Radioactive Decay

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Can anyone tell me what makes an atom unstable, and more prone to radioactive decay? From what I've read in books and on http://en.wikipedia.org/wiki/Nuclear_decay" [Broken], it's caused by flucutations in the quantum vacuum which disturb the nucleus. But why are some atoms more prone to be bothered by this disturbance?

Daisey
 
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  • #2
Astronuc
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Disturbances in the quantum vacuum?

How about it comes from having too many or too few neutrons for a given number of protons, which puts the nucleus at a slightly higher energy state than more stable configurations.

Too few neutrons, and the nucleus will undergo positron or more likely neutron capture.

Too many neutrons will result in beta decay or alpha decay if the nucleus is of an element like Bi or heavier.
 
  • #3
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Disturbances in the quantum vacuum?
Well, Yes. The exact wording in the Wikipedia article I quoted above is "In the case of an excited atomic nucleus, the arbitrarily small disturbance comes from quantum vacuum fluctuations."
 
  • #4
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Is it safe to say that if a nucleus has a different number of protons than neutrons, it is unstable? Or it is true that if an atom has a different number of electrons orbiting the nucleus than it has protons in the nucleus it is unstable?

Daisey
 
  • #6
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Naty1,

Thanks. A very nice article. :smile:

Here is a quote from the above CreationWiki link: "The instability of a radionuclide's nucleus may result from an excess of either neutrons or protons."

What is meant by "an excess"? Does that mean if there not exactly the same number of each?

Daisey
 
  • #7
Vanadium 50
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CreationWiki? The "encyclopedia of creation science"? I would not consider that a reputable source.
 
  • #8
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CreationWiki? The "encyclopedia of creation science"? I would not consider that a reputable source.
How would creation science differ from any other regarding Radioactive Decay??
 
  • #9
DrChinese
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How would creation science differ from any other regarding Radioactive Decay??
Doesn't really matter. They cannot be relied upon if they sometimes change things to suit their religious beliefs.
 
  • #10
DrChinese
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Is it safe to say that if a nucleus has a different number of protons than neutrons, it is unstable? Or it is true that if an atom has a different number of electrons orbiting the nucleus than it has protons in the nucleus it is unstable?

Daisey
No, that is not a good rule. It is a bit complicated and there are a lot of factors. Electrons don't really fit into the equation at all.

Almost every element has unstable isotopes. Generally, as a nucleus gets bigger than a certain size they become less stable very quickly. But a lot has to do with the shape of the nucleus and the exact mix of protons and neutrons. Adding a neutron to a stable nucleus may or may not produce a stable larger nucleus. And vice versa.
 
  • #11
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Titanium (Z=22) has 5 stable isotopes. Technecium (Z=43) has none. Not all nuclei that have an equal number of neutrons and protons are stable.
 
  • #12
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Adding a neutron to a stable nucleus may or may not produce a stable larger nucleus
I thought I remember reading somewhere that for the most part all nuclei are of the same size, no matter the atomic mass.
 
  • #13
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Not all nuclei that have an equal number of neutrons and protons are stable.
Bob, Is that also true for all elements below Bismuth (83)?
 
  • #15
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The nuclear radius is approximately roA1/3 fm, where ro = 1.2 fm = 1.2 x10-15 m, and A is the atomic mass.
Sorry. I stand corrected. I just found the quote. From Kenneth Fords book "The Quantum World", on pg 201 he states that all atoms (not nuclei) have about the same size.
 
  • #16
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Calcium 40 is the heaviest stable isotope for which Z = N, and Z = 20. All heavier stable isotopes have N > Z. Confirm here - http://www.nndc.bnl.gov/chart/
Astronuc,

That is a really fine web site. I went to look for Calcium 40 on that chart, and on the Decay Mode display, it shows 40Ca not colored black, meaning it is NOT stable. It's actually colored light blue, which I believe refers to the decay mode: EC+β+ Now, 42Ca IS stable (colored black).

Am I reading this wrong?
 
  • #17
Vanadium 50
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Doesn't really matter. They cannot be relied upon if they sometimes change things to suit their religious beliefs.
Exactly. An unreliable source isn't always wrong. Just...unreliable.
 
  • #18
vanesch
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To add my 2 cents. I think what the OP is alluding to (or at least, the article quoted by the OP is alluding to) is the fact that strictly speaking, stationary solutions of the hamiltonian are, well, stationary. So even excited states should be "stable" and remain there for ever, even if there exist lower states. But that is of course assuming that the hamiltonian describing the system is exact. The point is that, in *atomic* physics, it is the coupling to the QED field, which can be shown to be responsible for electronic transitions from excited states to ground states (spontaneous decay). I suppose (but I never read anything about it) that something similar is also at work in the nuclear case.
So spontaneous decay is attributed to the coupling between the empty (vacuum) field and the system at hand (say, the electron cloud in the case of atomic transitions, and the nuclear structure in the case of radioactivity), and it is indeed the quantum nature of the field to which it couples which makes the transitions possible (which would otherwise remain locked up in their stationary state).
 
  • #19
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Daisey-
You've been told about nuclear beta decay, both positron and electron. But there is another form of weak interaction decay. Sometimes an atom has too many protons and not enough neutrons, and the atom would have less mass if a proton were to decay. But it cannot because it doesn't have enough extra mass to create a positron and decay. So the proton waits around and captures an atomic electron. This is called electron capture. A good example is rubidium-83 "decaying" to krypton-83.
Here is another chart of isotopes that is easier to read:
http://en.wikipedia.org/wiki/Table_of_nuclides_(complete [Broken])
[Added in Edit]
There are a class of nuclei that are much more unstable than others. They are called odd-odd nuclei, because they have an odd number of protons AND an odd number of neutrons. A few of these can radioadtively decay by emitting either a positron or electron. One of these is copper 64, with 29 protons and 35 neutrons. Copper 64-can decay 3-ways: electron emission, positron emission, or electron capture. See
http://www.freebase.com/view/guid/9202a8c04000641f800000000bf63b1d/-/chemistry/isotope/decay_modes
 
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  • #20
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Daisey, google shell model which explains the magic numbers (the numbers of neutrons and protons in exceptionally stable nuclei) nicely. This question of yours has been on a table for a long time.
 
  • #21
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...So the proton waits around and captures an atomic electron...
Bob,

I have heard of that form of decay, but never understood it. That is very interesting. I was unaware a proton (or any nucleon) could capture an atomic electron. I thought the only type of capturing a nucleon could do was photon capture. Does the atom have to be unstable to allow the proton to do this, or can this happen in a stable atom?
 
  • #22
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When the mass (actually Mc2) of a proton-rich nucleus (p pronons, n neutrons) has a mass more than about 0.511 MeV more than a nucleus with p-1, n+1 protons and neutrons, it has enough total energy to radioactively decay by positron emission (from a bound proton). If the excess energy is less than 0.511 MeV, the proton can "decay" only by capturing an orbital (usually a K shell) electron. Such is the case with Rubidium-83, which can decay only by electron capture. Fortunately, the bare neutron's mass is about 1.3 MeV more than a bare proton, so the hydrogen nucleus cannot capture its only orbital electron and become a neutron. otherwise, the Earth's oceans would instantly become neutron soup.
 
  • #23
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Looking at the charts, it appears that many elements are unstable. Can I conclude that all common manufactured items in our macro world (cars, TV's, pencils, light bulbs, etc) are made of the elements that are stable?

I've found that most of everything that exists in our world has some type of beneficial feature. Besides creating bombs and nuclear energy, what benefit is there to all these unstable elements? Seems like such a waste, especially since some of these elements have such short half-lives (less than a second!)
 
  • #24
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Besides creating bombs and nuclear energy, what benefit is there to all these unstable elements?



Geiger counter used to measure radioactive emissions from a sample.Describe the uses of radioactive decay here.


Age Dating

Radioactive Tracers

Cancer Treatment

Sterilizing

Smoke Detectors

Genetic Studies


Beneficial Uses of Radiation by the Nuclear Energy Institute
 
  • #25
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So is there a reasonably well understood answer to the original question as to why some nuclei are less stable?? ...

the shell model seems to offer insights. I thought any radioactive decay involves quantum tunneling...which if true suggests energy barrier potentials would be significant.

Here is another description of the shell model:
the latter half of the paragraph just before the diagram is incomprehensible....

http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/shell.html
notes:
....remarkable patterns like the "magic numbers" in the stability of nuclei suggested the seemingly improbable shell structure.
and
If there are no nearby, unfilled quantum states that are in reach of the available energy for an interaction, then the interaction will not occur.
 

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