What is the SI equivalent of the rad?

The SI equivalent of the rad is the gray. The rad (radiation absorbed dose) is a traditional unit of measurement for the amount of ionizing radiation energy absorbed by matter, specifically in terms of radiation dose to human tissue. One rad is equivalent to 0.01 gray.

The gray is the standardized unit in the International System of Units (SI) and is used to quantify absorbed radiation dose. By adopting the gray as the SI unit, it provides a more universal and consistent method of measuring radiation doses across different contexts and helps facilitate international communication and data sharing in fields that involve radiation, such as medicine, radiography, and radiation safety.

In terms of the other choices, the coulomb is a unit of electric charge, the sievert measures biological effects of radiation dose based on its potential for causing harm (particularly to living tissues), and the roentgen measures exposure to ionizing radiation in air. While these units relate to radiation in various ways, they are not equivalent to the rad in terms of absorbed dose.