Wikipedia:Reference desk/Archives/Science/2018 October 14

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October 14

Nuclear shell model, alpha particles=nuclei within nuclei?

Since alpha particles come out of radioactive nuclei so often, and alpha particles are notoriously stable, why does the nuclear shell model insist that the shells have separate shells for neutrons and protons, why not even a shell made of alpha particles?...it seems likely to be energetically somewhat favorable, even within a nucleus, for alpha particles to wander around intact.Rich (talk) 00:07, 14 October 2018 (UTC)

I went looking and found a fringey-looking website that didn't link to any existing work, but makes kind of an interesting case. This looks more serious, and describes a weaker variant of the idea where nucleons simply can form alpha particles transiently within a nucleus, altering its properties; it's called alpha-clustering. That brings up scads of papers I know nothing about, describing many decades of work; a downloadable reference is [1] which sounds halfway between the first two. Better to search ArXiv and come up with [2] which looks promising, and many others. At this point I should cede the floor to an actual nuclear physicist... Wnt (talk) 02:33, 14 October 2018 (UTC)

While alpha particles are stable in sense that it is difficult to split them apart, their binding energy is still significantly less than that of heavier nuclei. In addition in heavier nuclei neutrons outnumber protons. So, it is difficult to imagine that heavy nuclei are literally made of alpha particles. Ruslik_Zero 19:19, 14 October 2018 (UTC)

Ruslik, your reasoning seems to me to imply the opposite of what you concluded. You seem to imply that not only would alpha particles be present as subnuclei, but heavier yet particles would be subnuclei. (By the way I didn't mean to suggest that a heavy nucleus would be made up entirely, or even almost entirely, of alpha particles, just some or a few.). Imo the binding energy argument argues against a nucleus consisting entirely of free protons and free neutrons. Thanks to both of you.Rich (talk) 20:11, 15 October 2018 (UTC)

I am not sure why you came to these conclusions? The nuclear forces have a short range and tend to saturate. So, as the coordination number increases, the binding energy increases. The coordination number in an alpha particle is around 3. It is higher in heavy nuclei. However returns diminish quickly due to the saturation. Ruslik_Zero 20:34, 15 October 2018 (UTC)

In a sense, I think alpha particles "fit through the gaps" in the nuclear shell model. After all, they have two protons and two neutrons with opposite spins, which mean they can always fall within a single energy level for either protons or neutrons, and often can be in the same state for all 4 (except spin). Supposing nuclear orbitals are like the somewhat more familiar electron orbitals, then knowing the angular momentum numbers means giving up any idea about the position, which is to say, if l is 1 and m_l is +1 for each of the four particles, we have no way to say if they are together in an "alpha particle" or not. I think that means that sometimes they are alpha particles, just by random chance? But to say they stay alpha particles would mean giving up some of the certainty about their quantized momenta. Speaking of which, does the sharpness of the nuclear gamma decay frequencies thereby place any Heisenberg constraint on whether they can be permanently bound as alphas in the nucleus? (This is so not my field, sorry!) Wnt (talk) 02:19, 16 October 2018 (UTC)