Friday, 28 February 2020

Next papers in our Frontiers In Physics Special Topic

I posted earlier in the month about the first papers to appear in the special topic on time-dependent methods in nuclear physics in Frontiers in Physics that I am co-editing with some colleagues.  Here are a summary of the next four, which have all appeared since then:

Far Off Equilibrium Dynamics in Clusters and Molecules (doi: 10.3389/fphy.2020.00027) by Phuong Mai Dinh, Marc Vincendon, Jordan Heraud, Eric Suraud, and Paul-Gerhard Reinhard, describes techniques and results from non-nuclear systems such as small metal clusters, atoms, and molecules.  The techniques are compared with analagous methods in nuclear physics, and the differences discussed, such as the need to deal with electron emission, and the interaction with laser fields, as well as the different interactions involved in the density functionals.  The plot shown here (from Fig 1 in the paper) shows the response in the dipole moment of a cluster of five sodium atoms to an excitation by a laser pulse.  The excitation and deexcitation is seen over a period of about 20 fs (though I dare say the compitational simulation takes some orders of magnitude to reproduce what nature does in real time).

Predictions of New Neutron-Rich Isotopes at N=126 in the Multinucleon Transfer Reaction 136Xe + 194Ir (doi:10.3389/fphy.2020.00038) by Xiang Jiang and Nan Wang uses the time-dependent Hartree-Fock method to provide a microscopic description of nuclear reactions of the xenon and iridium isotopes mentioned in the title, to look at some non-central reactions in which the reacting nuclei transfer some neutrons form one of the nuclei to the other during their reaction time, and then are left in excited states of another pair of isotopes following the transfer of neutrons.  A semiclassical approach (the GRAZING model) is then used to give a statistical de-excitation of the excited fragments to see what final set of isotopes are likely.  The purpose is to see if this kind of reaction can be used to study as-yet-unseen isotopes in current experimental facilities.  The results are that the proposed reaction should have a cross section large enough to produce new isotopes in presently feasible reactions, for isotopes around N=126 and Z~74.  The figure shows snapshots of the time-dependent part of the simulation of the two reacting xenon and iridium nuclei.
Quasifission Dynamics in Microscopic Theories (doi:10.3389/fphy.2020.00040) by Kyle Godbey and A. S. Umar discusses the microscopic picture of what happens when you try to make the kind of fusion reactions to discover new superheavy nuclei.  The two nuclei you are trying to fuse together might, depending on the exact details of how they impact on each other, might briefly fuse before falling apart again ("fusion-fission") or they might graze each other with the transfer of a few nucleons (neutrons, mainly - the mechanism discussed in the paper above by Xiang Jiang and Nan
Wang) or they might stick together without really fusing to an equilibriated compound nucleus and split apart into two fission fragments from a kind of molecular state.  This is quasifission and is discussed here for the purposes of understanding it as a kind of hindrance to forming new superheavy nuclei.  The authors show how to identify quasifission in mass-angle distribution measurements and map out the landscape of the briefly-made (super)heavy system, as shown in the figure showing how the entrance and exit channels are constituted in terms of the shapes of the combined system.

Fusion Dynamics of Low-Energy Heavy-Ion Collisions for Production of Superheavy Nuclei (doi: 10.3389/fphy.2020.00014) by Xiao Jun Bao is a mini-review surveying the literature for the various components of the mechanisms by which superheavy nuclei are formed, or fail to form (such as by the quasifission process discussed above by Godbey and Umar).  Aside from covering a broad survey of different theoretical methods, the paper highlights the uncertainty in the probability for a reaction to form a compound nucleus - an excited nucleus made from the colliding fragments which has lost the memory of what the intial fragments were.  That uncertainty, then, points to the need for more work in this area.  There is one figure in the paper, which neatly shows the possible reactions mechanisms for two colliding heavy nuclei.

Thursday, 27 February 2020

Acceleration is a vector

I had a meeting in the ATI (Advanced Technology Institute) earlier today, and as I stole along the walkways leading up to the second floor where my meeting was, I noticed the interesting pattern of the wear on the wooden flooring.  Here's a picture taken from the second floor, looking down at the walkway on the first floor.
What strikes me about it is that at the bottom, there is relatively little wear on the wood, while the corner shows considerable wear.  Probably with little exception everyone that walks along the unworn section also walks along the worn section, and each part has been thus been subject to the same footfall.  It is turning the corner, then, that causes the damage to the surface, and not walking in a straigh line.  While there may be a lot more going on in terms of gait factors, it seems to me that this is a nice pictorial example of the fact that force is proportional to acceleration, and acceleration is a vector.  When someone is travelling in a straight line at constant speed they are not acceleration, but to turn a corner, even at constant speed, requires acceleration, and hence a force, which wears away the surface of the walkway.  

While we have an Advanced Technology Institute on campus, we do not yet have a Retarded Technology Institute. 

Tuesday, 25 February 2020

Mahir Hussein memorial workshop

I have just seen advertised a workshop called "7th IEA International workshop on Clustering aspects in nuclei and reactions" to be held in São Paulo in September.  The workshop is being dedicated to Mahir Hussein, who died last year.  

Mahir was a nuclear reaction theorist whose work overlapped with much of the work that the Surrey theory group has specialised in.  He visited the group at least once, and gave jobs to some graduating Surrey PhD students over the years.  Though spending most of his research career in Brazil, he was originally from Baghdad in Iraq, and he graduated from Baghdad University in the 1960s.

Workshop details are here.  I've added it to my 2020 list of conferences and workshops.

Friday, 21 February 2020

Death and serious injury from dark matter

A few years ago, I wrote a blog post about a paper published on the subject of dark matter as a potential cancer-causing agent.  Today, a related paper was published on the potential of "macros" -- dark matter object of macroscopic size (or weight) that could cause death or serious injury in humans, in the form a bullet-like wounds.  The apparent absence of such dark matter deaths is then used to put limits on the actual occurrence of such forms of dark matter.  

Saturday, 8 February 2020

First papers published in our Frontiers special topic

Last year, some colleagues (Lu Guo (郭璐), Denis Lacroix, Cédric Simenel, Nicolas Schunck) and I started organising and co-editing a special topic in the Frontiers in Physics journal on "Advances in Time-dependent Methods for Nuclear Structure and Dynamics".  We hoped to get a fairly broad snapshot of the current active work in the area, and a review of important problems.  I'm pleased to say that the first couple of papers have recently been published:

Collective Inertial Masses in Nuclear Reactions,  Kai Wen and Takashi Nakatsukasa,  Front. Phys. 8, 16 (2020) describes a problem in going from a fully microscopic picture of nuclei (in which you treat each proton and neutron as an individual entity, and keep track of how they are evolving) to a collective picture (in which you characterise the whole nucleus in terms of a few parameters, such as its position, size, deformation).  One can derive collective equations of motion from the more complicated microscopic picture, thus providing a sure footing, but there is always a difficulty of how to deal with the mass in the kinetic energy term.  This paper discusses a particular method to derive these masses from the underlying microscopic theory, and that the results are consistent in the limit of several test cases, as well as showing interesting results for alpha-alpha scattering.

Time-dependent Approaches to Open Quantum Systems, Maasaki Tokeida, and Kouichi Hagino, Front. Phys. 8, 8 (2020) discusses the problem of describing a quantum system that interacts with its environment, or a quantum system which can lose particles - an "open quantum system".  The loss, or dissipation, of energy, or of particles, or information can be a complicated (and complex in the mathematical sense, for that matter) thing to describe, and the authors develop and present two methods to describe systems which lose information to the environment, using a time-dependent approach, and show how each can be applied to the nuclear physics case of a heavy-ion collision.

There are several more papers to come, and I'm looking forward to reading them all, and having them as a useful collection of papers for everyone working in the field.  The picture is from the Wen and Nakatsukasa paper, showing snapshots of a calculation of and alpha+alpha reaction

Friday, 7 February 2020

Hyperfine school

I've been off work sick for a few days with the lurgy.  Ugh.  Ah well, these things happen, and it is winter flu season after all.  It's also exam marking season, and I'm back in at work with lots of marking to do which I had planned to get done earlier in the week.  

I also have an email that I thought I'd share here.  It's from a MOOC (free, online, university-level course) on hyperfine interactions; those interactions between electrons and nuclei that give rise to tiny splittings of atomic energy levels.  From the point of view of nuclear physics, they're useful because they give us a probe of nuclear properties, using theory that is quite well understood (and so relatively model-independent)

It looks very interesting, and great that Ghent University are providing the material, and support, for this course.  I don't know what textbook (if any) they use, but I include a picture of me reading the book I found most useful on the subject as an undergraduate. I paste the whole email from the course organisers below, for those interested in taking part which can be done starting next week in a supported way, or any time, in a totally self-study way:

On 12 February 2020, a new edition of will start. It is a free open online course about the physics of hyperfine interactions and experimental methods based thereupon. You can take this course self-paced at any time of the year, or - if you start on Feb 12 or before - you can take it in sync with students at Ghent University. In the latter case, you can ask questions and get feedback just as these students do.
The course is scheduled for 12 weeks, and requires about 5 hours of work per week. Emphasis is on conceptual understanding, less on mathematical derivations. The level is advanced bachelor or (early) master. A general science background at bachelor level is the expected entry level.
This is the list of topics covered:
the nucleus
the physics framework (classical/quantum)
electric monopole shift
magnetic hyperfine interaction
electric quadrupole interaction
laser spectroscopy
Mössbauer spectroscopy
synchrotron radiation methods
nuclear magnetic resonance (NMR, a.k.a. MRI)
nuclear quadrupole resonance (NQR)
ENDOR (Electron Nuclear DOuble Resonance)
electron paramagnetic resonance (EPR)
low-temperature nuclear orientation (LTNO)
NMR on oriented nuclei (NMR/ON)
perturbed angular correlation spectroscopy (PAC)
Students get every week a set of videos, with associated tasks. Reports are submitted 24 hours before the weekly feedback webinar. During this weekly webinar, common problems encountered in the tasks are discussed, and questions raised by students during the past week are addressed. The webinars are livestreamed and recorded for later use.
If you want to offer this course as a formal course to students at your place, please get in touch and we’ll see how this can be organized.
Feel free to have a look at A short registration is required (temporarily or permanent, as you wish), after which you have access to all material.
You receive this one-time email because you are known to be interested in hyperfine interactions, or because you have been previously registered as a student in this course.
Yours sincerely,
Stefaan Cottenier
Ghent University