In my story an unknown energy source enters the solar system, as it speeds towards the inner planets it leaves behind a radioactive trace/trail. Think of the wake left behind by a speedboat. What equipment (ideally both on Earth and in space) could detect this radiation? I know they can't track it in real time, but I can't identify a piece of equipment that would detect and record this trail of radioactive energy speeding across the solar system. The concentration and beam like properties of the radiation would be evidence of intelligence, not just background space radiation.

Can anybody help? The best I've found is a gamma radiation telescope, would this do the job I need it to.