A Gray (Gy) is defined as the unit of absorbed dose equal to

• A. One joule per kilogram
• B. One joule per gram
• C. One watt per kilogram
• D. One coulomb per kilogram

Answer: (A)

The main concept is that the Gray measures how much energy is deposited by radiation in a given mass, i.e., absorbed dose. It is defined as energy (in joules) per mass (in kilograms), so 1 Gy equals 1 J/kg. This means if 1 joule of energy is deposited in 1 kilogram of material, the absorbed dose is 1 Gy; if the energy deposition changes or the mass changes, the dose scales accordingly.

Why the other options don’t fit: one joule per gram would be 1000 joules per kilogram, corresponding to 1000 Gy, not the unit defined. A watt per kilogram is a unit of power per mass (energy per time per mass), not total energy per mass. A coulomb per kilogram is a measure of charge per mass, not energy per mass, and it relates to different radiation quantities, not absorbed dose.