Shape Coexistence

I'm on the train on the way back from my second conference trip to York in as many weeks.  This time, it was a specialist workshop on shape coexistence in nuclei.  Shape coexistence is a slightly strange name, given to a natural consequence of quantum mechanics that seems a bit counter-intuative, so has been given a name.

Objects of everyday size and scale typically have something that you can call a shape.  Footballs are spherical in most of the world, or prolate spheroids in the USA.  Eggs are oviform, breasts are mammiform, and so on.  Only, if you were to look closely, at an atomic level, at any macroscopic object, you'd see that the surface was not quite as solid as you thought, but an undulating border between atoms belonging to the object, and those not.

Down at the tiny level of atomic nuclei, the undulation is so intertwined with the nature of the nucleus, that the concept of shape breaks down.  Many nuclei don't just sit there being a specific shape, but undulate so that they are a sometimes spherical, sometimes stretched spheres, sometimes squashed spheres, and occasionally other shapes.  This is just quotidian quantum mechanics, but because it's counter-intuative, it's got its own name, and associated conferences.  To be fair, it's as good a theme to use to understand any part of nuclear physics as any other, as long as one understands why we consider it a particular topic.

There were a lot of interesting talks, covering nuclear states that simultaneously exhibit multiple shapes, from both experimental and theoretical points of view.  I gave a talk, and it invoked some mild disparagement from a fellow theorist, as theory talks always seem to do.  It's too bad my interlocutor had to depart for a train before the end of the session I was in as we never managed to have a good chat about it.

As ever, I think about adding a figure to a blog post as I finish writing.  Clearly, the term "shape coexistence" is used only amongst nuclear physicists, and not in other areas.  This is perhaps not too surprising, as nuclei are the only quantum objects that are within their own size for a few orders of magnitude, so if that's the scale at which the effect happens, then there's nothing else for it to happen to.  The picture is taken from a Cern Courier article about work by many of the people at the workshop, including its organiser, showing some of the different "co-existing" shapes in an isotope of lead.

Solving the medical isotope problem

One of the nice things about this year's Institute of Physics nuclear physics conference is that we have a few attendees from universities which don't usually send people.  It seems, perhaps, that nuclear physics activity in the UK is growing, or at least that some of the more applied nuclear physicists are joining in community events.

Yesterday, I attended two interesting talks by people form such non-traditional universities;  Huddersfield and Cambridge (nice to be able call Cambridge a non-traditional university).  Both speakers were concerned with performing simulations of nuclear reactions induced by small-scale low-energy particle accelerators in order to produce medical isotopes.  One of the most important medical isotopes is technetium-99m (Tc-99m).  It is normally produced by taking molybdenum-99 from nuclear reactors, and letting that decay to technetium-99m.  The "m" means that the technetium is in an excited state, and it decays by X-ray emission.  By preparing a suitable technetium compound that targets particular parts of the body when ingested, the position of the emitted X-rays can be measured.

Through various reasons, including unplanned reactor shut-downs, the worldwide production of Tc-99m has decreased markedly.  The groups at Cambridge and Huddersfield are looking at new, non-reactor-based ways of producing it, and indeed other medical radionuclides.

The picture associated with this post is from a poster presented by Naomi Ratcliffe, from Huddersfield, available from the Huddersfield website.  It relates to some of the simulations of neutron yields that she presented in her talk.

York for the IoP conference

About this time of year every year, the Institute of Physics Nuclear Physics Group organises its national conference.  It's for everyone in the UK working on nuclear physics (mostly, but not exclusively with a bent towards academic as opposed to industrial aspects).  It's open to overseas attendees, too.  We usually invite a few overseas keynote speakers, and sometimes overseas applicants want to come and attend.  That's fine, as long as it fits in the conference venue, but the focus is on a national scale.

The location of the conference cycles between all the places where University nuclear groups are based, and this year we are in York.  Fortunately all the UK groups are based in pretty interesting places, except for my home institution, so I always get to travel somewhere more interesting than where I live at conference time.

The purpose of the conference is split into sharing the latest academic news and networking.  They probably both carry about equal weight.  Although we can keep up with each others' work by seeing what papers we publish in journals and the arXiv, there's nothing quite like being away from your desk,  and listening to people describe their work to get a good idea of what's going on.  On the social side, the conference is a great way to help build a sense of community in the UK groups.  Science research is sort-of international in scale, but also sort-of national.  At the basic level of science ideas, the community is worldwide, but part of research life is working within the organisational framework of your country, its institutions and funding mechanisms.  We spend some time together to help us act together as a community, and part of the conference will be a town meeting with representatives from the funding council STFC to talk through some of the political aspects of research.

Part of the networking and socialising will be in the form of visits to the pub in the evening, no doubt, and there is a conference banquet for the purpose of penalising the person to speak first in the morning after it.  I, fortunately, am speaking in the first morning, about the kinky lead phenomenon I blogged about before.

I decided to come up a bit early and book a separate hotel for a one-night holiday before the conference started.  At the top is a picture of me by York Minster, in deference to how tourists are expected to behave.

The Ubbelohde Effect

Last Monday, we had an interesting seminar in the Physics Department given by Angelos Michaelides from UCL.  He's a chemist, but his work spans chemistry, physics and biology, and his seminar attracted an audience from across the departments at the University.  He was talking about quantum molecular simulations involving hydrogen bonding.  He mentioned a nuclear effect that I was aware of, but used a name I hadn't heard before:  The Ubbelohde effect.

It is one of the (few) ways in which having a different isotope present can have an effect beyond the nucleus itself.  Replacing the nucleus in a normal hydrogen atom (i.e. a proton) in a molecule with a heavy isotope of hydrogen (a deuteron) will affect the molecules geometry as a whole.  In particular it can cause bond lengths elsewhere to change, and can affect the strength of hydrogen bonding between molecules.  Hydrogen bonding is important in all sorts of ways;  it's the bonding between water molecules that makes water relatively viscous, and it's the bonding between the strands of DNA.  If I understood correctly, the isotope effect on hydrogen bonds is what is called the Ubbelohde effect.

Having done some mild googling after the talk, I don't feel too bad about not knowing the name before.  The first hit when I search is a recent paper from the speaker, and the wikipedia page about Ubbelohde (a German chemist) is very terse and does not mention the effect.  I should probably edit it to change that.  It does, however, have an excellent picture of Leo Ubbelohde, reproduced here for your pleasure.

edit: I got the wrong Ubbelohde.  Sorry!  Thanks to my commenter for putting me straight.  The right one is Alfred Ubbelohde

The reliability of Carbon dating

Me, standing by some fresh water, in 2009

Carbon dating is a neat thing that our understanding of nuclear physics has brought us.  All carbon atoms have nuclei with 6 protons in them - that's what makes them carbon as opposed to any other element.  They can come in a few different isotopic varieties, with differing numbers of neutrons.

There are two stable isotopes - carbon-12, with 6 neutrons, and carbon-13 with 7 neutrons. Carbon-12 is by far the most common isotope (thanks to it being made in stars in one of the key stellar nuclear fusion reactions), but carbon-13 still is around in appreciable amounts.  It is interesting in its own right, and is used in all sorts of interesting ways but Carbon dating relies on an unstable isotope; Carbon-14.

Being unstable means that carbon-14 nuclei decay with an average half-life of about 6 000 years, meaning that if you have a sample of carbon-14, half of it will have decayed (to nitrogen) within one half-life. It is around on Earth in trace amounts, despite Earth being much older than 6 000 years, because it is formed in nuclear reactions in the the atmosphere.  These reactions occur when cosmic rays (mostly protons) from space collide with atoms of atmospheric gas, producing neutrons, which then react with nitrogen-14, knocking out a proton, and causing carbon-14.  The carbon reacts with oxygen to make carbon dioxide, which then makes its way in to the ecosystem by e.g. dissolving in water and falling as rain, which is then taken up by plants and animals.  We each have a proportion of radioactive carbon-14 nuclei in our bodies as a result.

Once we die, though, we no longer take up new carbon-14.  What carbon-14 is in our bones will then decay, and the proportion of carbon-14 to carbon-12 can be measured to tell how long it was since we (or whatever else we are measuring) was alive.  This is the basis of carbon dating.  It does rely on some assumptions - such as that the historic rate of carbon-14 production in the atmosphere has not changed much, that the resulting carbon-14 is equally distributed throughout the world, and that all living material takes up carbon-14 at the same rate as all other things.  There are ways in which it is known that some of the assumptions are not so good, and corrections are made because of them.  Various cross-checks can be made too, such as correlating carbon age with tree-ring age.

Thanks to a recent tweet by STFC's Head of Communications, I learned about a particular effect which can skew carbon dating results.  The tweet pointed to this story on a science news website.  The effect is to do with fish which have an anomalously low ratio of carbon-14 to carbon-12, which then get cooked by prehistoric people, leaving residue in the cooking vessels which can be carbon dated.  Rather unhelpfully, the news story says "Hard water contains high levels of calcium, and calcium contains no Carbon-14." While clearly true (calcium is one element, carbon another), I'm not convinced about it as an explanation.  There is a link on the page to the scientist behind the story, and while it seems most of the work has been presented at conferences with no write up, the abstract for one of the conferences describes the reasoning.

The effect is called the Freshwater Reservoir Effect.  It happens because rivers can have large amounts of carbon-containing minerals dissolved in them (for example, if the source of the river flows over particular kinds of rock).  These minerals are old, with no carbon-14.  The proportion of carbon-14 in the rivers is then low, and the proportion in the fish living in the rivers also low.  The proportion found when dating prehistoric clay bowls used to cook the fish in is also low.  If not for the freshwater reservoir effect, one would conclude that the bowl was actually much older than it is, potentially by many hundred years.

Blatant advert for PhD study at Surrey

If you are thinking of PhD study in theoretical physics in the UK, you could do worse than follow this link to an advertisement on the findaphd website.  We have funding for students to come and work with members of the group on theoretical nuclear physics, in topics ranging across structure and reaction theories, ab initio methods, time-dependent simulations, looking at nuclei from the lightest few-body systems up to neutron stars.  Mathematical and computational content vary according to project and taste. We're a friendly group, and Guildford is... an okay place to live.  The campus is pretty, at least.

What can we infer about nuclear matter from giant quadrupole resonances?

The title of this post is the subject of a recent paper, published in Physical Review C by Xavier Roca-Maza and co-workers.  It is an attempt to link what can be observed in actual atomic nuclei with a model fictitious substance called infinite nuclear matter.  The reason anyone might want to do this is twofold.  Firstly, this stuff - infinite nuclear matter - which doesn't quite exist in nature - is easy to calculate.  That might sound a bit weak, but the ability to calculate things at all in nuclear physics is a great boon, and one can characterise nuclear matter in various useful ways - such as how compressible it is.  But still - why calculate things that don't exist at all?  That leads to the other reason for calculating nuclear matter.

Infinite nuclear matter is an approximation to a few different things.  It's like the central part of atomic nuclei - the tiny combinations of protons and neutrons at the centre of atoms, and its also the same stuff that makes up neutron stars.  They interact according to the same rules of physics, as far as we know.  If we can understand the ways that the model infinite matter corresponds to both actual nuclei and actual neutron stars then we might learn something about the way protons and neutrons interact.

One difficulty in doing this is that real nuclei are so small, and are so dominated by the fact that they have surfaces, rather than being bulk matter, means that it's hard to make this link.  Fortunately, there are some things that actual nuclei do that don't rely too much on surface effects.  Prime amongst these are giant resonances.  These are ways in which nuclei vibrate when excited in experiments.  The recent paper discusses so-called isovector giant quadrupole resonances, in which protons and neutrons vibrate in opposite ways to each other with a particular geometry.

In their paper, the researchers systematically tweaked the properties of a model nuclear force and found a marked connection between the vibrational frequency of the giant resonance in lead nuclei, the radii of the proton and neutron matter distribution in the same nucleus, and a nuclear matter quantity called the symmetry energy, which measures the extent to which protons and neutrons like to exist in equal numbers.  They take care to rule out, as far as possible, dependence on the particular model of the nuclear force, and have added a little bit more to the story of understanding what makes protons and neutrons stick together.

Roca-Maza, X., Brenna, M., Agrawal, B., Bortignon, P., Colò, G., Cao, L., Paar, N., & Vretenar, D. (2013). Giant quadrupole resonances in ^{208}Pb, the nuclear symmetry energy, and the neutron skin thickness Physical Review C, 87 (3) DOI: 10.1103/PhysRevC.87.034301