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

Low-earth orbit has gotten full enough that collisions are a risk to consider when sending up a new satellite. How full are the Earth-Sun Lagrange points? How much could be put there?
2009-05-20 Wednesday 06:54:49
I can make a partially deflated helium balloon neutrally buoyant by trimming bits off its string. How big are the variations in the "right" counterweight in an ordinary room?
2009-04-22 Wednesday 12:52:43
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
There's a theorem in control theory that sets limits on how quickly the gain of a feedback system can change with frequency without driving oscillations. This is the logic behind using the integral and/or the derivative of the feedback signal in mechanized control systems. One factor in the current financial disaster is tension between short-term and long-term risk, a problem that also appears in smaller-scale "markets." Suppose this tension were reduced by, say, paying fund managers with investments less liquid than cash, in some quantifiable way. Are cash commissions guaranteed to have boom and bust oscillations? Do oscillation-free commission schemes exist?
2008-12-31 Wednesday 10:03:07
A jugful of water adds some thermal inertia and makes an empty refrigerator cool more efficiently. A tubful of water ought to do the same thing for drafty old house. How big is the effect?
2008-12-08 Monday 09:59:41
Gerrymandered Congressional districts tend to protect extremists at the expense of moderates. Will the House be more or less polarized after the turnover in yesterday's elections?
2008-11-05 Wednesday 12:39:29
The Economist claims that some 1200 people have died crossing from Mexico to the US since 1990, compared to about 300 killed trying to cross the Berlin Wall during the 28 years it was up. Fatalities per mile? Fatalities per legal crossing? Estimates of successful crossings?
2008-10-11 Saturday 09:54:20
The Economist claimed that an oil price spike on September 22 (or thereabouts) was a quirk of timing, due to some minor supply quirk that happened to happen in the last couple days to fill contracts for delivery October 1. When I hear about "the oil price" on the news, what am I hearing? Is there information in the spread between short- and long-term prices that isn't reflected in either market alone?
2008-10-03 Friday 00:57:19
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