What type of radiation does an EEG use?

It is electromagnetic radiation that the EEG works with. So-called "brain waves" are the signals we look at by attaching electrodes to monitor cerebral activity. Note that the EEG is passive device, and it doesn't "send out" anything, but only acts as a "receiver" for the signals. The brain creates its own electrical energy all the time, and we're just "listening in" by looking at the EEG.