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Are stable atoms stable?

Dorfl

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I know that some atoms are called "stable", while others are called "radioactive", but I can't find any source which says if stable atoms are truly stable—never spontaneously decaying—or if they simply have half-lives so long that they can, for all practical purposes, be ignored.

Does anyone know?

Thanks,
Dorfl
 
Dorfl said:
I know that some atoms are called "stable", while others are called "radioactive", but I can't find any source which says if stable atoms are truly stable—never spontaneously decaying—or if they simply have half-lives so long that they can, for all practical purposes, be ignored.

Does anyone know?
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Yes. There are a limited number of spontaneous decay routes available to an atom. If you know the mass of the atoms and its possible decay products precisely enough, you can determine whether or not the decay products are lower or higher energy states than the original atom. If they are all higher energy states, then the atom is truly stable.

Excepting, of course, the possibility that individual protons can decay. But if they can, they have half-lives much longer than the current age of the universe.
 
Ziggurat said:
Excepting, of course, the possibility that individual protons can decay. But if they can, they have half-lives much longer than the current age of the universe.
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My understanding is that proton decay has been observed, but I could be wrong.

And the problem of very long half-lives is real. For example, bismuth-209 was believed (experimentally) to be stable until someone did the math and predicted a very very long half-life; this was later confirmed. From Wikipedia:

While bismuth was traditionally regarded as the element with the heaviest stable isotope, bismuth-209, it had long been suspected to be unstable on theoretical grounds. This was finally demonstrated in 2003 when researchers at the Institut d'Astrophysique Spatiale in Orsay, France, measured the alpha emission half-life of 209Bi to be 1.9 x 1019 years, over a billion times longer than the current estimated age of the universe. Owing to its extraordinarily long half-life, for nearly all applications bismuth can be treated as if it is stable and non-radioactive. The radioactivity is of academic interest, however, because bismuth is one of few elements whose radioactivity was suspected, and indeed theoretically predicted, before being detected in the laboratory.
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drkitten said:
My understanding is that proton decay has been observed, but I could be wrong.
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This may be confusion due to two different processes having the same name. In particle physics, proton decay refers to the decay of a free proton into other things (Wiki suggests a pion and a positron) and occurs in several (I think) theoretical grand unified theories.
There is also proton decay in nuclear physics where a neutron deficient nucleus emits a proton to become more stable (though still generally pretty unstable). This has been observed a fair bit in the last half a century. This is also referred to as proton emission, for obvious reasons, which avoids any confusion.
 
Tubbythin said:
This may be confusion due to two different processes having the same name.
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No, it's simply that I was wrong. I may have been thinking of one of those pre-print sort of things where the researcher triumphantly presents his Bigfoot photo and expresses surprise about how much Bigfoot looks like a giant human thumb.

And then later quietly retracts the paper when he discovers that Bigfoot has a skin pattern identical to his own thumbprint.
 
Even though the half life is extremely (extremely) large, given a sufficient amount of the stuff wouldn't you find the occasional atom going "poof" in a reasonable timeframe? :confused: I know the basic exponential formula, but the numbers involved far exceed my puny calculator's abilities, so could someone calculate how many grammes you would need to have a 50% chance of at least 1 atom decaying in, say, 1 month?

ETA: I am referring to Bismuth 209
 
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Ziggurat said:
Yes. There are a limited number of spontaneous decay routes available to an atom. If you know the mass of the atoms and its possible decay products precisely enough, you can determine whether or not the decay products are lower or higher energy states than the original atom. If they are all higher energy states, then the atom is truly stable.
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There are a couple of exceptions to this. For most heavy nuclei---A > 150 amu---this calculation says that alpha decay should be possible. The fact that we call these nuclei "stable" just tells you that it's extremely, extremely improbable. To illustrate, in 2003 scientists were able to detect, for the first time, the decay of 209 Bi (usually reported as a "stable" nucleus) with a half-life of 2 x 10^19 years. See http://www.nature.com/nature/journal/v422/n6934/full/nature01541.html which also lists the other theoretically-unstable nuclei in Figure 1.

Similarly, there's a broad window where spontaneous fission decay should be possible, just suppressed by some absurdly long time constant.

For the light nuclei, though, Zig is right---stable means stable, unless protons themselves can decay.
 
CrikeyBobs said:
Even though the half life is extremely (extremely) large, given a sufficient amount of the stuff wouldn't you find the occasional atom going "poof" in a reasonable timeframe? :confused: I know the basic exponential formula, but the numbers involved far exceed my puny calculator's abilities, so could someone calculate how many grammes you would need to have a 50% chance of at least 1 atom decaying in, say, 1 month?
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Roughly speaking, one gram of anything contains 6x10^23 nucleons. For something with a half-life of T, you will see 6x10^23 decays per gram per T. So, if the nucleon lifetime is 10^33 years, you expect 6x10^23 decays per gram per 10^33 years. That's the same as 6 decays per gram per 10^10 years (10^10 years is about the lifetime of the Universe) or conversely 6 decays per 10^10 grams (10,000 tonnes) per year.

The leading experiment, SuperKamiokande, has been monitoring about 5x10^10 grams of water for about 10 years. They have not seen any proton decays yet.
 
ben m said:
Roughly speaking, one gram of anything contains 6x10^23 nucleons. For something with a half-life of T, you will see 6x10^23 decays per gram per T. So, if the nucleon lifetime is 10^33 years, you expect 6x10^23 decays per gram per 10^33 years. That's the same as 6 decays per gram per 10^10 years (10^10 years is about the lifetime of the Universe) or conversely 6 decays per 10^10 grams (10,000 tonnes) per year.

The leading experiment, SuperKamiokande, has been monitoring about 5x10^10 grams of water for about 10 years. They have not seen any proton decays yet.
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Looks like we cross-posted. I updated my post to indicate I was referring to Bismuth 209, but presumably I could apply the same approach to an atom as to an individual proton?
 
ben m said:
... there's a broad window where spontaneous fission decay should be possible, just suppressed by some absurdly long time constant.

For the light nuclei, though, Zig is right---stable means stable, unless protons themselves can decay.
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About how light is light? Do atoms go from stable to "stable" around the 150 u you mentioned for alpha-decay, or does the shift occur much earlier?
 
Avogadro's Number 6.0221415**23 is the number of atoms in a mole of anything. A mole is the gram-molecular mass and the mass of one mole of an atom can be calculated by roughly taking the molecular weight of the element as grams.

SO, for Bismuth 83 grams, roughly, in one mole. SO, in one year in a mole of Bismuth, 60,000 atoms ought to decay if I figured that right.

520K minutes in a year, roughly, so, in that mass one atom ought to decay in 1.5 hours.

So, I am not gonna feel like taking Pepto-Bismol puts me at any risk at all from radioactive decay of the Bismuth.
 
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BenBurch said:
SO, for Bismuth 83 grams, roughly, in one mole. SO, in one year in a mole of Bismuth, 60,000 atoms ought to decay if I figured that right.
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Almost! The atomic weight of Bi is 209, not 83, so 209 g/mol.
 
BenBurch said:
Avogadro's Number 6.0221415**23 is the number of atoms in a mole of anything. A mole is the gram-molecular mass and the mass of one mole of an atom can be calculated by roughly taking the molecular weight of the element as grams.

SO, for Bismuth 83 grams, roughly, in one mole. SO, in one year in a mole of Bismuth, 60,000 atoms ought to decay if I figured that right.

520K minutes in a year, roughly, so, in that mass one atom ought to decay in 1.5 hours.

So, I am not gonna feel like taking Pepto-Bismol puts me at any risk at all from radioactive decay of the Bismuth.
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Well I "calculated" 0.1g would result in a single decay in 1 month. No idea if that is even in the ballpark, but it seems not too far from your numbers. However I would have thought that much more was needed if it went unnoticed for so long.
 
I'm 54.
I'm getting the impression from this thread that I can count on most of the universe not decaying before I'm dead?
 
Ziggurat said:
For purposes of nuclear decay, I'd call anything lighter than iron "light", and anything heavier than iron "heavy". Roughly speaking, of course. The reason is that iron has the lowest energy (per nucleon) of any atom:

http://en.wikipedia.org/wiki/Binding_energy#Nuclear_binding_energy_curve
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That much? After high school astronomy I'd gotten used to thinking of everything above Helium as "heavy" :-P So 50-ish u is about the point where atoms cease being absolutely stable.
 
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