule of radiation energy per kilogram of matter. It measures the energy deposited by ionizing radiation in a unit mass of absorbing material and is used in radiotherapy, food irradiation, radiation sterilization, and radiation protection.
The gray was named after Louis Harold Gray, a British physicist who pioneered the measurement of X-ray and radium radiation and their effects on living tissue. Gray died in 1965, and the unit was officially named in his honour when it was adopted by the 15th General Conference on Weights and Measures in 1975.
The gray was adopted as part of the International System of Units in 1975, confirmed by the 15th General Conference on Weights and Measures. This followed decades of work by the ICRU and international standards bodies to replace the older rad and roentgen units.
The gray measures absorbed dose, the physical energy deposited in tissue, while the sievert measures equivalent dose, which accounts for the biological effectiveness of different radiation types. For X-rays and gamma rays the values are numerically equal, but one gray of alpha radiation equals 20 sieverts because alpha particles cause greater biological damage.
A whole-body acute exposure to 5 grays or more of high-energy radiation usually leads to death within 14 days. The LD50, the dose at which half of those exposed would be expected to die, is 5 Gy, equivalent to 375 joules for a 75 kg adult. The LD1 is 2.5 Gy and the LD99 is 8 Gy.
For curative treatment of solid epithelial tumours, typical doses range from 60 to 80 Gy. Lymphomas are treated with 20 to 40 Gy. Preventive doses for breast, head, and neck cancers are typically around 45 to 60 Gy, delivered in fractions of 1.8 to 2 Gy at a time.