Ionizing radiation, or hard radiation, is a form of radiation which carries sufficient energy to ionize an atom or molecule. It can be both natural or artificially generated. Background radiation was determined by measuring the amount of ionizing radiation in a particular location.
The asteroid ship Yonada, as it flew through space, left a trail of debris and hard radiation. (TOS: "For the World is Hollow and I Have Touched the Sky")
Ionizing radiation had the ability to interfere with a starship's sensors. In 2370, Geordi La Forge suggested that the asteroid containing the USS Pegasus be blanketed in ionizing radiation, which also was found naturally in the same area, in order to prevent its discovery by Romulans. (TNG: "The Pegasus")
When Geordi La Forge was investigating the mysterious disappearances of his former crewmates from the USS Victory on Tarchannen III in 2367, he performed a screen for ionizing radiation, without success. (TNG: "Identity Crisis")
In 3189, the USS Discovery was barraged by ionizing radiation upon entering the Verubin Nebula. (DIS: "Su'Kal")