What does the symbol mR signify in radiation measurement?

The symbol mR stands for milliroentgen, which is a unit of measurement used to quantify exposure to ionizing radiation. Specifically, one milliroentgen is one-thousandth of a roentgen, a former standard unit of exposure that measures the ionization produced in air by gamma or X-ray radiation.

In radiation safety practices, understanding units of measurement is crucial for assessing exposure levels and ensuring that they remain within safe limits. The milliroentgen allows for precise measurement especially in contexts where exposure levels are relatively low, making it practical for monitoring and controlling radiation in various environments.

The other choices reference different units or terms that do not exist in the standard nomenclature for radiation measurement. For instance, micro roentgen is represented by a different symbol (µR), and "major" or "medium" roentgen are not recognized units in the field. This emphasizes the importance of knowing the specific terminology used in radiation safety to properly evaluate and communicate exposure levels.