Bismuth is NOT a stable element; it is, in fact, radioactive.

[NNadir]

Many people who are familiar with science have been taught that bismuth is the heaviest stable element in the periodic table. I know I have always believed this, even if it seemed odd that a element with an odd atomic number should enjoy this distinction.

Unlike the heavy elements in the periodic table near it which are all highly toxic, lead, thallium and mercury, bismuth is a relatively non-toxic element, at least to eucaryotic cells. (Procaryotes don't fare as well.) In fact, the largest single use of bismuth is as a medication; it is the active ingredient in Pepto-Bismol, a well known over the counter medication that generates few, if any, concerns.

Today I was contemplating lead cooled fast nuclear reactors, a type of reactor that has not been built but is being contemplated for the Gen IV nuclear reactor program. Naturally I was perusing the Table of Nuclides and, because bismuth is a product of neutron capture followed by beta-decay in lead-208, (i.e. such a reactor transmutes lead into bismuth) I was inspired to look at the nuclear properties of bismuth.

Imagine my surprise when I saw this note:

"Bi-209 has been known as stable nuclide. But, alpha decay of Bi-209 with a half time of 2*10^19 years was found, recently.
Reference : PIERRE DE MARCILLAC, NOEL CORON, GERARD DAMBIER, JACQUES LEBLANC & JEAN-PIERRE MOALIC, " Experimental detection of a-particles from the radioactive decay of natural bismuth," Nature 422, 876-878 (2003); doi:10.1038/nature01541."

People often act irrationally when they hear the word "radioactive." For this reason let me state that it's not time to take your pepto-bismol to a radioactive waste dump . This is an extremely slow radioactive decay. If http://www.astro.ucla.edu/~wright/age.html">the age of the universe is taken to 13.7 billion years, only about 50 billionths of the bismuth formed near that time would have experienced nuclear decay. If one were to eat a kilo of bismuth, one would experience about 250 nuclear decays in a day. For comparison purposes, a typical 70 kg human being experiences about 4200 nuclear decays per second from the 140 grams of potassium such a person contains.

I have no idea whether anyone else will find this interesting, but I thought it was neat.

[Dudley_DUright]

1. I find it interesting (but then I am a physicist by trade)

That is a very long half life, as you point out.

[NNadir]

2. Ah well, then it was worth posting. n/t

[benburch]

5. Did I ever mention to you that I am an ex-Fermilabber?

Data Acquisition guy from the late 70s/early 80s.

[Dudley_DUright]

10. No you did not

I spent a year and a half in the early 90's at the DOE lab just down the road, Argonne National Lab. :hi:

[bemildred]

3. Two times ten to the nineteenth years is a heck of a long time.

But this raises the question of whether anything is NOT really radioactive. I believe neutrons decay over time too, for example.

What is the longest half-life at which a substance should still
be called "radioactive" or "unstable"?

[NNadir]

4. Free neutrons are radioactive and decay by beta decay to give protons.

They have a half-life of about 10.4 minutes. Neutrons in nuclei however are usually not unstable, unless the nucleus itself is unstable.

The question of what is and is not radioactive is an interesting question. People often speak of uranium as being "dangerous radioactive waste," for instance, without recognizing that uranium is a naturally occurring element that is quite ubiquitous and not particularly radioactive when compared with many other radioactive substances. In fact, uranium, at least when it is separated from its radioactive daughters, is a much more powerful chemical toxin than it is a radiotoxin. Potassium is also radioactive, owing to the presence of potassium-40 which has a half-life of 1.227 billion years. People eat bananas quite happily, because without potassium, its radioactivity aside, they would die.

People often focus their attention on the elements in so called "nuclear waste" that will persist for long periods, things like Cs-135, for instance, which has a half-life of 2.3 million years. However, cesium-135 is far less radioactive than its sister nucleus, cesium-137, which will completely decay to back ground levels over a few centuries. Cesium-137 is dangerous; cesium-135 is not particularly so.

In fact when life first appeared on earth, potassium was about 12 times more radioactive than it was now, about twice as much uranium-238, and about 84 times as much radioactive uranium-235. In fact there was so much U-235 in ancient times that naturally occurring nuclear reactors are known to have formed at Oklo, in Africa.

http://www.ocrwm.doe.gov/factsheets/doeymp0010.shtml

In fact, it can be shown that under certain circumstances, the use of nuclear power - public perception aside - will make the world less radioactive than it would be otherwise, although a period of about 1000 years is involved. This works because fission changes elements that stay radioactive a very long time, like uranium and thorium, into elements that do not stay radioactive particularly long - they quickly "burn out."

Leaving nuclear power aside, and looking only at natural radioactivity, it happens that nuclear stability theory predicts that no two nuclei of the same mass number can both be stable, even though such pairs clearly exist in nature. (Three of the "stable" isotopes of Selenium, for instance, Se-74, Se-76 and Se-80 are members of such pairs. Germanium-74 and Ge-76 both exist and are thought "stable" and Kr-80 also exists and is thought "stable.")

Careful examination, now that the instrumentation for the detection of radioactive decay has become so sensitive, of the question has borne theory out in some cases. For instance, the pair Xenon-136 and Barium-136 both exist. For a long time, it was thought that both were "stable nuclei" but recently it was discovered that Xenon-136 is in fact unstable. Like Bismuth it has an extremely long half-life, greater than 9.3 X 10^19 years. Should the universe persist for a very, very, very long time - which it may not do - all of the xenon-136 in the universe will decay to make barium-136 through the intermediary of cesium-136.

The situation is probably very much parallel to the chemical conception of kinetics vs thermodynamics.

[Tesha]

6. Do you realize that in that time, BUsh may actually make...

> Two times ten to the nineteenth years is a heck of a long time."

It's not such a long time. Why, don't you realize that in that
time, Bush may actually make 2.7 *CORRECT* decisions?

Tesha

[NNadir]

7. This is speculative. Physicists have NEVER detected such a thing.

I don't think that people are even looking for a correct decision from Bush any more.

[tocqueville]

8. that's nothing compared to potassium 40

K-40 Half-life: 1.28 x 109 years

K-40 Decay Mode:

Beta decay (89.3%). The beta maximum energy is 1.31 MeV

Electron Capture (10.7%).

Gamma Rays: 1461 keV (10.7%)

Daily intake of potassium element: 3.3 grams

Amount of potassium element in body: 140 grams (1.5 pCi/g or 55 Bq/kg of body weight)

Typical K-40 activity in body: 0.1 uCi; This means that there are over 200,000 atoms of K-40 that decay in the body each minute!

Typical K-40 activity in soil: 10 to 20 pCi/g

Dose from Potassium-40

The dose to a typical member of the population is approximately 15-20 mrem/year due to the K-40 in the body and 10 mrem/year due to the gamma rays emitted by K-40 in the environment (primarily the soil).


which is not negligible, it's about 11% of your yearly exposure - and you cannot do anything about it :

protection measures :

place a lead sheet between you and your wife in bed, specially if she has a big body volume

avoid bananas (look what happened to the chimp)

[NNadir]

9. I have far more mass than my wife, and I think I'll skip discussion of the

issue.

With two kids, the last thing I need is a discussion of a lead sheet before getting into bed with my wife.

:hide: