Tertiary Source

What's the antimatter content of a banana?

2008-10-21 11:06:37

Antimatter is strange, exotic stuff, right? Only produced in dangerous physics experiments? Leads to complete annihilation with ordinary matter?

Sort of. It's a question of quantity.

Consider the element potassium. Potassium, like sodium, is an alkali metal; potassium ions in solution play an important role in several different biochemical processes. You have to eat potassium or you'll die, but this is true for lots of different plants and animals, so potassium deficiencies aren't common in well-fed people. Natural potassium is made of three different isotopes. About 117 parts per million natural potassium is potassium-40, 40K, which is unstable with a lifetime 1.25 billion years. This radioactive potassium is left over from the formation of the solar system; in the 4.5 billion years since the earth coalesced, 97% of the the original 40K has decayed.

A "medium" banana (whatever that means) has about half a gram of potassium, or 7.7×1021 atoms. Of these, 9.0×1017 (about sixy micrograms) are 40K. If you picked just one of these nuclei, you'd have to wait a billion years (on average) to see it decay; in our banana we have lots of atoms we can watch all at once, so there will be about 23 decays per second. Of these decays, 89% are β- decays to calcium, and 11% are electron capture decays to argon. Only one decay in 105 actually emits an antielectron. So an ordinary banana contains an antielectron for a brief instant about once every 75 minutes.

Of course, all the β- and electron capture decays are accompanied by an electron antineutrino, which leaves the banana at the speed of light. Which is a bigger contribution to the antimatter number density in a banana: an antielectron that stops and annihilates, or an antineutrino that instantly escapes?

Comment on What's the antimatter content of a banana?
Name:
Email:
URL:
(I promise not to do anything unsavory with your contact information.)
You can also send regular email.

Recent questions

[none]

Older questions

A black hole at the end of its Hawking evaporation radiates with some very large power. But the high-power period doesn't last very long, since the mass is so small. Apparently the overall luminosity is small enough that it'd be hard to see a radiating black hole outside the solar system. What limit can you set on the density of very small black holes knowing only that, say, there are none in the room right now?
2009-02-04 Wednesday 22:19:29
Much of the work of digestion is done by bacteria in the gut. Could an appropriately engineered infection permit someone to extract nutrition from normally indigestible materials, like grass and wood?
2009-01-29 Thursday 19:37:59
NPR has reported that part of the auto-industry bailout may be a "car czar." How many "czars" are there in the government, anyway?
2008-12-09 Tuesday 10:34:41
A stopped clock is exactly right twice a day. With some error-weighted averages, a stopped clock is more accurate on average than a clock running, say, ten minutes fast. What averages do and don't have this property?
2008-11-19 Wednesday 23:40:45
My fan-driven humidifier makes my house feel colder. How much heat does it pull to vaporize the water it does?
2008-11-14 Friday 16:59:35
Supposedly volcanism on Io is caused (or enhanced) by tidal heating from Jupiter. What power does tidal heating dissipate in the Earth-Moon system?
2008-10-23 Thursday 13:03:59
What makes glassy smooth patches on the surface of a larger body of water? Why are the patches partially stable against rippling from outside?
2008-10-23 Thursday 06:05:41
Under what circumstances are disease screenings "preventative medicine"?
2008-10-21 Tuesday 17:14:47
A paper in PLoS Medicine argues that an artificial scarcity of high-profile journals may distort what appears in the scientific literature, in favor of spectacular, surprising, and possibly wrong results. The analysis seems to favor medical and biological publications. Do the conclusions apply to physics publishing?
2008-10-21 Tuesday 14:31:48
You can't interact with a physical system without disturbing it, though you can ignore the disturbance in the limit where ħ is small. Similarly, you can't interact with a market for some product (by asking or buying or offering or selling) without affecting the "market price" seen by others, but you can neglect that change unless your exchanges comprise a large fraction of the market. There is some correspondence here. Clearly it'd be easy to take this correspondence as justification for saying some really dumb things. Are there any useful insights there?
2008-09-26 Friday 14:50:23