Tuesday, 11 June 2019

Advances in Time-Dependent Methods for Nuclear Structure and Dynamics

Together with some co-editors, I've started up a Research Topic in the journal Frontiers in Physics.  It's called Advances in Time-Dependent Methods for Nuclear Structure and Dynamics, which I hope is a reasonably self-descriptive title for the kinds of articles we are looking for for the special issue.

There's a little picture of me on the topic web page among the list of editors.  Fame at last.  If you want to submit an original research article, a review, or a perspective piece on future research areas, please get in touch.  We (the other editors and me) hope it will be a thorough snapshot of time-dependent methods being used today in nuclear physics research.

Saturday, 8 June 2019

Student placement in Beijing

 Marko, at the entrance to the nuclear physics building
I'm heading home from my last trip of the season to visit students on their MPhys Research Year placement.  This time it's been to Beijing where we have a student, Marko, working on some calculations of nuclear fission.  He is being hosted by Peking University, working in the group of Junchen Pei.  Usually on these trips, I am visiting a student working in an area not exactly the same as my own.  In this case, Marko, together with Pei, is building on some work that I did with a PhD student a few years ago, and it's exciting for me to see this work being continued.

I now have a relatively quiet summer (travel-wise, at least).  According to the rules for the MPhys Research Year, our students get visited twice by a member of academic staff, and I will make a second round of trips in September.

Thursday, 23 May 2019

Visiting students in Canberra

This year, we (University of Surrey Physics Department) have sent two of our undergraduate masters students out to the Australian National University in Canberra for a paid work placement in the nuclear physics department.  In the last few days I've been visiting them to find out what they've been up to, to check on progress, conduct an assessment and to make sure everything is generally okay.

The two students are working on reaction mechanisms, trying to understand the process of fusion better, particularly as fusion processes compete with reaction mechanism that lead to the combined nucleus, formed when two nuclei react, not sticking together.  This is especially important in understanding the reactions that lead to making new superheavy elements, where the competing processes dominate.

I'm happy to report that the students are enjoying life in Canberra and at ANU in particular, and I've enjoyed learning about the work they are doing.  I should be back for a second visit in September, which will coincide with a workshop taking place here (HIAS 2019).

Last night, as part of the visit, I took the students and two of the supervisory team out to dinner.   From left to right in the picture we are Prof Mahananda Dasgupta, Dr Ed Simpson, Stefan Parker-Steele, Wiktoria Wojtaczka, and me, Dr Paul Stevenson.

Tuesday, 21 May 2019

Re-writing nuclear physics textbooks

Following a Summer School in Pisa in 2017 with the ambitious title "Re-writing Nuclear Physics textbooks: Basic Nuclear Interactions and Their Link to Nuclear Processes in the Cosmos and on Earth" the notes from the lecturers have been written up and published in a special collection (a "focus point", so they call it) across several issues of the EPJplus journal.  They are freely available until 20th July 2019 here.  Seems like a useful resource to point students at and, indeed, to read oneself.

Monday, 20 May 2019

The early-universe lithium-beryllium wars

There's a spat playing out on the nucl-ex (nuclear experimental) part of the arXiv preprint server.  I have no special knowledge of what's going on, but the timeline of the papers on arXiv is:

1. Moshe Gai of UConn sent a paper to the arXiv "The Interaction of Neutrons With 7Be: Lack of Standard Nuclear Physics Solution to the "Primordial 7Li Problem"."  The paper describes an experiment to look at the capture of neutrons by beryllium–7 nuclei at energies similar to the conditions of the early universe, in order to try to understand if this reaction can account for the disagreement between the observed amount of lithium–7 in the universe compared to models of the big bang in which the lightest few elements were formed.  The paper appeared on 24th Dec 2018 and in the submission comments, Prof Gai says
Talk presented on behalf of the SARAF US-Israel, Switzerland Collaboration at Nuclear Physics in Astrophysics (NPA8), 18-23 June 2017, Catania, Italy, that was reviewed by two referees and accepted for publication in NPA6, EPJA, 2017
So it appears to be a paper written for and accepted for a conference proceeding, but that did not appear in the proceeding.  The preprint gives results from the experiment in a summarised list, promising that a full paper is planned for publication.

2. On 3rd April 2019 Dorothea Schumann submitted at "Comment to "The Interaction of Neutrons With 7Be: Lack of Standard Nuclear Physics Solution to the Primordial 7Li Problem", published by M. Gai in arXiv 1812.09914v1".  Dr Schumann is one of the authors listed as being in the collaboration on behalf of which the original Gai paper was claimed to be written and the comment is also coauthored by 5 other members of the original collaboration list.  The comment says that "The Hebrew University PI of this collaboration has dissociated himself together with his team from this experiment and from the collaboration in Fall 2016" though which members of the list given by Gai are from the Hebrew University is not obvious.

Moreover, the comment goes on to say that Gai had no permission from the collaboration to publish the experimental data, and that the cross sections given by Gai as deduced from the experimental data are unreliable for reasons that Dr Schumann will provide "on request".   The comment finished with the strongly worded statement
We  consider  unauthorized  release  of  questionable  data  an  intolerable  damage  of  the  scientific reputation  of  the  collaborators  personally,  the  involved  research  institutions  in  general  and  the trustworthiness of published data in the entire scientific field. We fully dissociate ourselves from any scientific content published by M. Gai on the project “The Interaction of Neutrons With 7Be: Lack of Standard Nuclear Physics Solution to the Primordial 7Li Problem" and request the paper currently posted on arXiv immediately to be retracted.
3. Today (why I am now noticing the conversation) Prof. Gai has posted, on the arXiv, "Gai Reply to Comment by Schumann et al. [arXiv:1904.03023]".  This is quite lengthy, but the gist of it is given in the abstract:
Statements included   in   the   comment   published   by   Schumann et al.(arXiv:1904.03023) are contradicted by documents that were communicated to one of the co-authors of the comment (Dr. Koester). These documents are reviewed but cannot be disclosed here due to copyright (they are available on request). A summary of the scientific dispute between the collaboration and Dr. Schumann,was submitted on September 24, 2018, to the Directorate Support of the Paul Scherrer Institute (PSI) and can be provided on request.
Here the disagreement is characterised as being between Schumann and "the collaboration".  Gai goes on to elaborate on his side of the story, though clearly there is more information that the authors have not disclosed (and I have not taken them up on their request, since I don't imagine that I am exactly going to arbitrate on this), but it's rather unfortunate that it is playing out in the arXiv.

I suppose next that either there will be a response on the arXiv from Dr. Schumann or another co-author, or perhaps better, that the institutes involved will convene a panel, if they haven't already, to adjudicate.

[N.B. I've used the titles Prof for Gai as per his institution' website, and Dr for Schumann as for hers.  Apologies to either if that is an incorrect title.]

Friday, 17 May 2019

Well hello neptunium-220

The discovery has been announced, in Physical Review Letters (here, but paywalled) of a new isotope of element number 93, Neptunium (Np).  It's Np–220, with 93 protons and 127 neutrons.

This isotope is a long way from the nearest stable isotope, as can be seen on this section of the nuclear chart, with proton number Z increasing along the vertical axis and neutron number N along the horizontal axis:

Np–220 is at the top left of the chart.  The stable isotopes are the black ones, with the nearest either involving losing many protons to go towards bismuth and lead, or to gain many neutrons to get towards the stable uranium isotopes.

It's possible to make these very far-from-stability isotopes by reacting together two lighter nuclei, in which the stable isotopes tend to have a N:Z ratio which is close to that of Np–220.  The actual reaction used was to fuse argon–40 with rhenium–185, making a very excited Np–225 nucleus, and then looking for decays in which 5 neutrons are emitted.

The experiments were performed in Lanzhou, China, and the resulting observation of alpha decay of Np–220 led to the conclusion that the extra stability conferred by the N=126 magic number survives into this far-from-stability region.

Thanks, as ever, to Ed Simpson, for providing the #1 online chart of the isotopes at http://people.physics.anu.edu.au/~ecs103/chart/index.php

Wednesday, 24 April 2019

My first video abstract

With my collaborator, Yoritaka Iwata, I have just published a paper in the New Journal Of Physics.  The paper is about conditions under which two nuclei can react in such a way that the final state of the reaction looks just like the initial state, despite a strong interaction taking place between all the nucleons involved.  If we're right, then our calculations might have implications for inert parts of stars' cores, or one day in nuclear fusion reaction.

This is the first time I've published in the New Journal of Physics.  They offer the possibility of having a video abstract with our paper, so I opened, probably for the first time, the iMovie software on my computer, made some animations, wrote a script and put it all together.  The final result is on the journal website.

Wednesday, 17 April 2019

Peter Butler FRS

The Royal Society has announced its newly-elected set of Fellows. There is a nuclear physicist amongst the cohort:  Prof. Peter Butler from the University of Liverpool has been made a fellow for his work on experimental nuclear physics.  In particular, they cite his work in reflection-asymmetric nuclei, and the leadership of prorgrammes at laboratories around the world such as at Jyväskylä and CERN.

Congratulations, Peter!

Thursday, 4 April 2019

Mathpix is pretty neat

Here's a neat piece of software I spotted via a re–tweet from @eddedmondson on Twitter:
It's called Mathpix and it lets you take screen captures of equations which it then turns into LaTeX code.  I thought I'd try it out with something quite stretching in the form of a slightly poorly scanned pdf of Max Planck's paper from 1900 Ueber das Gesetz der Energieverteilung im Normalspectrum.  Here is a screencap of the original equation:

As you can see, it's not perfectly scanned in, especially in the horizontal lines of the fractions.  Mathpix, though, did a fine job.  It said "We had trouble reading that.  Try zooming in for a better result." but the result was exactly right.  Its LaTeX result is

$E=\frac{8 \pi c h}{\lambda^{5}} \cdot \frac{1}{e^{\frac{c h}{k \lambda \vartheta}}-1}$

and its own graphical rendering comes out as

So pretty good really.

Wednesday, 3 April 2019

Welcome to O-11

The discovery of a new isotope was announced last week in Physical Review Letters (paper here, but it seems no open-access version exists, even on the arXiv).  Oxygen–11, aka 11O, has 8 protons (because it's oxygen) and 3 neutrons (to give it overall mass number 11).  That's a pretty extreme form of Oxygen, whose lightest stable isotope has 8 protons and 8 neutrons.

To make it, the experimenters from the NSCL (National Superconducting Cyclotron Laboratory at Michigan State University) started from a beam of stable oxygen–16 nuclei which they collided on a beryllium–9 (4 protons, 5 neutrons) target.  This bombarding produced a range of nuclei lighter than the oxygen–16 beam from which a magnetic separator was used to focus the oxygen–13 component of the debris into a secondary beam.  This was then sent to another beryllium–9 target.  Some of the reactions between the oxygen–13 and beryllium–9 nuclei caused two neutrons to be knocked out of the oxygen–13 to give oxygen–11.  Oxygen–11 quickly decays to carbon–9 (6 protons, 3 neutrons) and two protons.  These decay products were detected;  their coincident detection and the ability to reconstruct from the detection the properties of the parent oxygen–11 nuclei, repeated through thousands of events enabled the research team to confirm that indeed oxygen–11 had been produced.

Oxygen–11 is so unstable that it decays not by one of the three traditional radioactive decay mechanisms (alpha, beta, or gamma) but by losing two of its protons, and hence undergoing "two proton radioactivity".  This puts it on the borderline of actually existing as a nucleus at all.  The nucleus is so short-lived and unstable that it does not have a well-defined mass (or equivalently its ground state is not a stationary state of the nuclear Hamiltonian).  The experimental values of the observed mass of 11O is seen to have a spread of values ranging over about 3 MeV/c2.

The picture above is taken from Ed Simpson's excellent Colourful Nuclear Chart.  It currently has a blank space where oxygen–11 will no doubt soon go, just above nitrogen–10, and to the left of oxygen–12.  The black tramlines in the plot show the so–called magic numbers which are numbers of protons and/or neutrons which confer extra stability to the nucleus.  These cross each other at oxygen–10.  I doubt if oxygen–10 will really turn out to be noticeably stable compared to its neighbours.  There is already enough evidence that the magic numbers don't apply in light nuclei so far from stability.  I wonder if the odd stray oxygen–10 nucleus was made in the experiment at NSCL, in too little a quantity to get a good measurement of.

Tuesday, 2 April 2019

Nuclear Physics Meetings in 2020

 Bormio: Location of meeting in Jan 2020
Here is a post, which I will add to over the next 18 months or so, listing (mostly low-energy) nuclear physics meetings taking place in 2020 that I hear about:

20/01–24/01: 58th Winter Meeting on Nuclear Physics, Bormio, Italy
It's blurb calls it a long-standing conference, and indeed at #58, it may hold a record for the most-held nuclear physics meeting.  The remit is very broad, including what might once have been called nuclear physics, but is now particle physics.  It is preceded by a one-day pre-conference school for students, covering the basics of the main physics areas dealt with in the conference.

Bormio is in the Italian Alps, and I understand that there is ample time in the programme for leisure activities, such as skiing.

24/02–28/02: Conference on Neutrino and Nuclear Physics (CNNP), Cape Town, South Africa
A conference for those working on the interaction between neutrinos and nuclei, whether it be for beta decay, reactor neutrino studies, dark matter searches, solar and supernova modelling and detector technologies.  Or anything else closely related.  This is the followup to a first CNNP meeting held in Catania in 2017 [website]

01/06–05/06: Nuclear Photonics 2020, Kurashiki, Japan
A conference on the emerging field of direct interaction and manipulation of nuclei with photons, coming about thanks to the new experimental sources of high-intensity lasers and monochromatic gamma rays.  Not sure there is a website yet.  Here is the first circular.

14/06–19/06: ARIS 2020, Avignon, France
ARIS stands for Advances on Radioactive Isotope Science (sic), and the conference series grew out of a combination of the earlier ENAM (Exotic Nuclei and Atomic Masses, I think) and RNB (Radioactive Nuclear Beams) conferences.  It's quite a general, large conference for work coming out of radioactive beam facilities -- i.e. most of the big nuclear physics labs.  At the time of writing they've sent out a save the date email, but there is not a website to link to yet.

Thursday, 28 March 2019

Visiting a student at RIKEN

Part of my trip to Japan involves visiting a Surrey undergraduate MPhys student who is on his Research Year placement at the Radioactive Ion Beam Facility at RIKEN, just outside Tokyo.

The student, Richard, is working on the Rare-RI Ring[link to open access paper on the apparatus], a storage ring in which very unstable neutron-rich isotopes are injected, following their creation at the production target.  Here, by measuring their cyclotron frequency, their masses can be deduced.  Ultimately, good knowledge of these masses is important to understand the r-process of nucleosynthesis which takes place in (probably) colliding neutron stars and certain kinds of supernova.  Richard's task it to work on a replacement for one of the detectors which detects the precise location of the ion beam for one which perturbs the flight of the particle less as it makes the measurement, and hence increases the accuracy.

Here in the picture is Richard standing by the detector, where he is currently spending his days making measurements to characterise how it is working.

As well as his work, Richard has been enjoying getting to know Tokyo at the weekends.

Tuesday, 26 March 2019

JAEA Symposium at Tokai

I am in Japan this week, first at the 54th ASRC International Workshop "Nuclear Fission and Structure of Exotic Nuclei" which is partly based on the fact that the researchers here at JAEA (Japan Atomic Energy Agency) have a target made of Einsteinium-254.

I've given a talk on calculations to see what prospect there is for making the next unknown elements in the periodic table, and I've been interested to see that others are doing similar things.

This is my first time at JAEA in Tokai, and I was pleased that I managed to navigate here from Haneda Airport on Monday morning, though I waited at the wrong platform at Shinagawa station for a connecting train for a bit until I realised that I was probably at the wrong platform.

After this workshop ends today, I'm going to RIKEN, in Saitama prefecture just outside Tokyo, to visit an MPhys student from Surrey who is on placement there, and then home on Friday.

As ever, being at a conference has filled me with ideas and plans, and led to discussions for collaborations.  Not to see how much of that I can bring to fruition.

Thursday, 14 March 2019

Congratulations Katie Ley

Some congratulations are in order for The University of Surrey's Katie Ley, who won Gold Medal in Physics in the STEM for Britain Parliamentary Showcase this week:

Katie is a PhD student at Surrey, working in applied radiation physics.  Here is a recent paper of hers on thermoluminescent dosimetry.  It's behind a paywall, unfortunately, though at some point a freely-dowloadable version should appear on our university repository, and you should be able to find that from the journal website with Unpaywall.

Friday, 8 March 2019

Daphne Jackson at Surrey

What better day than International Women's Day 2019 to write a post about a nuclear physicist who was in the Physics Department here in Surrey some time before me, Daphne Jackson.

Daphne Jackson was the first female professor of Physics in the UK, here at Surrey — appointed in 1971, at the age of 34.  Initially she worked on the local Surrey specialty of nuclear reaction theory, before applying the theoretical work to nuclear medicine, in diagnosis and treatment.

She was a strong promoter of women in physics, particularly supporting those who had had a career break for family reasons from which the route back in to academic research used to be practically impossible (and is still very difficult).  In 1985 she started a fellowship scheme to enable women to return to research careers.  The scheme was continued after her untimely death in 1991 by the Daphne Jackson Trust, which continues today to help returners (both male and female) to academic research careers.

I attach to this post a couple of pictures I took this morning.  One is of a picture of Prof Jackson, which is hanging up in our Jackson Room.  I took the picture from the side to avoid reflections, hence the strange choice of angle.  The other picture is of a monograph that she wrote with Roger Barrett on fundamental nuclear physics in the mid 70s.  It's still referred to today:  Here is Google Scholar's take on citations in 2018-2019.

Wednesday, 6 March 2019

The zirconium-88 neutron caputre cross-section mystery

 Figure: The neutron capture cross section of measured nuclei.  The new measurement of zirconium-88 is the second-highest point.  From Shusterman et al., Nature 565, 328 (2019) doi: 10.1038/s41586-018-0838-z
Imagine you are a neutron,  newly–released from a fissioning nucleus in a reactor.  Your new–found freedom involves flying around in an environment full of other nuclei.  If you crash into one, it might grab you and keep you, converting the nucleus to the next-heaviest isotope.

Fortunately for you, you can see all these nuclei and they all look very small, and easy to avoid crashing into.  Except there are a few sneaky ones which can grab you as you fly past. According to a new paper published in Nature by Shusterman et al., zirconium–88 is one such nucleus, for reasons that are not at all clear.

The protons and neutrons in 88Zr define a nuclear density which extends up to a radius of a little over 5 fm (= 5 x 10–15m) which amounts to a cross–sectional area of ≈ π x (5 x 10–15)2 = 2 x 10–29 m2.  According to Shusterman et al., a 88Zr nucleus presents an effective cross–sectional area of  9 x 10-23 m2. That's around a million times larger than the extent of the matter that makes up the nucleus.  The effective radius of 88Zr for neutron capture is around 5 x 10–12m.  This is around the same radius as the innermost electrons circling round a zirconium atom.

So a neutron only needs to go in the vicinity of zirconium–88 for it to get captured.  This result is a surprise.  The predictions previous to the experimental work gave a predicted cross section around 4 orders of magnitude smaller.

The reason why 88Zr appears so large to neutrons is a mystery, then.  Presumably there is a strong resonance which makes the nucleus really likely to accept a neutron coming into it with just the right energy (the measurement is integrated over a range of energies).  But then, all nuclei have resonances like this.  Why is it so strong in 88Zr?  We don't know.

This result has consequences in modelling of stellar nucleosynthesis, where the absorption of neutrons is one of the key processes happening in novae and supernovae.  It also means that significant quantities of 88Zr in nuclear reactors — created during the fission process, for example — would be a reactor "poison", sucking up the slow neutrons which are the basis of the nuclear chain reaction.

A plot from the paper is shown here.  The large value for zirconium–88 is the second-largest measured ever.  The largest cross-section, in xenon–135 was measured in 1948.

Friday, 1 March 2019

Quantum Textbook Shelfie

Following a Twitter post from my colleague Jim Al–Khalili yesterday,
and a subsequent round of discussion of the books and which ones we had on our shelves, here is my shelfie of the quantum mechanics textbooks I have on the shelves in my office:

Physics textbooks are the sort of thing I coveted as an Undergraduate but couldn't afford, and now can afford but don't have, or don't make the time, to read.  I've more or less stopped acquiring physics books, though on the occasions that I go to second hand bookstores in college towns, I do sometimes fail to resist the temptation to buy one or two.

Of all the books in the picture here, it's the yellow one whose spine is hard to read – An Introduction to Quantum Physics by A. P. French & E. F. Taylor – that I've had the longest.  My brother bought it for me while I was still a sixth-form school student, who was hoping to go to university for further study (which I duly did).  It was a wonderful and thoughtful present, and I attempted to read it, though I'm not sure I really got very far.  Looking back, I recall what a struggle it was to learn most of undergraduate physics.  Now I can pick up an unseen area of UG physics relatively easily, despite a generally diminished brainpower.  Thinking like a physicist has, I suppose, seeped so far into my unconscious.

My copy of Pauling & Wilson's Introduction to Quantum Mechanics is a rare one that I saved up for and bought when an undergraduate student (most of the rest I bought later, second hand). Perhaps Pauling and Wilson is not the most suitable for a late-20th-century student, with its presentation unchanged since it was written in 1935. Shortly after I bought my copy, Pauling died, and I have a cut-out of his obituary from the Independent from 22nd August 1994, in the summer before my final year of undergraduate study.

Here's the obituary.  If you click on the picture it should come up in high enough resolution to read.

Thursday, 28 February 2019

Learning matplotlib

I've been doing some calculations today on the structure of some heavy transactinide nuclei.  This has involved running a code (ev8, from W. Ryssens et al, Comput. Phys. Commun. 187 175 (2015)) and manipulating the output data, which I would like to plot in the kind of sector plot that nuclear physicists use when plotting the potential energy surfaces of nuclei using Hill–Wheeler coordinates.

Here's an example of such a plot, which I've taken from the arxiv paper 1809.04406 (also by W. Ryssens et al., though the al is slightly different to the first paper above):

I have all the necessary data and have mapped it to the right sort of coordinates, but I am struggling to get matplotlib to encase it in the right circular sector.  So far, I've got it looking like this:

Any suggestions on how to mask this into a sector, would be most welcome.  I'd quite like to add a labelled axis along the gamma=60º line, too, if you have any ideas.  And add in those grey axis lines.  I can handle changing all the colours and font sizes that I need to do.

I should probably be posting this to stackexchange, right?

edit: This blog post might prove helpful, so I link to it for my future reference: http://blog.rtwilson.com/producing-polar-contour-plots-with-matplotlib/

Monday, 18 February 2019

Google Scholar has again gone through a process of forgetting or purging a number of citations to my papers.  This time last week it said I had 1,589 citations to my work.  Today, 1,170 (both figures down from 2,130 in January last year).  Google Scholar is fairly opaque in how it does things, and I don't know what records it deleted and why, but it seems unfortunately very unreliable.   As a record of my papers, it's useful.  For citations?  It's becoming increasingly less so.  Anyone else noticed the same with them?

After noticing (some years ago) strangely fluctuating numbers coming from Google Scholar I started periodically recording the number of citations it reported for me on a spreadsheet.  Hence I can present the graph attached here.  The latest downward line is smaller than the one I noticed mid-way through last year, but still takes the number of citations back to that of around 5 years ago!

Thursday, 14 February 2019

Nuclear Spot The Difference #6 revisited

Regular readers of this blog will no doubt remember a previous spot-the-difference in which the understandable confusion between Art Malik and Jim Al-Khalili was explored.  With the recent acquisition of a beard, it has been claimed that nuclear physicist Jim now looks like comedian Alexei Sayle.  I present the following pictures to allow the reader to judge

 Sayle Al-Khalili

Here is Jim from his pre-nuclear days, singing "Ullo John, got a new motor?"

Monday, 11 February 2019

Relativity makes mercury liquid

The link between the theory of General Relativity and the orbit of the planet Mercury is pretty well-know.  What I learned recently is that it is a consequence of Special Relativity that the element mercury is liquid at room temperature.

In a paper by Krista G. Steenbergen, Elke Pahl, and Peter Schwerdtfeger (J. Phys. Chem. Lett. 8, 1407 (2018)) the authors perform calculations of bulk mercury using both relativistic and non-relativistic quantum mechanics, and show that the use of relativity lowers the melting point of mercury by 160 K.  The relativistic calculations give a melting point close to nature (within 10K).  They are sophisticated quantum mechanical calculations in which bulk many-body effects are taken into account.  You can't simulate melting and freezing by considering the interaction of two or three atoms, but need at least tens of them to get a realistic picture of bulk matter.  That's not easy to calculate, and the authors have made a convincing case that they have done so, and that the dominant effect in causing mercury's unusually low melting point comes from relativity.

The picture below is from the paper, showing the result of a simulation with a disordered liquid phase on the left, becoming an ordered solid on the right.

Thursday, 31 January 2019

Farewell to Kai

Last night I made a rare trip to the pub (okay, the same pub I went to last Saturday for dinner) where the Surrey nuclear theory group said goodbye to our postdoc Kai Wen, who is off to pastures new (Tsukuba University in Japan).  I took what young people call a "selfie", so I take the opportunity to post it here.  Best wishes for the future, Kai!

 L–R: me, my son Hamish, Kai, Pierre, Mehdi, Arnau

Tuesday, 29 January 2019

Raj K Gupta (1938–2019)

I read from various friends and colleagues on Facebook that Prof Raj K Gupta of Panjab University, Chandigarrh, has died.  I have read several touching stories about him on Facebook from physicists who knew him and worked closely with him over the years, and it is clear that he had a strong and positive impact on many younger physicists across India.

I worked with him (long-distance) via a collaborator, Suresh Patra, who came to Surrey for several month to work on giant resonances with me.  I then got involved a bit in a project on clustering and cluster radioactivity that Suresh worked on with Prof Gupta, it being the latter's specialty.  In the end, I published a few papers as co-author with Prof Gupta, as part of this collaboration, though I did not meet him until after this time, during a conference visit to New Delhi.  My research interests have evolved since that time, partly into heavy-ion fusion reactions, where I see his name regularly in publications about ioi-ion potentials in which he developed some important results.  Judging by those Facebook posts his scientific and personal legacy will both continue after his death.

Thursday, 24 January 2019

Macramé periodic table

I took a brief break from marking exams and dissertations today to go to Senate House to see the macramé periodic table made by artist Jane Stewart.  Very impressive, and must have been quite a labour to make it, and a careful one, at that.  It's there until tomorrow afternoon, then off on its world tour, so maybe you will be able to catch it at a science festival or its tour of universities.

Monday, 14 January 2019

del-squared-cactus

A quick post today as I take a break in marking computing assignments.

I saw on Twitter a post about not recognising Greek letters in physics equations:

and a response from someone who teaches physics

Looking through many of the responses, it seems that it is a common problem among physics students to not have learnt or be able to pick up from context the names of the symbols used in physics equations if they are Greek letters (or other unfamiliar symbols presumably).

I think I do try to teach the names of the symbols I use when I use them, but maybe I don't.  Maybe I just say them out loud as I read or write them down and hope that's enough, but it seems from the Twitter responses above that it's probably not.  Would be interested to hear anyone's experiences on the matter.

Thursday, 10 January 2019

Pear-shaped fission

Over the Christmas break I noticed that a paper appeared in Nature giving an explanation of why fission fragments tend to favour particular daughter nuclei over others which might naively be expected to dominate.

When a heavy nucleus, like Plutonium–240, with 94 protons and 146 neutrons, fissions, it splits up into two lighter nuclei.  Different daughter nuclei can be produced in different fission events, but there is a distribution which centres on a light fragment with around 54 protons and 85 neutrons, and a heavier one taking the rest of the nucleons.  The reasons why particular daughter products are favoured are to do with how the slow process of the parent nucleus stretching before forming two nascent fragments and finally splitting is energetically possible.  It has been a bit of a puzzle why ~54 protons should be more favoured than 50.  50 is a "magic number" for protons;  an especially stable configuration which you might expect to be a preferred end product when looking at energetically possible outcomes.

The paper by Scamps and Simenel calculate that the key reason that 50 is bypassed is that octupole shapes (pear shapes) are favoured at around proton number 54, and these are the gateway shapes formed as the nucleus fissions and the neck of the fissioning nucleus splits to give the narrow end of the "pear" in the daughter nuclei.  This effect is more important than the final stability of a 50–proton nucleus.

The paper is in Nature, which is behind a firewall, so I'm not sure that it is available to a general audience.  With the unpaywall browser plugin, I see that there is a "bronze open access" version available here.  The first description I find when searching of what bronze open access means is that it is an potentially fleeting open access status without the backing of a perpetual license that some publishers are implementing.  This somewhat critical description is something I found in ... Nature.

The kind of calculations performed in this paper are very close to what I do, so I'm (a) pleased to see that it gets published in Nature (b) pleased they cite work done by my PhD student a few years ago and (c) annoyed that I didn't followup my student's work after he left to work for Sainsbury's by making this same analysis that got published in Nature.   The attached picture is a snapshot from a movie they provided in the article as supplementary material.

Guillaume Scamps and Cédric Simenel (2018). Impact of pear-shaped fission fragments on mass-asymmetric fission in actinides Nature, 564 : 10.1038/s41586-018-0780-0

Thursday, 3 January 2019

Two new isotopes: Boron-20 and Boron-21

I noticed a new paper appear on the arXiv this morning, announcing the discovery of two new isotopes;  boron–20 and boron–21.  Boron has atomic number 5, so each boron nucleus has 5 protons.  There are two stable isotopes; boron-10 with 5 neutron and boron-11 with 6 neutron.  Boron–20 and –21 have 15 and 16 neutrons respectively.  That large asymmetry between the number of neutron and protons makes the isotopes unstable.  They are so unstable that the last neutrons do not even stick on to the nucleus for long and they decay by emitting one or two neutrons.  The live long enough to be identified as a resonance state in experiment, which is exactly what happened in the experiment that led to their discovery.

 Figure from arXiv:1901.00455
The experiment took place at the Radioactive Isotope Beam Facility (RIBF) at RIKEN Nishina Center, in Japan.  A beam of calcium–48 ions was fired at a beryllium target at very high energy, causing some of calcium nuclei to have a number of their protons and neutrons ripped off to give lighter isotopes of various sorts.  From this cocktail of fragments, the experimenters extracted nitrogen–22 and carbon–22 which were then directed onto a second target (of carbon) where some reactions knocked protons out of the nitrogen–22 and carbon–22 nuclei to form the previously-unknown boron isotopes. The snapshot taken from the paper in the arXiv shows the reactions taking place.  The plot is a section of the Segrè chart in which isotopes are shown with increasing proton number in the y-direction and increasing neutron number in the x-direction.  The arrows show the proton knockout reactions leading to the boron isotopes.  The red line marks out the neutron drip-line, separating those nuclei which are stable with respect to losing a neutron, and those which are not.

Though the paper just appeared on the arXiv today, it was published in Physical Review Letters on 27th December.  Odd to put it on the arXiv only after it has been published elsewhere, but at least it means it reaches a wider audience (including me!), albeit belatedly.  Unfortunately the arXiv is not as widely adopted in nuclear physics as astro or high energy physics.

Tuesday, 1 January 2019

PhD in theoretical nuclear physics -> fitness trainer

One thing that some students ask when it comes to signing up for PhD study is what is the job market like afterwards?  One thing is for sure – a PhD does not mean that the only thing you can do is to go further and deeper into academia in the specialty area of your PhD research.  Here's one example of a previous PhD student of mine.  After her PhD in theoeretical nuclear physics (leading to publications here, here, here, here, here, here, here, here, and here) Emma went to work on climate science, where she devloped a successful career.  Most recently she has started a new business as a fitness trainer for children.  Here's a video featuring her for her company Kidz Impact.  I note that the link to nuclear physics is still present: "Kidz Impact and Teen Impact are trading names of Strong Force Limited"