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When one considers the decay chain of heavy elements, alpha decay forms a significant part of the decay series.

This make me wonder about the internal structure of atomic nuclei. When atomic nuclei are created by fusion is the structure of the two combining elements “scrambled”, or are they stuck together as clusters.

Question: Are atomic nuclei composed of an agglomeration (clustering) of helium nuclei and for odd numbered elements a clustering of helium nuclei and a hydrogen nucleus? Something like big marbles in a bag, where each big marble is a helium nucleus and a small marble is a hydrogen nucleus. Under such a scenario a helium nuclei (alpha particle) could potentially be more easily separated from the other clusters in the nucleus during alpha decay.

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Yes, here is a little more detail:

Within the nucleus, there exists a shell structure to the protons and neutrons inhabiting it, similar to the shell structure of the electron orbitals outside the nucleus. Those different shells contain nucleons with different energies. At the same time, there is a natural tendency for those protons and neutrons to in some sense "agglomerate" into alpha particle associations, because of the extremely high binding energy of such clumps.

But it isn't geometrically possible for protons and neutrons to 1) pack themselves down perfectly into a hexagonal-close-packed ("fully dense") form, 2) maintain the alpha particle associations, and 3) obey the shell structure rules as you add more and more nucleons to the nucleus. Compromises are necessary; this means that there will be certain nuclei in the periodic table that are lucky and have high binding energies and others that are less tightly-bound, and still others which are so unstable that they decay quickly and are not found in nature.

In a practical sense, this means you can model alpha decay as a highly asymmetrical case of fission, which one of the fission products is an intact alpha particle i.e., a helium nucleus.

This also means that there are nucleon configurations (or packing schemes) which are energetically unfavored but which can be created through collisions with other particles, yielding something called a shape isomer which you can think of as a metastable nucleus with a lump sticking out of one side or having a football-like shape instead of being spherical. The shape isomer can have a half-life for decay of microseconds, seconds, minutes, hours, days or years, and can produce (for example) a highly-energetic gamma ray when it decays.

A certain isotope of technetium, for example, can be produced in an accelerator to yield a shape isomer with a decay half-live of order ~6 hours and which produces highly penetrating 140 kiloelectron-volt gamma rays. These are routinely prepared and used as tracer elements in various medical procedures.

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  • $\begingroup$ Do the shape isomers always decay and fiss, or can they also settle into a stable spherical configuration? $\endgroup$
    – tparker
    Jun 26 at 15:52
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    $\begingroup$ It depends. most revert to their more stable shape. $\endgroup$ Jun 26 at 19:24
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I think yes, because of the high binding energy of alpha particle the matter is clustered inside heavy nuclei. At least there are such models.

You can read about them here:

https://iopscience.iop.org/article/10.1088/1742-6596/111/1/012001/pdf

https://www.ggi.infn.it/talkfiles/slides/talk3742.pdf

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