Joking, then to Essex (via Woking)

Last night I took part in Guildford Bright Club.  It's a kind of academic outreach activity in the form of stand-up comedy.  The idea is that university researchers, be they professors who find it hard to keep their eyes open during talks or new PhD students, with eyes blinking in wonder, the Bright Club format provides a place for them to engage with the sort of audience who thinks that going to a comedy night is perhaps more fun than going to a more formal public evening lecture.

I had misgivings about the name Bright Club, as being a bit superior (we're the bright ones), until I realised that it was a play on Fight Club, and my misgivings abated a bit.  It's a fun idea, and the events have a paid-for comedian to compere and some musical accompaniment.

Last night there were six researchers in a set of two halves, with comedy music to round off each half.  I was billed right at the end, and carefully sipped a single beer the whole evening so as not to have an inappropriate kind of Dutch courage that the stupid joke I suddenly though of might be funny.

I think it went okay.  I got some laughs.  I sort of lapsed into a stupid pastiche nerdy physicist voice at the end, which I hand't really meant or wanted to, but all-in-all it went well, I think.  I was allowed a free drink for performing, though was quite disappointed that it didn't extend to a free cocktail.  A shame, since we were in a cocktail bar.  Still, I guess I know my worth now; more than a glass of water but less than a martini.

On the other hand, today, I, without even the reward of an alcopop, went to Theydon Bois, near the far northeastern end of the Central Line to talk to a University of the Third Age (i.e. retired people interested in general intellectual things) group about nuclear physics.  I gave a version of the talk about nuclear applications that i give to general audiences, so was slightly abashed to discover that the core of the group is formed from retired (or semi-retired) scientists from Queen Mary University.  It turned out too that I was in a double bill with a member of the UA1 experiment that discovered the W and Z bosons, who is, as well as an ex-UA1 member, also a U3A member.  When he presented pictures of Feynman diagrams, I wondered if I hadn't pitched my talk a little low.

I was hoping that someone at the U3A meeting would be able to teach me something in exchange for the undoubted words of wisdom that I was giving them.  So I appealed at the end of my talk that if anyone could teach me how to tie a bow tie I would be most grateful, as I was going to a black tie event that evening.  One kind chap came up and showed me him tying it on himself.  He did it quite well, but with a little difficulty, and admitted that he was no expert and would have trouble doing it on me.  I should have stood there and tried, following his instructions over and over, but the call back to answer questions helped me wimp out of this display.

So after the talk, I headed off to the Royal Institution to the Friday Evening Discourse in which Jim Al-Khalili was talking.  Jim was kind enough to invite me along as his guest not only to the talk, but to a drinks reception beforehand.  I tried my usual trick of asking the people at the door if they could help me tie my tie, but they couldn't and I turned up the black tie reception with my black bow tie crossed over in a sort of rakish cravat affair.  It was deemed pretty cool.  The net effect is in the picture for you to see.  I enjoyed Jim's talk, and particularly that he acknowledged me at the end as "Professor" Paul Stevenson, despite the fact that lack that lofty status. I was sitting next to our faculty dean, whom Jim had also invited, and I'm sure he'll slowly get to think of me as a professor, and will be shocked into promotive action when he discovers I'm not.

Anyway, I'm on the train home, as you can see from the picture, and about to arrive at GLD, so I will bid you goodnight

Boundary conditions, marking, music and the prize crossword.

Hello.  Here are a few disjointed things:

I don't exactly intend to turn this blog in to a pastiche of any other famous physics blogs but still, I do want to mention that I sent off the solution to the Saturday Independent crossword today to see if I might win the prize for solving it.  Not that I particularly lack dictionaries, which are the prize, but more for the kudos.  The funny thing about this Saturday's crossword was that it didn't have the usual π rotation symmetry.  This was quite disturbing.  I expect there was a clever reason why not.  Indeed, there was a sort-of anarchic theme to the crossword, which I didn't really understand, so I probably missed something, though I think I got all the clues correct.

I had a nice email from one of my PhD students to point out that we had a paper published today, on boundary conditions in solutions of the time-dependent Schroedinger equation.  I'll try to make a proper post about that soon to explain what it all means.

On Thursday I'll be appearing at Guildford Bright Club doing a stand-up comedy set.  Thanks to a quick run-through yesterday, I know I have plenty of preparation still to do, but I think it'll be pretty good, so come to Bar Des Arts in Guildford this Thursday to check it out if you're free

Finally, I don't exactly intend to turn this blog in to a pastiche of any other famous physics blogs but still, I was listening to Radio 3 while doing some marking earlier, and came across a bizarrely wonderful piece of organ music.  There's a recording of it by the composer, which is a joy to watch, so I attach it here:

I am a Material Girl

When I was at STFC headquarters last week, I was pleased to be recognised there as someone who helps advertise UK advances in nuclear physics.  I was a little crestfallen when asked if I'd seen the UK paper in Physical Review Letters that had come out the day before, and about how it was an exciting story about nuclear astrophysics.   The paper referred to was an excellent paper by the nuclear astrophysics group at York, but I had published a paper in the same journal on the same day, that seemed to have gone unnoticed.  Sob.  Still, go York group.  I post more because I've just listened to the lead author on that York paper, Alison Laird, speak on Radio 4's Material World programme.  Go check it out here!

The size of lead

Overlap of the different quantum orbits of
the extra neutrons in lead isotopes with the
proton orbits

Sorry to have been a bit silent since the new year.  The day after my last post, I went for a week-long holiday, and since I've been back blogging has not quite made the top of the list of things to do.  Still, following mention of a paper I recently wrote with my student and his co-supervisor, I've been meaning to write a bit more about the physics of what the paper contains.  I think the concepts involved are mostly quite simple, and so I hope I can do a reasonable job of explaining what it's about.

Nuclear physics is the study of the really tiny nuclei that are at the centre of every atom, and atoms are what make up all material.  Atoms are small enough that the conclusive evidence that they exist only came around 100 years thanks to advances in physics.  Nuclei, though, are far smaller.  The usual analogy is that if the atom is the size of a cathedral, then a nucleus is sized like a fly buzzing round inside.  Despite the tiny size, though, we can measure the size of a nucleus and see that some nuclei are different sizes to some others.

A nucleus can be charaterised by the number of protons and neutrons it contains, as these two more basic objects are the sole constituents of nuclei.  The number of protons tells you what chemical element the nucleus corresponds to and the number of neutrons tells you what isotope of that chemical element you have (see Elizabeth Williams' excellent recent explanation for more of the basics).

It turns out that if you keep adding neutrons or protons to a nucleus, you expect it to grow in a steady way (quite unlike the way atoms grow when you add electrons), so that a nucleus with more protons and neutrons will be larger than one with fewer with a very regular kind of pattern.  By and large, this is true.  In fact, I've just presented a rather circular argument - it's expected more because it has been observed to be generally true, rather than us having guessed in before-hand.

One particular anomaly in nuclear sizes comes in isotopes of lead nuclei.  All lead nuclei have 82 protons, but can exist with a number of neutrons ranging from around 100 to at least a little over 130.  At least, this range has been seen in experiments, though many of the combinations are short-lived an radioactive.  If you keep adding neutrons to lighter lead isotopes, and measure the size of the proton distribution, which is much easier than measuring the size of the neutron distribution, then you observe that the protons get pulled out a bit by the extra neutrons, and the radius of the proton matter distribution increases, and it does it in a steady way, all the way up to 126 neutrons.  As soon as you go over 126 neutrons the size starts increasing much more rapidly.

Various reasons have been given for why this is so.  Some are in terms of particular nuclear models which either do or don't correctly reproduce this "anomalous kink effect," as it has been termed.  Other reasons use a description based on the fact that quantum mechanics dictates that neutrons and protons have to exist in certain shells or orbits (roughly analogous to planetary orbits) and that different models predict different radii for different shells.  What we have suggested, in our new paper, is that although their might be some truth in the previous explanations, the key factor seems to be not the size of the orbits themselves, but how much the orbits make the extra neutrons interact with the existing protons.  It turns out that tighter orbits, with smaller radii, can generate more of a pull on the protons and make their density distribution larger, than larger orbits that don't interact so much with the protons.

On the level of some of the advanced and complex ways now available to theoretical nuclear physics, it is on the basic side.  Sometimes the simple explanations turn out to be correct.  We at least hope that this sheds some light on the puzzle.

Goddard, P., Stevenson, P., & Rios, A. (2013). Charge Radius Isotope Shift Across the N=126 Shell Gap Physical Review Letters, 110 (3) DOI: 10.1103/PhysRevLett.110.032503

My nuclear year

Well, it's New Year's Eve and I've yet to post a suitably festive seasonal post to Blog of the Isotopes.  This is not because I switch off from blogging mode over the holidays, but more because I foolishly left both my phone and computer behind when I went to stay with my family over the holidays.  There was much jesty mocking at my expense about my ability to withstand such a trial.  I'm back at home now, ferrying clothes back and forth to the campus launderette since my washing machine seems to have packed up,  and I'm currently sitting in my office.  I figure now is a good time for a holiday post, and to review my 2012 as far as things nuclear physics goes.

I've always enjoyed all, or at least most, aspects of the academic life;  both the teaching and the research, and to some extent the administration (I liked being admissions tutor), but this year I tried to make a real effort to push the research side, hopefully not at the expense of anything else.  By and large, I think I've been successful.  If not on a global scale that will get my poached by Harvard then at least it has been a good year as far as I am concerned and my judgement of how far my own abilities will take me.  My list of publications in 2012 include four papers in Physical Review.  One of them is the (joint) second most cited paper in Phys. Rev. C in 2012.  It also led to a trip to Brazil - my first time south of the equator, and to a new collaboration, so that's all to the good.  The two current PhD students for which I am principal supervisor have had joint papers with me accepted for publication, both of which should appear in January, and so I am very pleased for them, as well as me.  I had an enjoyable conference trip to Bulgaria, with a small holiday in Plovdiv attached on the end, and made a visit to my old stomping ground of Oak Ridge, Tennessee, which I always enjoy, being able to catch up with old friends.  That trip was to visit a Surrey MPhys student who was on placement there.  Too bad we aren't sending a student there next year for me to visit.

The biggest news of the year for me, though, was some sad news, that I didn't post about at the time.  My PhD was co-supervised by two people;  Jirina Stone from Oxford, where I was based, and Michael Strayer from Oak Ridge, Tennessee, where I spent about half of my overall time while studying for my PhD.  Like many PhD-supervisor relationships, as with many general research collaborations, the working partnerships are friendships, too, and so it was with Mike.  While I lived in Tennessee I spent a lot of time around his house hanging out, and became friends with his family, too.  After my PhD, he employed me for a year as a postdoc, and was very helpful and sympathetic when I had to partially complete it in the UK following the serious illness of my then-wife.  For the next few years, I would go back and spend time working in Oak Ridge with him, until he moved to a position at the US Department of Energy in Washington.  At that point we more or less lost touch (I am really bad at keeping in touch with people, as school and university friends will surely attest).  It was thus with great sadness that I learned that Mike committed suicide this year.  At the time of his suicide, he was under indictment by the US government for potential misuse of government funds in the publication of a trade magazine for whom his wife worked.  I can only speculate on the mental state that led Mike to that dreadful solution, and how bleak he must have felt.  I feel only sadness that it came to this.  Irrespective of the rights and wrongs of the case against him, there was some irony to me, in a year in which much attention has been paid to the fact that the scientific publishing business has taxpayer-funder scientists writing articles for free for privately-owned journals only to be sold back to the taxpayer, that some part of the scandal involving Mike seemed to involve the fact that scientists were being asked to write for free, but that the staff working for the publisher would benefit financially.  It seemed to me, too, that the explicit mention of the "foreign" publisher of the magazine (being the UK's Institute of Physics) was rather prejudicing things, but I cannot claim to know all the details in this case.  Sorry I didn't keep in touch with you, Mike, and sorry it came to this. 

Thorium and proliferation

I was just reading this interesting comment article in Nature about the dangers of using Thorium to make fuel for nuclear weapons, when I realised it was written by ex-Surrey PhD student Stephen Ashley, who is now a postdoc in the nuclear engineering group in Cambridge.  Good job, Steve!

Given that the article is a comment piece, already designed for general consumption, I don't see much point in me précising it here, but it's an interesting read.  And I thought at first the picture in the article showed someone holding a bowl of corks from wine bottles.  

Hideous Kinky

So, there's this interesting effect going on with the size of lead nuclei as you add more neutrons, whereby the radius suddenly goes up at a much larger rate as you go beyond 126 neutrons.  This is commonly called the "kink effect" in the isotope shift.  People have been calling it that in published papers for years.

I recently wrote a paper with a student and another academic about the reason that this happens.  I think we came across the correct reason, and that it's simultaneously simple and profound.  Thinking those things, I thought it was worth writing up for publication in journal Physical Review Letters, which is the sort of place one sends stuff which should be of general interest to the wider physics community, and is also quite prestigious and a bit difficult to publish in.

The good result is that the referees liked the paper and it will be published there.  The bad news is that our title for the paper, "Why is lead so kinky?" was deemed unsuitable by the journal.  Even before sending it to referees, they asked us to change the title to something which reflected the content of the paper.  In every dictionary I looked in (including the one I bought when I lived in America) the word kinky meant having a kink in the sense of a bend, in exactly the way the radius of lead isotopes does.  Sure, there is also a meaning, usually listed in the dictionary as informal or slang, in which kinky means having an outré sexual taste, but obviously we didn't mean that.

In the end, we had little choice but to change the title, but it seems a shame.  The title seemed entirely appropriate to me, and we were answering the question posed.  On the other hand, I don't want to complain too bitterly;  after all, the paper was accepted, and the science was considered sound.  Too bad that the American Physical Society are in favour of bowdlerising.  Except when they aren't...