Thursday, 28 July 2016

Nuclear Structure Day 3

This morning's run
It's more than half way through the Nuclear Structure 2016 conference.  Thanks to Kelly's comment on my previous post, I indeed took a run straight down Walnut Street to the river so that I could run along the "Greenway" path that runs alongside it.  I was a bit worried by the very steep hill I ran down to get to the bridge across the main road and then the stairs down to get to the riverside.  It meant that there would be a lot of uphill on the return run, and of course so there was.  I ended up running 2km, so no great shakes, but I certainly felt like I got a lot of exercise while doing it.  

There have been a lot of good talks at this conference.  I particularly enjoyed Gaute Hagen's talk yesterday which showed some recent results from his group's calculations using the coupled cluster method to calculate the properties of the doubly-magic 48Ca nucleus.  They use interactions from chiral effective field theory which one can think of as a fairly fundamental way of describing the nucleon-nucleon (NN) interaction (though it turns out that they are still a little uncontrolled so that there are many such interactions they can choose from) and the whole method falls under the name ab initio, meaning really that they use free NN interactions rather than in-medium interactions to produce the structure of nuclei.  I think there is still some work to make NN interactions sufficiently fundamental to justify the ab initio moniker. But okay, they are heroic calculations that were justifiably published in Nature Physics.  It was good, I think, that they made some effort to get the radius of their nuclei right.  The radius always seems to take second place to binding energy when people are trying to reproduce the properties of nuclei with their theories.  There are strong links between the neutron-proton radius difference and e.g. the expected properties of neutron stars, linked via the equation of state of nuclear matter.  Anyway, the result Gaute presented suggest that the neutron skin is on the lower end of what is usually predicted, which is certainly an interesting result, and I (and many others) await the CREX experiment which is planned to make the best ever measurement of the neutron radius.  The proton radius is relatively easy to measure via electron scattering.

charge radii
The radii of nuclei in this region show really interesting behaviour, as shown by Kei Minamisono in the talk before Gaute's, and by Ronald Garcia-Ruiz's talk immediately after Gaute's.  The second picture here shows a snapshot I took during Kei's talk.  If you click on it you get a slightly bigger version.  The points show the charge radius – so the proton distribution, basically.  The black triangles (second line from top) are for calcium, in which there are always 20 protons.  The radius, between neutron number 20 and 28, shows a kind of inverted parabola with odd-even staggering.  Very few theories can reproduce this.  Then there is a strong linear increase after the N=28 magic number.  Add one proton for scandium, or subtract one for potassium and the details don't just mirror calcium shifted up or down a bit, but look quite different.  There are rich structure effects going on in here that I don' think we fully understand.  Certainly there are approaches (such as density functional theory) which have reproducing radii well within their remit, but simply don't get the details right.

Lee Evitts
Yesterday also saw a Surrey PhD student (who spends all his time actually working at the TRIUMF lab in Canada despite formally being enrolled at Surrey) who is also a graduate of our MPhys programme, Lee Evitts, give a talk to the couple of hundred delegates present.  He did a good job, talking about his results of spin-zero excited states in nickel isotopes, and what they tell us about the nature of those nuclei.  In particular he was looking at electromagnetic transitions between spin-zero states, which are very peculiar as they cannot proceed by the emission of a gamma–ray photon, which is the usual way that electromagnetic transitions proceed.  This makes the experiments harder as the probes tend to be messier.  Lee's experiment used proton scattering off of the nuclei to let the associated Coulomb field cause the transition to take place.