Thursday, 22 April 2021

Earth Day and Isotopes

Apollo 8's Earthrise Image (NASA)


Today is Earth Day - one of the highest profile of the special named days that occur throughout the year.  High profile enough, at least, to prompt newspapers to run articles on helping to save the planet.

This is a nuclear physics blog, and I rarely write about the kind of issues Earth Day highlights.  The role of nuclear power as a low-carbon electricity source is probably the most obvious intersection of the nuclear and green worlds, and one that has people more expert than me in energy generation campaigning one way or the other on it.

One lesser-known way that nuclear physics makes an impact on environmental issues is in the use of different nuclear isotopes to help understand natural processes going on on Earth now, and back into the distant past.  

For each element in the periodic table, there are many possible different isotopes.  Each element is characterised by the number, Z, of protons in the nucleus: Z=1 for hydrogen, Z=2 for helium, Z=3 for lithium etc.  But for each element there can also be a different number of neutrons in the nucleus.  Hydrogen, for example, comes in 2 different stable isotopes 1H with 1 proton and 0 neutrons, and 2H with 1 proton and 1 neutron.  

The chemical behaviour is largely the same for each different isotope of an element, because the chemistry is mainly determined by the number of electrons in an atom of the element, and that number matches the number of protons in the nucleus.  But there can be slight differences between isotopes in chemical and other processes.  Isotopes with more neutrons are heavier, and they have shorter bond lengths when making molecules than their lighter counterparts.  These differences are enough to make some physical processes - such as the evaporation of water molecules, or the way nitrogen-rich nutrients are metabolised - happen at slightly different rates depending on the isotopes present.

In the case of water molecules, the rates of evaporation and precipitation of different kinds of heavy water (e.g. with heavy hydrogen and/or heavy oxygen isotopes) happens differently depending on the temperature, and by studying ice cores in Greenland's ice sheets, geophysicists have been able to reconstruct the Earth's temperature back long before scientific instruments began recording temperature.   The nuclear isotope ratios can be used to measure back to around 100,000 years ago - vital information in understanding the development of the Earth's climate to understand where we are today.

In the case of nitrogen isotopes, by looking at the reaction rates of different nitrogen molecues (ammonium, without oxygen, and nitrates, with oxygen), biogeophysicists can see evidence in the fossil record of when the Great Oxidation Event (GOE) took place, in which oxygen was released into the atmosphere and life forms began to develop which made use of it.  This is looking back a bit more than 2 billion years, so back to half the lifetime of the Earth ago.

The existence of isotopes was posited in 1913 by radiochemist Fredrick Soddy, with a range of new isotopes being discovered by mass spectrography in the following years.  Finally in 1932 the existence of the neutron was confirmed and the reason for the existence of different isotpes was understood.   Those involved at the time had no idea, of course, that the work would help us piece together the geohistory of our planet, help us understand our historic climate, and hence help us model how it will develop in future.  Such cases of applications of intitially blue-skies research are the way it goes (so the moral is: fund blue-skies research!)

For more details of the oxygen isotope ratios in ice cores see Frozen Annals by W. Dansgaard.  For the nitrogen isotope ratios going back to far prehistory, see Nitrogen Isotopes in Deep Time by Colin Mettam and Aubrey L. Zerkle

Sunday, 18 April 2021

A couple of new papers: On fission, and nuclear sizes

 I haven't mentioned here about a couple of new papers I have been involved with which have appeared over the last month:

• First is a paper on nuclear fission (here in Physical Review C, here open access arXiv version).  The work was done primarily by a PhD student in Beijing, but I contributed a little with discussions, expertise in the code and interpretation of results.  In it we try to understand what goes on microscopically (at the level of individual neutrons and protons) when fission takes place.  We go beyond some previous work (e.g. that of my previous PhD student here and here). Through random fluctuations we see reproduction of the different final products that appear in the distribution of fission products.

• Next is a paper on the isotope shift across shell gaps (here in Journal of Physics G, here open access arXiv version). This is work done by an extended group of collaborators, and again I contributed discussion, interpretation, suggestion of which calculations to do, with the lead authors doing those calculations.  It also builds on some work I did with the same PhD student as the fission work, published here.  I think the nicest thing about this paper is the showing how the underlying mechanism of the isotope shift (change in radius of nuclei as one adds neutrons) can be described in complementary ways by two somewhat disparate theories which each have their own language and mindset for thinking about nuclear structure.  It is also neat in that the idea of understanding how the size of nuclei change as you add more neutrons is in the (science) news right now thanks to the recent results from NASA's NICER telescope on the properties of neutron stars.

Here's a pretty picture from the fission paper representing how different fission events progress through different paths of shape of the fissioning nucleus:

Tuesday, 13 April 2021

Conference week

 I mentioned earlier that I had a couple of conference coming up, happening in the same week.  This is the week:  There is the joint IoP Astroparticle Physics, High Energy Particle Physics, and Nuclear Physics Groups' conference (website:, Twitter hashtag: #EdiIOP2021) and the YQIS Young Quantum Information Scientists' conference (website:

With the IoP conference being in the UK and the YQIS conference in the US, I could in principle attend the IoP conference and the the YQIS meeting with only a small amount of overlapping time.  In practice, with childcare responsibilities that's not very practical, but everything is being recoreded, and I am trying to make a sensible combined programme of talks that I want to see and then arranging which ones I am able to watch live, and which I will watch after the fact.  This ability to watch pre-recorded lectures is just what many of our students are now finding useful in our taught undergraduate classes. 

Yesterday, when the conferences started, was a Monday, meaning my day for looking after my youngest children, so I wasn't planning on doing much live participation, though it also happened to be the day when my own talk was scheduled at the IoP conference, so I arranged to have the boys looked after for that time and gave the talk.  I perhaps could have strapped the baby to a sling and walked around while giving the talk, but it turned out to be easier to arrange a little time swap with my partner in childcare duties. 

Today, having cycled all the kids to school / nursery, I am able to attend the live sessions, which means that I have started with Jim Hough's talk on gravitational waves (screenshot below).  It's amazing how we have been able to observe so many events of merging black holes over the last few years, coming from a situation not so long ago when black holes were suspected to exist, but not definitively observed, even indirectly.  

I think it's a bit of a shame that the conference is set up using Zoom's webinar mode, in which I can't see who else is in the audience, can't send them a quick message to say hi, or do any of the other 'conferring' that I would do at a conference.  I know there is a formally-arranged coffee break as part of the schedule, but I don't quite get the point of limiting our ability to interact with other attendees.

Here is a snapshot from Prof. Hough's talk.  Right now there is a talk I'd like to listen to about the FAIR laboratory and the work going on / planned there, but the speaker's audio has a strange bass echo that makes it unlistenable to me.