A Conversation with Professor Arthur Champagne about Nuclear Physics
By Joshua Grubbs, published in our 2016-17 issue
June 2015, Professor Champagne became the director of the Triangle Universities Nuclear Laboratories (TUNL), but his work with the lab goes back many years. After joining the Physics Department at UNC - Chapel Hill in 1990, he has led many projects in collaboration with TUNL, including notably within the Laboratory for Experimental Nuclear Physics. His former work comprises teaching as a professor at Princeton University and serving as the Chair of the Physics Department at UNC - Chapel Hill. In an interview with Vertices, Professor Champagne discusses recent developments in the field of nuclear physics and how they relate to his work at TUNL.
How has the field of nuclear physics changed in the past decade?
“Every 8 years or so the field comes together to draft a Long Range Plan and one thing that I notice in looking at the 2015 plan vs. 2007 is that the earlier plan asked many detailed questions while the new one asks a few overarching ones. So one difference is that we’re beginning to see the bigger picture and this is being driven by advances in technology. We can make beams of radioactive nuclei that allow us to make more exotic combinations of protons and neutrons. This allows us to pull out some of the subtleties of the strong interaction as it shows up in nuclei (as opposed to say a proton or a neutron). It also allows us to study stellar explosions in more detail - supernovae produce a large variety of short-lived nuclei and by studying them, we can study the details of the explosion. There’s a new national facility (the Facility for Rare Isotope Beams or FRIB) that will greatly extend the reach of experiments with short-lived nuclei.
“In the area of QCD, experiments using electrons can provide a lot of detailed information because the electromagnetic interaction is well understood. Also, the higher the energy, the more detail you can see within the proton. The accelerator at the Thomas Jefferson Laboratory has just been upgraded to double its energy and there are a suite of new detectors that are starting to come online.
“In symmetries and neutrinos, there are now experiments operating deep underground to search for a rare nuclear decay - double beta decay that releases no neutrinos (2-neutrino double beta decay has been observed). Observation of this decay would indicate that the standard model of particle physics would need to be rewritten. There are also new experiments to test time-reversal symmetry and symmetries in the weak interaction.
“Advances in computing have allowed for calculations that could only have been dreamed about 10 years ago. These include the hydrodynamics and nuclear interactions that occur when the core of a massive star collapses, and connecting QCD (structure of protons and neutrons) to the structure of nuclei. All of these things have become possible within the past decade.”
How has TUNL contributed to the body of knowledge in nuclear physics? Is there a specific investigation or study that has changed the way physicists think about the world?
“First a word about what TUNL is. There are 3 accelerator facilities:
A) The Tandem Van De Graaff Accelerator (the original TUNL accelerator). This accelerator is used for measurements related to nuclear astrophysics and has unique capabilities to produce high-quality neutron beams. These are used for measurements of nuclear reactions and structure, applied physics and in the design of searches for neutrino less double beta decay and dark matter.
B) LENA - the Laboratory for Experimental Nuclear Astrophysics. Most of the reactions in stars involve the fusion of hydrogen to create heavier elements. LENA has the most intense proton beam in the world for measurements of these reactions. Beam intensity matters because reactions in stars are very slow - some can take thousands or millions of years on average in a star and we obviously need to speed the process up.
C) HIGS - the High Intensity Gamma Source. HIGS collides an electron beam with FEL light from the Free Electron Laser to produce gamma rays. It’s currently the world’s most intense source of gamma rays with tunable energies. This is used for studies of QCD at lower energies than Jefferson Lab, which covers the middle ground between nuclei and the structure of nucleons. HIGS is also used for studies of nuclear structure, by using gamma rays to populate excited energy states, as well as for applications. For example, there’s interest in using gamma-ray beams to search for radioactive materials being smuggled in shipping containers.
“HIGS has provided some interesting new data on how protons and neutrons stretch in electric and magnetic fields - the gamma rays carry these fields. This in turn provides information about the larger-scale substructure of the proton and neutron. About 10 years ago, LENA produced data on the nitrogen-14 + proton —> oxygen-15 + gamma reaction. This reaction regulates energy production in stars more massive than the sun and in solar-type stars near the ends of their lives. We found that the rate of this reaction is about 40% less than previously thought and one spinoff from this is that certain age estimates for the galaxy have to be increased by about a billion years. TUNL is also the lead institution in the Majorana Demonstrator project (MJD) - a proof-of-principle for a large-scale neutrinoless double-beta decay search. Finally, TUNL is also a major player in the nEDM experiment (neutron - Electric Dipole Moment). If the neutron behaves as though it has equal and opposite charged poles, then both mirror and time-reversal symmetries would be violated.”
What are some of the applications of nuclear physics that relate to everyday life?
“The answer that everyone is most familiar with is medicine - medical imaging and radiation therapy. People are also very familiar with radioactive dating, but measuring trace amounts or radioactivity can also be useful in climate studies. I mentioned national security and a related topic is stockpile stewardship - how do we ensure the reliability of weapons without testing them? TUNL is involved in this work. We also do experiments to test the microscopic properties of membranes used to purify water and we look at how increasing levels of CO2 in the atmosphere affect plants (in collaboration with the Phytotron). The latter is done using an isotope of Carbon as a radioactive tracer. Many of our students have gone on to careers in industry and government - a degree in experimental nuclear physics is a good entry to other technical fields.”
How can we use this research to answer the growing demand for energy?
“The most apparent connection between nuclear physics and energy is through nuclear power. Currently there’s very little of this done at TUNL, but there is work going on in advanced fuel cycles - making cleaner fission reactors; and on nuclear fusion. The latter has proven to be elusive, but fusion, which powers stars, is thought to be more efficient and cleaner. There are environmental impacts of energy production and nuclear techniques have been very useful in looking at how pollutants migrate and get into bodies of water. Again, this is not something that we currently do, but we have the infrastructure for it.”
Champagne, Arthur. E-mail Interview. 16 Nov. 2016.