So we were talking about bananas on the Asylum channel of the teamspeak, and then I whipped out my nuclear background and this happened:
When a person eats a banana, they incur a specific amount of radiation damage due to the radioactive fraction of the potassium present in the banana. Of the potassium in that banana, about 0.0117% of it is potassium-40, which unstable on a nuclear level. This radioactive potassium will deliver approximately 0.1 uSv (microsieverts) of damaging radiation exposure to the victim that consumed the banana.
The potassium-40 decays with a half life of about 1.25 billion years, which, to save you the math, means that with 1g of potassium-40, 31 atoms per second will decay, undergoing beta decay. Potassium-40 is interesting in that it undergoes all 3 possible types of beta decay! Very few isotopes can do this. 89.28% of the time, it decays into calcium-40 and emits an electron with 1.33 MeV (mega-electron volts) an an antineutrino. 10.72% of the time, it decays to argon-40 by electron capture, and emits a 1.460 MeV gamma ray and a neutrino. About 0.001% of the time, it will decay to argon-40 via emitting a positron and a neutrino.
0.1uSv is delivered by a single banana through its radioactive potassium, but how much is dangerous? Well, if you absorb about 400 mSv (millisieverts), and it happens fast enough, you might turn red and feel sick. To actually have a chance of dying at all, you need to absorb about 2Sv (2000 mSv or 2000000 uSv). In this case many would get significantly ill but recover. Since we want a guaranteed kill here, we will need at least 8Sv, or 8000000 uSv, which is enough to ensure you'll die in the next week or two of aplastic anemia in a best case scenario.
8Sv is going to take about 80 million standard, 150g bananas worth of potassium to deliver through ingestion. Also, you'd have to do this in about a day to ensure that you don't heal from the damage fast enough to avoid death.
I would also like to note that, aside from the recruitment process, this is my first post to these forums. Next week, if I can get a hold of an astrophysicist to help me out, we'll explore the necessary mass of bananas required to collapse under its own weight sufficiently that it ignites a nuclear chain reaction and gives birth to a star.