“Modes of decay” and gamma rays.

In comments, crosspatch was checking out the isotope list wondering where the gamma radiation was coming from. He was putting them in wolframalpha and it was returning “mode of decay: beta” for every one.

Let me spend a minute on decay. Nuclei are “unstable”, meaning radioactive, when they don’t have a stable balance between the number of protons and neutrons. All known elements with more protons than lead are unstable in every isotope.

(There are heavier stable elements theorized, but we can’t make them yet.)

Every form of decay is the universe’s way of seeking a stable nuclear configuration. 

In general, as protons increase, neutrons must increase even more to have a stable isotope.  The very first radioactive isotope makes me seem a liar.  Tritium, Hydrogen-3, has one proton and two neutrons.  It’s unstable.  It decays by pure beta emission and becomes Helium-3, which has two protons and one neutron.

Which naturally is stable.  The universe is full of stuff like this. 

But, in general, the more protons, the more neutrons needed.

Uranium is element #92.  U-235 has a total of 235 protons and neutrons, so it has 92 protons and 143 neutrons.  In fission, when one splits apart it usually splits into two smaller nuclei and, let’s say, releases three neutrons in the process.  (These can go on to split more U-235 apart.)

So we have two fission fragments that total 92 protons and (now) 140 neutrons.  You might expect an even split, which would give you 2 Palladium-116’s.

Almost never happens.  Thus the twin-peak fission curve, aka the “Dolly Parton”:


But say you do get identical Pd-116 twins.  You’ve now got two radioactive atoms, beta emitters, with a half-life of 11.8 seconds.  Almost all fission fragments emit beta.  Why? 

 The new nuclei have too many neutrons for the number of protons.  Somewhere between “one too many neutrons” and “wah-hey! too many neutrons”.  

Okay, beta particles are electrons to all extents and purposes.  So in beta decay a neutron throws out the beta and turns into a proton.  This brings things in balance, or closer to balance.

As a rule, the more unbalanced they are the shorter their half-life.  Lots of fission products are never a problem because they don’t last long enough to trouble us.  But their “daughter products”–the nuclei they turn into–are often radioactive as well. 

Okay, this is getting lengthy.  Didn’t get past one mode of decay, much less into gammas.  Will resume later. 

Any questions are  of course welcome.


About wormme

I've accepted that all of you are socially superior to me. But no pretending that any of you are rational.
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16 Responses to “Modes of decay” and gamma rays.

  1. crosspatch says:

    Ok, I just went back and changed by search criterion from “decay” to “decay chain” and I am still not coming up with any gamma in any of the decay products or their daughter products (if any).

  2. oldHP says:

    Journal article has exactly (?) what you want.


    Gamma-ray and half-life data for the fission products

    $ 41.95
    Jean Blachot and Charles Fiche
    Centre d’Etudes Nucléaires de Grenoble 85 X, 38041, Grenoble Cedex, France
    Centre d’Etudes Nucléaires de Cadarache B.P. No. 1, 13115, Saint Paul Lez Durance, France

    Available online 17 September 2004.
    Presented here are gamma-ray and half-life data for the fission products. The first table lists the energies and intensities of up to 5 of the more abundant gamma rays for each fission product. The second table lists gamma rays in order of increasing energy. The first section of this table covers nuclides with half-life less than one hour. The second section covers nuclides with half-life greater than one hour. Each listing consists of gamma-ray energy, intensity, and half-life. The third table lists all the fission products in order of increasing mass. Data for each nuclide include half-life, uncertainty and reference key for half-life, number of gamma rays, reference key for gamma data, total gamma-decay energy (including internal-conversion energy), and internal-conversion energy expressed as fraction of total gamma-decay energy.
    References available through January 1977 have been covered.

    • wormme says:

      I didn’t think he’d want to pay for it. At work we use a gamma spec system regularly, we’ve got multiple sources of every emitter. Just couldn’t scrounge anything up here. That didn’t require dough.

  3. waytoomanydaves says:

    Geez, I feel like a 7 year old watching the big kids play the pinball machine.

    Silly question from a layperson, if I may…

    As these atoms bleed off particles and transmute from one element to another, are they forming compounds or allotropes? Or is the environment of fission and decay too chaotic for that?

    • wormme says:

      Not a silly question!

      The answer is yes. Radioactive isotopes’ chemical behaviors are exactly like the non-rad version of that element. Of course, as soon as they hook up with a molecule they can decay again, and out they go.

      If any environment is energetic enough (or near-absolute zero cold enough) to prevent non-rad molecules from forming, naturally it will prevent “rad” molecules

      • waytoomanydaves says:


        Gud lawd, what a freakishly wicked-toxic brew that must be… full of exotic ions AND careening particles of kinetic obliteration PLUS being just really, really hot (thermally).

        (As you can see, your answer just aided my mental visualization considerably.)

        Looking forward to your next piece on decay modes.

        • wormme says:

          Well, you also want to keep in mind that these radioisotopes are chemically insignificant to the environment. When we get a “click” on a frisker…it’s registering the decay of a single atom!

          I think Charlie Martin calculated the mass of Iodine-131 released so far to be 26 grams. Think about it…Fukushima iodine has contaminated Japanese crops above legal limits and has been detected all around the entire world. You think we can chemically detect it over here? Hah.

  4. crosspatch says:

    Gotta dig for it, but it’s there:

    131I decays with a half-life of 8.02 days with beta and gamma emissions. This nuclide of iodine atom has 78 neutrons in nucleus, the stable nuclide 127I has 74 neutrons. On decaying, 131I transforms into the stable 131Xe.

    The primary emissions of 131I decay are 364 keV gamma rays (81% abundance) and beta particles with a maximal energy of 606 keV (89% abundance).[3]

    • wormme says:

      Yeah, we haven’t even gotten into the “abundance” thing yet. Probably won’t have to. It’s when your radiation is coming from a pure isotope that you’ve really gotta be aware of its characteristics. Iodine/cesium mix is readily detectable by both friskers and dose rate meters.

      If you look up Be-7 and its abundance, you can figure out why it’s such a headache for us to find. And tritium is a pain, of course.

  5. Pingback: More on the basics of beta decay. | World's Only Rational Man

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