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

Arguments about the safety of high-energy experiments sometimes cite the rate of high-energy few-particle collisions when cosmic rays hit the moon. What's the production rate for making transuranic elements from this process? What fraction of the universe's element 118 was made in earthbound accelerators?
2009-06-11 Thursday 15:34:41
Today's Stardate gave the "exact time" of the full moon. This is presumably when the moon is at opposition. But "full moon" historically described when the moon was at its brightest in the sky. Does the moon's libration ever make the moon observably brighter at some time other than at opposition?
2009-06-07 Sunday 10:27:20
With continuous thrust, it takes less energy to eject an object from the solar system than to crash it into the sun. This isn't the case for satellites in low-earth orbit, since they skim the atmosphere already. For geosynchronous satellites? Where is the boundary?
2009-02-13 Friday 08:41:34
A metric for cheap eating: calories per dollar?
2009-01-27 Tuesday 11:48:35
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
How strong is the correlation between income tax witholding / refunding and tolerance for government spending?
2008-10-14 Tuesday 23:05:05
When using χ2 minimization to estimate parameters from some data set, what fraction of the fits stop converging in the "wrong" place?
2008-09-28 Sunday 16:55:01
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
What's the pressure inside an unopened soda bottle (or can)? What's the rate at which CO2 comes out of solution at ambient pressure? How much does the bubbling change the temperature of the liquid?
2008-09-23 Tuesday 19:19:09
NPR has reported twice in the past couple weeks that NASA managers have expressed anxiety about the upcoming mission to service the Hubble telescope. Because the Hubble orbits further above the atmosphere than the ISS, where most shuttle missions have gone recently, the chance of the shuttle colliding with some space junk is "double" that on recent missions. Doubled from what? to what?
2008-09-20 Saturday 08:28:36