Must be super radioactive…

Should kill every human in the area instantly

Counterpoint: What if Superman has an element in his body that is crucial to his makeup that is specifically destroyed/harmed by the radioactivity, while that radioactivity passes harmlessly through any other element?An anti-krypton element of sorts, if you will.

Krypton, for example, could be emitting a wave with incredibly low amplitude, but incredibly high frequency. This combination would struggle to make contact with the atoms in normally-dense objects here on Earth, but would strike an ultra-dense object like anti-krypton like a truck.