A student is investigating radioactivity in a physics lab. She removes the radioactive source from the room and places it in its lead-lined box in a secure store. She then resets the Geiger-Muller (GM) tube and counter to zero.
After switching the counter back on and waiting for 5 minutes, she notices that it still registers a small number of counts.
Give one reason why the GM counter still registers a reading when the radioactive source has been removed.