Tuesday, 8 June 2021

Papers 064304 and 064305

By something of a coincidence, I have two papers appearing consecutively in Physical Review C this month.  An unplanned coincidence because one was submitted in April, accepted in May and published in June, while the other was submitted in January, accepted in April, and published in June.  

The older (published last Friday) paper also has a longer history:  It is work done by my PhD student Matthew Barton.  Matthew finished his PhD in 2018 and this paper was part of it.  His thesis was on the use of the time-dependent density matrix (TDDM) method to look at beyond mean-field aspects of nuclear dynamics with a goal (initially) of understanding fission processes.  Previous implementations of the (rather computationally intensive) TDDM method involved a few approximations which we thought we could overcome.  Although we did sort-of overcome them, we only did partially, in the sense that the result was too time-consuming to apply to fission.  One of the approximations we got rid of was to not assume that an uncorrelated ground state was good enough to start studies of collective motion from.  It seemed sensible, especially with fission as a goal, to start from as close to the right state as possible. 

This means running the time-dependent calculation in such a way as to turn on part of the interaction at the beginning of the simulation time so that the generation of the starting point ground state for the 'real' calculation occurs as the nuclear interaction is turned on.  It uses the Gell-Mann Low theorem to build up the ground state correlations.  In the end just this part of the calculation was quite a heroic effort and had not been done before in the same level of consistency we had done it.  Even though it was a sub-project of Matthew's thesis, we thought it worth publishing, and eventually got round to writing it up in time for a submission at the beginning of this year.

The paper is published as Matthew Barton, Paul Stevenson, and Arnau Rios, Phys. Rev. C 103, 064304 (2021) doi: 10.1103/PhysRevC.103.064304

Here's a figure from the paper showing the Gell-Mann Low theorem in action, turning on the correlation part of the formalism to build up a more complicated wave function and finding a lower energy state than the inial configuration which was itself a variational minimum within the space of uncorrelated wave functions

The newer paper's history (or at least my involvement with it) dates back to October last year when my colleague at Surrey, (Emeritus) Prof Phil Walker got in touch to ask me a question about state mixing and how it might change the wave functions of K-isomers.  State mixing is a bit like the correlations of the TDDM paper above, in the sense that we are trying to 'mix' or 'correlate' some functions that we have calcualted based on a model to make them more like nature has them.  Nature has already done the mixing or correlation.  As a statement, I cringe slightly when I write things like that, as if there is something somehow natural that they should be thought of as uncorrelated and nature then mixes them.  It's a language we use in nuclear physics, and in physics in general, that implies an event - states are mixed, symmetry is broken etc, when the action of mixing or breaking is only done in our theories so that we better reproduce what nature is.  Anyway - there is some logic to thinking this way, because sometimes natures produces unmixed states or unbroken symmetries and we can use those as a way to understand the more complicated cases. 

In the case of this paper we have some excited state of nuclei which either look like quite pure simple wave function which can be written down as 'unmixed' configuration, but that get mixed with other ones if there is a chance near-degeneracy between states with the same quantum numbers.  Then the wave functions of each state gets mixed together.   The effects of two state mixing is widely understood (by afficionados) to mix together the values of angular momentum projection (known as K in the context of nuclear isomers), changing the decay rates of the states compared to if there were no mixing.  Phil had come to me to see if I could help him understand whether a three-way mixing might be causing what appeared to be quite an anomalous decay rate for a particular isomeric state in Ta-179.  I helped by doing the three-state mixing calculation, but Phil did all the "understanding".  Anyway, his hunch was right, and the calcualtion should this really nicely. 

Here's a picture of the effect of the extra mixing caused by the third state:

The paper is P. M. Walker and P. D. Stevenson, Phys. Rev. C 103, 064305 (2021) doi: 10.1103/PhysRevC.103.064305

I owe my current position to Phil Walker, by the way, as it was he who first employed me as a post-doc at Surrey.  Thanks Phil!  Glad to still be collaborating with you.

Wednesday, 26 May 2021

Bull Sessions

 My colleague Jim Al-Khalili, who exceeds my 21 years in the Department of Physics at the University of Surrey asked a question about the term "Bull Sessions" on Twitter a couple of days ago:

I first heard the term, I think, after arriving at Surrey, when I found that the nuclear group had these informal talks in which lots of discussion was intended, called "Bull Sessions".  Usually they were based on talks from researchers in the group, and differed in style from the kind of more formal invited seminar from a visiting speaker.

You can follow the replies to Jim's tweet to see what other people know about the term.  I was slightly surprised to learn the following in the Oxford English Dictionary, which mentions the phrase, dating it back to at least 1920.  It appears unrelated to the animal, or the papal bull, but is related to the sense of bull meaning "Trivial, insincere, or untruthful talk or writing; nonsense" from which bullshit derives.  So, in that sense, it means an informal discussion.  It's a US term, originally.

The etymology of bull in this sense is listed as unknown according to the OED, but lists some similar sounding and meaning words in Old French, Modern Icelandic and Middle English which might (I suppose one is supposed to infer) be related. 

Interesting!  I don't know how it got into the Surrey group, but my guess is that possibly Ron Johnson brought it back from the US after doing a postdoc there and becoming head of the Surrey group (as well as one of Jim's PhD supervisors).

Tuesday, 25 May 2021

RIP Steven Moszkowski 1927–2020


I just heard the news that Steven Moszkowski died in December.  I heard of his death from my PhD supervisor, Jirina Stone, who has been working with him in recent years (with a highly-cited paper co-authored by Moszkowski, Jirina Stone, and her husband Nick Stone published in 2014).  I met Steven one, as far as I can remember, but I knew his name for his work on effective interactions for use in nuclear structure calculations - particularly the surface delta interaction (SDI).  I was reminded of him recently when a final year project student I've been working with this semester was working on a solvable model of octupole states in lead-208, as worked out by Piet van Isacker.  It made use of this SDI interaction, since although it is not a fundamental nucleon-nucleon interaction, still captures enough of the correct physics to be very useful, and is conceptually and mathematically simple.

Like my meetings with many of the older generation of nuclear physicists, I felt a bit inadequate to collaborate with them, and have clearly now missed my opportunity.  As I say, I only met Moszkowski one time (that I recal).  From the short memorial page to him at his institution, UCLA, it sounds like there were a lot of interesting stories he could have told me.  RIP, Steven.

Thursday, 22 April 2021

Earth Day and Isotopes

Apollo 8's Earthrise Image (NASA)


Today is Earth Day - one of the highest profile of the special named days that occur throughout the year.  High profile enough, at least, to prompt newspapers to run articles on helping to save the planet.

This is a nuclear physics blog, and I rarely write about the kind of issues Earth Day highlights.  The role of nuclear power as a low-carbon electricity source is probably the most obvious intersection of the nuclear and green worlds, and one that has people more expert than me in energy generation campaigning one way or the other on it.

One lesser-known way that nuclear physics makes an impact on environmental issues is in the use of different nuclear isotopes to help understand natural processes going on on Earth now, and back into the distant past.  

For each element in the periodic table, there are many possible different isotopes.  Each element is characterised by the number, Z, of protons in the nucleus: Z=1 for hydrogen, Z=2 for helium, Z=3 for lithium etc.  But for each element there can also be a different number of neutrons in the nucleus.  Hydrogen, for example, comes in 2 different stable isotopes 1H with 1 proton and 0 neutrons, and 2H with 1 proton and 1 neutron.  

The chemical behaviour is largely the same for each different isotope of an element, because the chemistry is mainly determined by the number of electrons in an atom of the element, and that number matches the number of protons in the nucleus.  But there can be slight differences between isotopes in chemical and other processes.  Isotopes with more neutrons are heavier, and they have shorter bond lengths when making molecules than their lighter counterparts.  These differences are enough to make some physical processes - such as the evaporation of water molecules, or the way nitrogen-rich nutrients are metabolised - happen at slightly different rates depending on the isotopes present.

In the case of water molecules, the rates of evaporation and precipitation of different kinds of heavy water (e.g. with heavy hydrogen and/or heavy oxygen isotopes) happens differently depending on the temperature, and by studying ice cores in Greenland's ice sheets, geophysicists have been able to reconstruct the Earth's temperature back long before scientific instruments began recording temperature.   The nuclear isotope ratios can be used to measure back to around 100,000 years ago - vital information in understanding the development of the Earth's climate to understand where we are today.

In the case of nitrogen isotopes, by looking at the reaction rates of different nitrogen molecues (ammonium, without oxygen, and nitrates, with oxygen), biogeophysicists can see evidence in the fossil record of when the Great Oxidation Event (GOE) took place, in which oxygen was released into the atmosphere and life forms began to develop which made use of it.  This is looking back a bit more than 2 billion years, so back to half the lifetime of the Earth ago.

The existence of isotopes was posited in 1913 by radiochemist Fredrick Soddy, with a range of new isotopes being discovered by mass spectrography in the following years.  Finally in 1932 the existence of the neutron was confirmed and the reason for the existence of different isotpes was understood.   Those involved at the time had no idea, of course, that the work would help us piece together the geohistory of our planet, help us understand our historic climate, and hence help us model how it will develop in future.  Such cases of applications of intitially blue-skies research are the way it goes (so the moral is: fund blue-skies research!)

For more details of the oxygen isotope ratios in ice cores see Frozen Annals by W. Dansgaard.  For the nitrogen isotope ratios going back to far prehistory, see Nitrogen Isotopes in Deep Time by Colin Mettam and Aubrey L. Zerkle

Sunday, 18 April 2021

A couple of new papers: On fission, and nuclear sizes

 I haven't mentioned here about a couple of new papers I have been involved with which have appeared over the last month:

• First is a paper on nuclear fission (here in Physical Review C, here open access arXiv version).  The work was done primarily by a PhD student in Beijing, but I contributed a little with discussions, expertise in the code and interpretation of results.  In it we try to understand what goes on microscopically (at the level of individual neutrons and protons) when fission takes place.  We go beyond some previous work (e.g. that of my previous PhD student here and here). Through random fluctuations we see reproduction of the different final products that appear in the distribution of fission products.

• Next is a paper on the isotope shift across shell gaps (here in Journal of Physics G, here open access arXiv version). This is work done by an extended group of collaborators, and again I contributed discussion, interpretation, suggestion of which calculations to do, with the lead authors doing those calculations.  It also builds on some work I did with the same PhD student as the fission work, published here.  I think the nicest thing about this paper is the showing how the underlying mechanism of the isotope shift (change in radius of nuclei as one adds neutrons) can be described in complementary ways by two somewhat disparate theories which each have their own language and mindset for thinking about nuclear structure.  It is also neat in that the idea of understanding how the size of nuclei change as you add more neutrons is in the (science) news right now thanks to the recent results from NASA's NICER telescope on the properties of neutron stars.

Here's a pretty picture from the fission paper representing how different fission events progress through different paths of shape of the fissioning nucleus:

Tuesday, 13 April 2021

Conference week

 I mentioned earlier that I had a couple of conference coming up, happening in the same week.  This is the week:  There is the joint IoP Astroparticle Physics, High Energy Particle Physics, and Nuclear Physics Groups' conference (website: http://appheppnp2021.iopconfs.org/home, Twitter hashtag: #EdiIOP2021) and the YQIS Young Quantum Information Scientists' conference (website: https://indico.frib.msu.edu/event/31/).

With the IoP conference being in the UK and the YQIS conference in the US, I could in principle attend the IoP conference and the the YQIS meeting with only a small amount of overlapping time.  In practice, with childcare responsibilities that's not very practical, but everything is being recoreded, and I am trying to make a sensible combined programme of talks that I want to see and then arranging which ones I am able to watch live, and which I will watch after the fact.  This ability to watch pre-recorded lectures is just what many of our students are now finding useful in our taught undergraduate classes. 

Yesterday, when the conferences started, was a Monday, meaning my day for looking after my youngest children, so I wasn't planning on doing much live participation, though it also happened to be the day when my own talk was scheduled at the IoP conference, so I arranged to have the boys looked after for that time and gave the talk.  I perhaps could have strapped the baby to a sling and walked around while giving the talk, but it turned out to be easier to arrange a little time swap with my partner in childcare duties. 

Today, having cycled all the kids to school / nursery, I am able to attend the live sessions, which means that I have started with Jim Hough's talk on gravitational waves (screenshot below).  It's amazing how we have been able to observe so many events of merging black holes over the last few years, coming from a situation not so long ago when black holes were suspected to exist, but not definitively observed, even indirectly.  

I think it's a bit of a shame that the conference is set up using Zoom's webinar mode, in which I can't see who else is in the audience, can't send them a quick message to say hi, or do any of the other 'conferring' that I would do at a conference.  I know there is a formally-arranged coffee break as part of the schedule, but I don't quite get the point of limiting our ability to interact with other attendees.

Here is a snapshot from Prof. Hough's talk.  Right now there is a talk I'd like to listen to about the FAIR laboratory and the work going on / planned there, but the speaker's audio has a strange bass echo that makes it unlistenable to me.

Tuesday, 30 March 2021

A talk in Athens on Zoom

 I gave a talk on Zoom this morning to a group based in Athens, Greece.  I didn't have to, or didn't get to, go to Athens for the talk, and was just sitting in my daughter's bedroom where I have a desk set up for home working.  It's my first talk of this year, and I talked about some calcualtions I have been doing on octupole vibrations in nuclei around lead-208.  I started work on them during the first lockdown, and have not looked at them too much in the last six months, but I would like to get them into a shape suitable for publication - which mainly means polishing off one last aspect of the calculation, and then deciding how best to present the "story" to the wider world.

It's nice to be able to give a talk to people in Athens without the time-consuming, carbon-consuming air travel to Greece, but I do slightly miss the whole experience of visiting another research group, joining them for discussions, joining them for lunch ... 

I'm not sure I have anything very Greek in the house for lunch.  It'll probably be a peanut butter and banana sandwich.

I at least did get a lesson on how to pronounce Greek letters properly.  I have always said "Phi" and "Psi" to rhyme with pie, but the proper (Greek) way is to rhyme with tree.  Well, I'll see if I can switch over to the right way in future.

Here's a snapshot my host took, on a very equationy slide.  Normally I do not go into as much mathematical detail as this, but I wanted to how how "simple" the theory was.  Hopefully the words I used to accompany it conveyed that okay.  Just looking at equations without context is never simple.