Friday, 31 January 2020

Brexit Day

I'm feeling pretty sad today that we are leaving the EU.  At 2300 this evening we will no longer be a member state.  So long, EU.  It was good while it lasted.

picture credit:  It's from this page in the Daily Express "Brexit POLL: Should it be illegal to fly the EU flag from UK public buildings.  VOTE HERE!" (I voted no)

Thursday, 30 January 2020

Historical reminiscences on Skyrmions

There's a new paper on the arXiv today by Ian Aitchison, giving an historical account of the development of Skyrme's nonlinear field theory for nucleons and nuclei.  It explains some of the motivations - in terms of a desire to consider boson fields as more basic than fermion fields, with the latter being derived from the former; a desire not to have point-like particles; and a desire for conservation to arise from structural properties of an object rather than a symmetry.  In that last point he was apparently (as explained in the paper) inspired by William Thompson (Lord Kelvin), who was much enthralled by vortices in liquids and the emergence of a conserved quantity (Wirbelbewegung) and who came up with a vortex theory of atoms as a result.

The paper is quite informal, so I'd say it's fairly accessible to interested physicists, but with parenthetical remarks in it like "(this is essentially a consequence of the hedgehog ansatz)" it's clearly not quite targeted at the lay-person.  An enjoyable read for me to start the day with before getting on with marking assignments.  

Here's a picture from the paper, of a machine to make smoke rings to study the vortices that so interested Kelvin.  

Thursday, 23 January 2020

Happy Birthday Gran!

I've just walked to the post office on campus to send off a card, written by my kids and also signed by me, to wish my grandmother a happy 90th birthday.  That happens on Sunday, so I hope my second-class stamp will get it there by then. 

Not only has my grandmother reached a major decadal milestone (or is about to), but all her 5 siblings are alive and well.  I don't think they are all getting together for the birthday, as far as I know, and I can never keep track of who is and isn't talking to whom in my family, but I hope that I inherited the relevant genetic traits from her to keep me fit, active, and mentally with-it to such an age.  

I am currently 45, so around the age my grandmother was when I was born.  Funny to think that the grandmother I remember from my earliest years was only in her late-40s - an age which seems relatively young to me, now that I am there!

The picture was taken 2 years ago; the last time I visited her in Troon.  That's my second daughter, Alba, posing next to her.  For a sense of scale, If I were standing on the other side of my grandmother, our three heights would form roughly a straight line.

Happy birthday Gran!

Friday, 17 January 2020

Paul-Henri Heenen 1947–2019

I received the latest copy of Nuclear Physics News today and was saddened to read of the death of Paul-Henri Heenen.  I knew Paul-Henri mainly through his work on building the Generator Coordinate Method on top of the nuclear mean-field to give a rather complete model of nuclear structure starting from an effective interaction and giving results at the bulk level (nuclear masses, sizes, and shapes) and spectroscopic level (details of excited state energies and transition rates) all in one picture.  I cite his review, co-authored with Michael Bender and P.-G. Reinhard in practically every paper I write. 

I met Paul-Henri many times at different conferernces and meetings.  I was particualrly struck by his humility when discussing his work, partly because it is an all-too-rare trait.  Despite the fact that his work was heroic in its sophistication and completeness, he was honest about the deficiencies.  Many people I know are quick to point out only the good points of their own work, and the bad points of others'. 

RIP Paul-Henri.  The picture is taken from the website.

Wednesday, 8 January 2020

IoP SciNotes

Browsing the Institute of Physics list of journals, I noticed a new one, IoP SciNotes, which is "dedicated to the rapid publication of shorter research outputs across the physical and environmental sciences, including new results of any type (novel, negative or reproduced) or descriptions of scientifically valuable methods and datasets."

Allowing negative results, reproductions of existing results, or just descriptions of datasets, among other things, is an important thing to do.  Negative results are important to document, but unless you can spin them in the right way, they can be hard to publish.  Datasets that you may never get round to analysing yourself now have a home, and short descriptions of methods or techniques that don't quite feel enough for a full paper, too.  

The journal is open access and will introduce an article processing charge in future, but for early submitters, there is no charge.

The subject matter covers all of physics (but not astronomy thanks to an agreement with an existing similar journal in astronomy), and I would certainly encourage my nuclear physics readers to ensure that the journal reflects our field as much as any other.  I should start with my own contribution...

Tuesday, 7 January 2020

A new relativistic nuclear dynamics code

I note a new paper in press in Physics Letters B by Z. X. Ren et al. (doi: 10.1016/j.physletb.2019.135194) which presents new calculations of alpha cluster dynamics using a new code developed using relativistic density functional theory.  This is a method very similar to much of my research, and it's great that the authors have a production code using the relatvistic theory (my work is non-relativistic). 

This paper on alpha cluster dynamics looks at overlapping problems to my last paper on the subject.  I note that mine was submitted on 05/09/2019 while theirs on 09/09/2019 :-) and it's good to see different methods being used for the same kind of thing.  Our plots of the Fourier strength of the quadrupole vibrations in the Be-8+alpha system ought to be directly comparable:

Spectral power for quadrupole resonances
in He-4 + Be-8 system from Ren et al. (in press)
Insets show spectral density for He-4 + Be-8
quadrupole vibrations (x-axis units are MeV) from
Stevenson et al. (in press)

In the Ren at all work, then solid peak should correspond to the same physical quantity as (probably) the solid peak in my work in the upper inset in the right figure.  Ren's units give the period of the vibration in fm/c, while I give energy in MeV.  The peak from Ren's calculation has a period of ~120 fm/c.  These strange time units are the amount of time it takes light to travel 1 femtometre (10-15 m).  That's a very short amount of time (10-15 / 3×108 ≅ 3.33 × 10-24 s = 3.33 ys = 0.0033 zs).  So 120 fm/c is 0.4 zs (zeptoseconds).  The frequency (number of such vibrations per second) is f=1/0.4 zs which is 2.5×1021 s-1.

According to Planck's quantization rule, these vibrations have a corresponding energy E=hf, where h is Planck's constant.  so E = (4×10-21 MeV s)(2.5×1021 s-1) ~ 10 MeV, so we appear to agree with each other pretty well.  That's not really surprising, since the nuclear forces we use have to be reasonable enough to reproduce the right kind of scale of nuclear properties, but it's still reassuring. 

Thursday, 2 January 2020

Squeezing Calcium Nuclei

Over the Christmas holidays, a paper appeared on the subject of giant monopole resonances in Calcium isotopes.  The title of the paper is Compression-mode resonances in the calcium isotopes and implications for the asymmetry term in nuclear incompressibility (doi:10.1016/j.physletb.2019.135185), by Kevin Howard et al., based on experimental work that took place at RCNP Osaka.

Giant monopole resonances are a subclass of "compression-mode" resonance; excited nuclear states in which the nucleus is vibrating in a spherically-symmetric way, inflating and deflating like a balloon.  A principal interest of these modes is that their properties tell us something about how much a nucleus likes being squashed.  To a first approximation, they don't like it at all.  Nuclei are (like liquids) really hard to compress.  It's not impossible, though, and measuring just how hard they are to compress tells us something about the underlying nuclear forces via the equation of state of nuclear matter.

Motivating Howard et al.'s experiment is some recent published results from Texas A&M University (TAMU) in which the energy of the giant monopole resonances across calcium isotopes were found to increase as the mass number of the calcium isotope increased.  Usually, one expects that as nuclei get heavier (and larger), the giant resonances occur at lower energy.  One can think of the vibrations as having to have nodes at the surface of the nucleus, and the wavelength of the vibration gets longer in a heavier (larger) nucleus.  Thanks to Planck's quantization rule, longer wavelength means lower energy.  More specifically in this case, there is (also) expected to be a decrease in energy when one specifically is making the nucleus more asymmetric between protons and neutrons, as happens when going from the calcium-40 to calcium-48 -- the chain in question here.  

Howard et al.'s work, then, was aimed at checking the surprising result.  Their answer is in disagreement with the TAMU data, showing instead the normal order.  The results are here in the figure from the paper (the whole thing is published open access via the link above).  The decreasing solid blue line show the results from the Howard et al. work, while the increasing red solid line is the TAMU data.  The other lines are models of various levels of sophistication.  While the new data is reassuring, in that it agrees with our understanding of nuclear forces, either a third set of experiments, or a reevaluation of the TAMU data looks in order.

The year 2020 is the year of calcium-40; the nucleus with 20 protons and 20 neutrons :-)