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

The gamification of research

As I drove my daughter to dance class this morning, we listened to a program on Radio 4 called Hardeep's Game of Life.  It was rather interesting (okay, Flora didn't think so and wanted to listen to Radio 3, as usual).  The program was about how turning worthwhile actions that are not much fun into games can help promote them.  For example, if you get some "points" for taking exercise, and you care enough about the points, then you can be motivated to take more exercise.  The whole idea has been dubbed gamification, and discussed how many social networking games rely on the kind of addictive qualities which make gamification help motivate people to do things that they otherwise find it hard to find motivation for.  There was a somewhat amusing example of a game, called Cow Clicker, which was a kind of pastiche of some online games, and involved getting points by clicking on a cow, which you are only allowed to do once every six hours.

The thing that really occurred to me during the program, though, was that things like league tables for Universities and the metrics-driven "key performance indicators" demanded by senior managers at universities have turned much of academic life into a gamified proxy of what it actually should be.  I seem to have taken up the gamification.  After all, I'd like to get promoted, have a nice life, be able to buy a house, things like that... Thanks to my profile on google scholar citations, I can see how "successful" I am at the game of academic research.  It tells me how many citations I am getting - and look; I already have gained a quarter of the citations I had in 2013 as I had in all of 2012 and it's only been two months.  Isn't that great?  Maybe.  I worry I am just trying to win a game, and that somewhere the idea of doing research because it is interesting, correct or worthwhile has been lost.  

By the way, the image attached to this post is from a cellular automaton called the Game of Life, taken from this page at San Diego State University.  It's an interesting topic in its own right.  Another time...