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

Venoms have evolved independently in several different vertebrate lineages: in snakes and lizards, and in the platypus, and in one other mammal. What are the transitional stages for venom to appear? How would the appearance go in, say, a population of people?
2009-04-01 Wednesday 17:41:46
The Greenland and Antarctic landmasses are under a mile of ice. Supposedly if this weight were removed the landmasses would "rebound" out of the mantle. How high would they go?
2009-02-27 Friday 23:30:51
To an outside observer, an object dropped onto a black hole never actually crosses the horizon; it approaches the horizon asymptotically and light from it becomes more and more redshifted. Suppose I drop a room-temperature blackbody in a hole. How long does it take before the redshifted temperature is colder than the Hawking temperature of the horizon?
2009-02-27 Friday 23:24:26
Computers have fast storage with limited space (RAM) and slower storage with fewer overall limits. How could this set of constraints change and make computing fundamentally different? How would similes about how "your brain is like a computer" change? Is there a better model for the brain?
2009-02-23 Monday 15:23:33
A metric for cheap eating: calories per dollar?
2009-01-27 Tuesday 11:48:35
Yesterday's solstice was the day when the northern hemisphere gets less solar irradiance than any other. In most countries this is "midwinter," but in the United States it's "the first day of winter." When did this label change?
2008-12-22 Monday 06:58:50
How strong is the correlation between income tax witholding / refunding and tolerance for government spending?
2008-10-14 Tuesday 23:05:05
Suppose I wanted to check the accuracy of an ensemble of weather forecasts (e.g. "30% chance of rain tonight"). What's the right metric for doing this?
2008-10-09 Thursday 10:35:34
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
I have heard it repeated a few times now that, in some counting, the human body is "mostly" bacteria. Is this number of cells? biomass? genetic information? Wikipedia cites a 2005 "mini-review" in Anaerobe. How much of the human body and its function are bacterial in other interesting metrics?
2008-09-22 Monday 10:40:07