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Caesium 137 is an unstable isomer

  1. Mar 8, 2013 #1
    -Caesium 137 is an unstable isomer and decays into (95% of nuclear decyas) Ba-137m via beta decay which then decays into Ba-137 via gamma ray emission.

    I have a couple of questions from this:

    1) Am I correct in thinking that radioactivty can basically be classified into two types :
    i)an unstable nucleus (dictated by the ratio of proton number to nucleon number)
    ii) an excited atom - occupying an energy level other than its ground.

    and so in the above case, the first decay is an example of an unstable nucleus decaying into a more stable nucleus(situation i), whereas the second case is simply a result of the barium 137 being fromed in an excited state and thus can only ocupy this state for a limited amount of time - the transiition occuring via emission of a gamma ray(situation ii).

    2) So also, that i includes beta and alpha radation only (and not gamma) whereas ii includes only gamma photons (and not beta and alpha).

    3) Also in case ii) if one is to measure the half-life of Ba-137m are you basically measuring the lifetime of its metastable energy state?

    Many Thanks for any assistance !
     
    Last edited: Mar 8, 2013
  2. jcsd
  3. Mar 9, 2013 #2

    mfb

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    It is not just the ratio of proton to neutrons, other factors are relevant as well.
    An excited nucleus.
    Photon emission can be so quick that it gets hard to distinguish your cases.
    Excited nuclei can undergo alpha or beta decay as well.
    If you are measuring the lifetime of a metastable state you are measuring the lifetime of a metastable state, indeed.
     
  4. Mar 9, 2013 #3
    Yes I though this, but in terms of defining the nucides e.g- if you are to write out the equations, for e.g - both decays in the above example,than the first would not include gamma

    Okay thanks, would you be able to give some examples of situations when this happens?

    Just confirming thats meant to be half-life ?

    Thanks alot, really appreciated.
     
  5. Mar 9, 2013 #4

    SteamKing

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    cesium-137 is not an isomer. It is an isotope. An isomer is a different structural arrangement of a compound, particularly an organic compound.
     
  6. Mar 10, 2013 #5

    mfb

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    half-life and lifetime are just two different ways to describe how quick the nuclei decay:
    After 1 half-life, 50% of the nuclei are remaining.
    After 1 lifetime = 1/ln(2)*halflife, 1/e = 37% of the nuclei are remaining.

    217mPa, 234mPa
    155m1Lu, 156mLu, 157mLu, 162m1Lu, 166m1Lu, 168mLu, ...
     
  7. Mar 10, 2013 #6

    Astronuc

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    In the context of radionuclides, or nuclear isomers, an isomer is a metastable state of a nuclide, in context of the OP, 137mBa, which decays by gamma emission (or isomeric transition) to 137Ba.

    A chemical isomer is different molecular structure with the same stoichiometry.

    A nice overview article from Physics Today, June 2005 on the Ups and Downs of Nuclear Isomers (pdf)
     
  8. Mar 10, 2013 #7

    SteamKing

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    First, Pluto is not a planet anymore.

    Now, I guess isotope is on its way out, too.
     
  9. Mar 11, 2013 #8
    Does this imply that only one n(discrete energy value) is occupied by the Ba-137m , that only one value is metastable, otherwsie I thought the time to de-excite would vary with n?(or perhaps I have interpreted the definition of life-time incorrectly).

    I have found that Ba-137m decays 11% of the time by internal conversion and 89% by gamma, is there anything that dictates /correlates with which is to occur? - like a change in n. And if one is measuring the life-time of Ba-137m then for the 11% to be accounted for, this assumes that a detector is equally sensitive to the electrons as it is for the gamma rays?

    Also am i correct in thinking that whilst lifetime can be defined for case ii, with case i you can only define a average lifetime?
     
  10. Mar 12, 2013 #9

    mfb

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    Ba-137 has many excitations, two of them have a long lifetime, they are called metastable: 137m1Ba and 137m2Ba.

    No. Strictly speaking, internal conversion requires electrons around the nucleus, but apart from special conditions in labs those are usually present.
    To measure the lifetime, you can measure the decrease in activity as function of time. This can be done with every decay channel you like. Lifetime is the property of the particle, not a property of a decay channel.
     
  11. Mar 12, 2013 #10
    Instead, say you are trying to obtain the frequency of the number of counts per second(N) for 01,2...
    Then if a sensor is not equally sensitive to electrons and gamma rays, the obtained frequency for each value of N will not be a true reflection of the total number of decays which will be greater than the sum of the total frequency adding the frequency for all N values. (so say the electrons are not detected at all, the difference will be due to this 11% decay path)
     
  12. Mar 13, 2013 #11

    mfb

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    That does not matter, if the lifetime is not too long. You can check how the counts decrease over time. If activity drops by a factor of 2 each day, the half-life is 1 day. This is independent of your efficiency (which can be hard to calculate, even if you cover all decay channels) and the total amount of radioactive atoms (can be tricky to get, too).
     
  13. Mar 13, 2013 #12
    Okay, I think I understand. What if instead of measuring how the acivity changes over time, you are after the mean count rate obtained from such data(frequency of the number of counts per second for each value). Surely in this case the mean would be less than its actual value, (with 0 counts per second) as obtained from the detecor having a greater frequency than it actually does decreasing the mean, due to the 11% unaccounted for (assuming these two ways - internal conversion and gamma rays are the two only decay paths).
     
  14. Mar 14, 2013 #13

    mfb

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    If you use the activity and the total number of atoms in your sample to determine the lifetime (necessary for long-living isotopes, for example - you cannot wait 1000 years in your measurement), you have to take all decay modes into account, right. In addition, you have to evaluate the detection efficiency - no detector will see all decays of any decay type.
     
  15. Mar 14, 2013 #14

    SammyS

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    Well, Steam, it looks like you were not entirely wrong regarding the misuse of the word "isomer" in referring to Caesium-137 . Caesium-137 is indeed an isotope of Caesium.

    It's the Barium-137m which is the metastable nuclide and is referred to as an isomer of Barium-137. See the Wikipedia article on Caesium-137 .

    (Thanks to Astronuc for giving us a link to a very enlightening article. I now have a better understanding of what goes on when I'm given Technetium-99m for a diagnostic CT-scan.)
     
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