New Royal Society Fellows

I see a list of newly-elected members of the Royal Society has appeared.  I carefully checked, but did not see my name there.  Oh well.  Searching on the word 'nuclear' brought up a link of Nobel laureate Donna Strickland's work to nuclear fusion, and the words 'Nuclear Magentic Resonance'.  Next year, then...

UK Lockdown seminars

The UK nuclear physics community has just announced its own series of lockdown seminars to deal with the present situation where were are all working from home.  The idea is to include the sort of people who different nuclear groups from around the UK were planning to have to speak before all live events were cancelled.  The webpage for the initiative is here. 

The first seminar is 3pm BST this Friday, with Alessandro Pastore from York, and there are three seminars every week - on Mondays, Wednesdays, and Fridays.  This fits in well with the nuclear reaction seminar series I'm already (mostly) watching (https://reactionseminar.github.io/schedule), which are on Tuesday and Thursdays.   It's almost like being at a conference...

HIAS Proceedings Online

Way back in ancient times (i.e. before Covid-19), academics used to go and meet up with each other to talk about science and stand around awkwardly in coffee breaks.  Such a thing happened to me last September when I was in Canberra, Australia.  I attended the HIAS conference at the Australian National University (ANU) in Canberra.  HIAS stands for Heavy Ion Accelerator Symposium and it is a regular meeting to showcase the kind of science they can do at their own on-site accelerator facility, though speakers are invited from competing and complementary facilities around the world.  They ask the participants to write up their presentations for the conference proceedings.  I duly did this, and the collected proceedings have been published today in the EPJ Web of Conferences vol 232.

As a (rather large) figure to accompany the post, here is Figure 1 from the first paper in the proceedings.  The paper is an overview of the kind of work they do at the ANU accelerator, written by Andrew Stuchbery.

My own contribution is about what heavy-ion reactions can tell you about the surface energy of nuclei (not much): P. D. Stevenson, EPJ Web of Conferences 232, 03005 (2020)

A rare pair of mirror nuclei

Yesterday a paper appeared in Nature which describes on the second case of a pair of "mirror nuclei" (which differ from each other by having the number of protons and neutrons swapped) in which the ground states have different spin.

The pair in question is strontium-73 (Z=38, N=35) and bromine-73 (Z=35, N=38) which have been measured to have spin-parity assignments of 5/2^– and 1/2^– respectively. The states of mirror nuclei are pretty close to identical, thanks to the isospin symmetry of the nuclear force:  To a good approximation the nuclear force looks the same between pairs of protons, pairs of neutrons and neutron-proton pairs.  There are a couple of ways in which differences appear in mirror nuclei - e.g. because there is also the Coulomb force in play which acts between protons but not neutrons, and these small differences can sometimes cause an effect like the one seen in the Sr-Br pair.  In this case, the small differences are enough to give a different ground state as there seems to be a very low-lying state close to the ground state in these nuclei and the small differences happen to be enough to swap the order of these levels in the two nuclei.  

The figure to the right is part of one of the figures in the supplementary material on the paper.  It's a section in the nuclear chart of isotopes in which the line of N=Z nuclei appears as a vertical line in the middle, and nuclei close to this line are shown - the ones for which mirror pairs are known to exist.  The two pairs coloured in black with little cracks in, are the two cases in which the mirror ground state symmetry is broken.  The other case which was previously known is ¹⁶F/¹⁶N.  In that case the fluorine isotope has its last proton unbound, and it only exists as a nucleus thanks to the Coulomb barrier.  The nitrogen valence proton is not unbound and this significant difference is enough to cause the difference in the ground states.  The same effect is not in play in the Sr-Br case. 

My first remote PAC

My working from home today consisted of participating in the Jyväskylä accelerator laboratory Programme Advisory Committee (PAC) where were review proposals for beamtime and then make recommendations to the laboratory for how to allocate resources requested in the proposals.  The hosts in Finland graciously moved the start time of the meeting to 10:15am Finnish time, so 8:15am UK time, and we had a good meeting, in terms of getting through the agenda with the same diligence as when we all attend the meeting in Jyväskylä in person.  We missed the peripheral, but still nice, and still important parts of the meeting.  The full social exchange, the visiting another country and interacting with it and people there outside the meeting, the chatting to colleagues working in the physics department at the University there during the breaks in the meeting, the nice meal afterwards... But the meeting did work well, and we saved our carbon emissions.  I also had less time away from my family.  Indeed since we are in our extended lock-in, my 3yo son even decided to spend some of the meeting sitting on my lap.  I hope, though, on balance, to be able to have the next meeting in person.

Online seminars during the coronavirus epidemic

My 3yo son, realising that many things are cancelled at the moment, asked me this morning "is working from home cancelled?"  Alas not.  As it looks like we'll all be working from home for at least a couple of months, various schemes seem to have popped up to start research seminars that are given over the newly dominant teleconferencing platform, Zoom.  

The nuclear reaction theory community has started a series of seminars, detailed on https://reactionseminar.github.io. I wasn't aware of it when the first seminar was given, but I plan to attend future seminars.  One will be given by my Surrey colleague Natasha Timofeyuk.  I'm not aware of a similar initiative by the nuclear structure theory community (structure + reactions is a traditional way of dividing up low-energy theoretical nuclear physics, though I sit in both camps).  I would certainly join one if it started, though I'm not quite keen enough to start such a project by myself.

Based at CERN there is a series called Seminar on Precision Physics and Fundamental Symmetries which includes nuclear physics, but is broader than that.  There will be two seminars per week (Tuesday and Thursday at times to suit the speaker) starting today: https://indico.cern.ch/category/12183/.

If anyone knows of any more, please advertise them in the comments below.

That's my son who asked the question in the picture, during one of our daily permitted excursions for exercise.  It was at the weekend when the weather was very cold, and a streak of hailstone is caught in the picture too.

edit: Thanks to JINA-CEE for responding to me tweeting this blogpost.  They point out their online seminars in nuclear astrophysics.  See https://www.jinaweb.org/events and look for the events tagged "IReNA online seminar"

Li-6 as a probe of giant monopole resonances

A new paper appeared on the arXiv this morning (in nucl-ex, cross-posted to nucl-th) titled Reexamination of 6Li scattering as a Probe to Investigate the Isoscalar Giant Resonances in Nuclei.  The Isoscalar Giant Resonances are excitations of nuclei in which the protons and neutrons move together en masse, in phase.  

The two main isoscalar giant resonances, and the two examined in the paper, are the monopole and quadrupole versions.  The Isoscalar Giant Monopole Resonance (ISGMR) is sometimes called the breathing mode, since it can be loosely imagined as a kind of inflating and deflating of the nucleus, though there is no air being pushed inside or breathed out to cause the motion.  The breathing mode is set off when particular kind of particle is sent scattering of the nucleus in such a way that no angular momentum is imparted.  In this case, a nucleus which started off as spherical will retain its spherical shape as it breathes in and out.

The other one of interest here is the Isoscalar Giant Quadrupole Resonance (ISGQR) which gets excited when two units of angular momentum are  transferred to the nucleus by the excitation.  This gives a different shape for the excitation in which the nucleus is squeezed in one direction while expanding in the orthogonal plane -- imagine the Earth squeezing down along the poles and expanding along the equatorial plane as a result, and then bouncing back as it tries to restore its equilibrium shape.

A key interest in making these excitations is that they probe rather generic properties of nuclear matter.  By squeezing a nucleus you can immediately probe the matter all the way through it and try to understand how "stiff" it is - how hard it is to compress.  The answer here has to come from the underlying forces between the nucleons, and we can learn all about that force by exciting these resonances.  

The traditional particle of choice for striking on nuclei to form the ISGMR is the alpha particle.  This works well because alphas only excite isoscalar excitations (those in which the protons and neutrons behave in the same way, unlike isovector excitations in which they act out of phase, and which can strongly mask isoscalar excitations).  A difficulty, though, is that the scattered alpha particles, which have to be observed to deduce how they interacted with the nucleus, have to be measured at very small scattering angles.  In other words, these measured alpha particles are very close to the beam line which is full of alpha particles which did not interact, or even worse, with stray alphas that scatter out of the beam line but not due to the interaction being targetted.   

The paper, then, uses lithium-6 (3 protons, 3 neutrons) to scatter off the target nucleus and cause the excitation.  The benefit here is that Li-6 breaks up easily into an alpha particle and a deuteron, and the interaction with the target nucleus will often cause this to happen.  One can then look for the alpha particles coming off, which are no longer contaminated by beam alphas (since the beam is now lithium).  The cost is a more complicated analysis of the scattering process and the possibility of different excitations of the target nucleus.  From the paper, though, the results seem very nice.  Here is a plot of the excitation of carbon-12, showing the tiny error bars:

 There is some structure to the peak.  Even ignoring what is happening below ~13 Mev, there appears to be one main peak and at further peaks in the shoulders - at least one prominent one in the higher-energy side.  This could be due to all sorts of things, but with carbon-12 not being spherical (it is oblate deformed, like the Earth) then there should a strong coupling to the ISGQR when the ISGMR is excited, and that is likely what dominates the structure.