The short is part of the game story/worldbuilding.
During your game-play, you may encounter regions with elevated levels of radiation or snowstorms containing radioactive particles. So, I came up with an analog device from the 1970s or 80s that is not yet digital. This will be the first rad detector that you will be using.
I did some research on radiation and came across a table that detailed radiation absorption and its side effects over time. The scale was in Gray units, the successor of the Rad unit. However, I decided to stick with Rad units to keep the theme consistent.