Pear-shaped fission

Over the Christmas break I noticed that a paper appeared in Nature giving an explanation of why fission fragments tend to favour particular daughter nuclei over others which might naively be expected to dominate.

When a heavy nucleus, like Plutonium–240, with 94 protons and 146 neutrons, fissions, it splits up into two lighter nuclei.  Different daughter nuclei can be produced in different fission events, but there is a distribution which centres on a light fragment with around 54 protons and 85 neutrons, and a heavier one taking the rest of the nucleons.  The reasons why particular daughter products are favoured are to do with how the slow process of the parent nucleus stretching before forming two nascent fragments and finally splitting is energetically possible.  It has been a bit of a puzzle why ~54 protons should be more favoured than 50.  50 is a "magic number" for protons;  an especially stable configuration which you might expect to be a preferred end product when looking at energetically possible outcomes.  

The paper by Scamps and Simenel calculate that the key reason that 50 is bypassed is that octupole shapes (pear shapes) are favoured at around proton number 54, and these are the gateway shapes formed as the nucleus fissions and the neck of the fissioning nucleus splits to give the narrow end of the "pear" in the daughter nuclei.  This effect is more important than the final stability of a 50–proton nucleus.  

The paper is in Nature, which is behind a firewall, so I'm not sure that it is available to a general audience.  With the unpaywall browser plugin, I see that there is a "bronze open access" version available here.  The first description I find when searching of what bronze open access means is that it is an potentially fleeting open access status without the backing of a perpetual license that some publishers are implementing.  This somewhat critical description is something I found in ... Nature.

The kind of calculations performed in this paper are very close to what I do, so I'm (a) pleased to see that it gets published in Nature (b) pleased they cite work done by my PhD student a few years ago and (c) annoyed that I didn't followup my student's work after he left to work for Sainsbury's by making this same analysis that got published in Nature.   The attached picture is a snapshot from a movie they provided in the article as supplementary material.

Guillaume Scamps and Cédric Simenel (2018). Impact of pear-shaped fission fragments on mass-asymmetric fission in actinides Nature, 564 : 10.1038/s41586-018-0780-0