quote:

Originally posted by nvmichael:
PaulS, that is interesting. Thanks. I googled it ( ever hear anybody say I yahooed it?)

Radioactivity, antimatter, and… bananas?


Bananas, like many things we eat, breathe, and touch, are slightly radioactive. That’s because bananas contain potassium, and one of the naturally occurring isotopes of potassium — potassium-40 — is radioactive. That banana that you thought was just sitting idly on the kitchen counter is silently emitting subatomic particles! It even emits antimatter, as we’ll see in a bit.

Potassium-40 undergoes beta decay. This type of decay occurs when a nucleus trades in one of its neutrons for a proton, in exchange for a few hangers-on which are emitted from the nucleus — an electron and a neutrino. The electron (also known in this context as a beta particle) is the kind of radiation that potassium-40 emits about 90% of the time it decays. Most of the rest of the time, it will decay by capturing an electron. But very rarely (0.001% of the time), when potassium-40 decays, it will produce a particle of antimatter, a positron!

A positron is the twin of an electron, with the opposite charge. Electrons have negative charge, and positrons have positive charge. Their other properties — mass, the total amount of charge, etc, are identical. (As a rule, antimatter particles are always have an oppositely charged normal-matter twin.) For this reason, positrons are also called beta particles in this context.

So if a banana is radioactive, how often does it emit one of these betas? Let’s work it out.

A banana has about 450 mg of potassium. Almost half a gram. A gram is about the weight of a paperclip. So, if you have two bananas for breakfast, you’ve just eaten a paperclip-worth of potassium. Woah! That’s quite a bit of potassium.

We can use Avogadro’s number (6.022*1023) and the atomic weight of potassium (39.1 amu) to convert that 450mg of potassium into the equivalent number of atoms of potassium. From that, we get 8*1021 atoms. (That’s an 8 with twenty-one zeroes after it. That’s quite a lot of zeroes. Atoms are tiny.)

Now, let’s figure out how many of those atoms we expect to be potassium-40. Potassium is made up of several isotopes. Each isotope has a different number of neutrons, which affects how stable the atom is. The most common isotopes, potassium-39 and potassium-41, are stable. They have the right number of neutrons to keep them happy, so they don’t decay. They make up the vast majority of naturally-occurring potassium. About 0.01% of potassium is potassium-40. So that gives us 8*1017 atoms of potassium-40 in a single banana.

Ok, so the conclusion is that bananas have more potassium-40 atoms than the human brain can really conceptualize. That’s not so useful as a point of reference. Let’s try to figure out how often one of those decays. Perhaps that is a number that’s on a more human scale.

The half-life of potassium-40 is 1.2 billion years. This means that if you left a pile of potassium-40 untouched for 1.2 billion years, half of it would decay in that time. From this, and our previously-calculated number of atoms of potassium-40, we can calculate the average time between decays. That gives us 15 decays per second. 90% of that time, the decay will emit an electron. So an atom in your banana spits out an electron on average 13 or 14 times a second!

What about the decay that emits a positron? That happens much less frequently. On average, your banana will emit a positron once every 111 minutes — that’s 13 times a day!

Let me say that again, because it’s just that cool: A banana emits a particle of antimatter 13 times a day.

Keep in mind that the positrons and electrons won’t get very far. Most of them will slow down and stop inside your banana, or in the case of positrons, annihilate with an electron. So your banana isn’t shooting particles around the room. But there’s a lot more going on in your banana than you might have imagined. Ponder this for a moment the next time you pick up the humble, radioactive banana for a snack.

Numbers from the National Nuclear Data Center and Mayo Clinic.

quote:

Originally posted by jdollar:
i love bananas but i never buy them green- don't know if i will live long enough for them to ripen!