Monday, 14 January 2019

del-squared-cactus

A quick post today as I take a break in marking computing assignments.

I saw on Twitter a post about not recognising Greek letters in physics equations:

and a response from someone who teaches physics

Looking through many of the responses, it seems that it is a common problem among physics students to not have learnt or be able to pick up from context the names of the symbols used in physics equations if they are Greek letters (or other unfamiliar symbols presumably).

I think I do try to teach the names of the symbols I use when I use them, but maybe I don't.  Maybe I just say them out loud as I read or write them down and hope that's enough, but it seems from the Twitter responses above that it's probably not.  Would be interested to hear anyone's experiences on the matter.

Thursday, 10 January 2019

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 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 with 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.

ResearchBlogging.org 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

Thursday, 3 January 2019

Two new isotopes: Boron-20 and Boron-21

I noticed a new paper appear on the arXiv this morning, announcing the discovery of two new isotopes;  boron–20 and boron–21.  Boron has atomic number 5, so each boron nucleus has 5 protons.  There are two stable isotopes; boron-10 with 5 neutron and boron-11 with 6 neutron.  Boron–20 and –21 have 15 and 16 neutrons respectively.  That large asymmetry between the number of neutron and protons makes the isotopes unstable.  They are so unstable that the last neutrons do not even stick on to the nucleus for long and they decay by emitting one or two neutrons.  The live long enough to be identified as a resonance state in experiment, which is exactly what happened in the experiment that led to their discovery. 

Figure from arXiv:1901.00455
The experiment took place at the Radioactive Isotope Beam Facility (RIBF) at RIKEN Nishina Center, in Japan.  A beam of calcium–48 ions was fired at a beryllium target at very high energy, causing some of calcium nuclei to have a number of their protons and neutrons ripped off to give lighter isotopes of various sorts.  From this cocktail of fragments, the experimenters extracted nitrogen–22 and carbon–22 which were then directed onto a second target (of carbon) where some reactions knocked protons out of the nitrogen–22 and carbon–22 nuclei to form the previously-unknown boron isotopes. The snapshot taken from the paper in the arXiv shows the reactions taking place.  The plot is a section of the Segrè chart in which isotopes are shown with increasing proton number in the y-direction and increasing neutron number in the x-direction.  The arrows show the proton knockout reactions leading to the boron isotopes.  The red line marks out the neutron drip-line, separating those nuclei which are stable with respect to losing a neutron, and those which are not.  

Though the paper just appeared on the arXiv today, it was published in Physical Review Letters on 27th December.  Odd to put it on the arXiv only after it has been published elsewhere, but at least it means it reaches a wider audience (including me!), albeit belatedly.  Unfortunately the arXiv is not as widely adopted in nuclear physics as astro or high energy physics.

Tuesday, 1 January 2019

PhD in theoretical nuclear physics -> fitness trainer

One thing that some students ask when it comes to signing up for PhD study is what is the job market like afterwards?  One thing is for sure – a PhD does not mean that the only thing you can do is to go further and deeper into academia in the specialty area of your PhD research.  Here's one example of a previous PhD student of mine.  After her PhD in theoeretical nuclear physics (leading to publications here, here, here, here, here, here, here, here, and here) Emma went to work on climate science, where she devloped a successful career.  Most recently she has started a new business as a fitness trainer for children.  Here's a video featuring her for her company Kidz Impact.  I note that the link to nuclear physics is still present: "Kidz Impact and Teen Impact are trading names of Strong Force Limited"