Did you know that bananas are radioactive? They contain potassium-40 isotope that is indeed radioactive. The amount of potassium in one banana is around one tenth of a microsievert.
Actually quite many common household items and foods are radioactive. Radioactive decay is everywhere and we are constantly exposed to that radiation. In the matter of fact, we ourself are also emitting some of that radiation.
But those amounts are nothing to worry about. As the banana is one of the most radioactive items in our everyday life there is actually an informal measurement called BED (banana equivalent dose) that has been used to demonstrate the radiation in common items versus those sources that are actually dangerous.
The daily average radiation we receive is around 100 BEDs, safety levels for nuclear powerplants are at 2500 BEDs and a lethal dose for a human being is 35 million BEDs.
So if you are worried about being killed by the radiation from bananas you can relax. You'd have to eat at least 10 million bananas at once to be in danger. If that sounds too much to swallow at once you can also try eating 274 bananas every day for seven years to see any symptoms. And if you were doing either of those the radiation would probably be the least of your problems anyway.