Strangely behaved silver led nuclear physicists to an important discovery
When nuclear physicists make a significant scientific discovery, its meaning tends to be hard to explain to the public. This is also the case with Mikael Reponen’s and Markus Kortelainen’s work, where an experimental and a theoretical physicist collaborated successfully in investigating the secret of an unknown nucleus.
Associate Professor Markus Kortelainen’s office is equipped with a flip chart and wall board, which are full of calculations and mathematical formulas in his neat handwriting. When Postdoctoral Researcher Mikael Reponen presented his measurement result at the end of 2019, Kortelainen became instantly interested as a theorist of nuclear physics.
Reponen had succeeded as the first scientist ever to measure the charge radius of the nucleus of silver- 96 (96Ag) isotope, and it seemed to behave oddly: the charge radius deviated from the systematic pattern of other silver isotopes.
”A theorist always becomes interested in a situation where something is not working as expected. What is wrong with the model? Should the model be improved?”, describes Kortelainen, who is one of the few experts of this field in Finland. Alongside with research, he is also in charge of all course-based teaching of theoretical low-energy nuclear physics currently provided in Finland.
Kortelainen ran some calculations for about a week and found out that even the most recent density functional theory based nuclear structure models cannot explain the measured results.
The findings resulted in an article published in the Nature Communications science journal and launched efforts to develop a new nuclear structure model to explain the properties of 96Ag nucleus better. Mikael Reponen, for his part, started preparations for the measurement of the next unknown silver isotope.
Charge radius is a small piece in a jigsaw puzzle
The discovery made by Reponen and Kortelainen is an important one, although they state that there are no direct practical application targets instantly available.
The isotope concerned belongs to the so-called exotic nuclei, which cannot be found in the nature but are produced through nuclear reactions in an accelerator laboratory. These nuclei are radioactive and thus different from the stable isotopes found in the nature.
Knowledge of the properties of rare nuclei give researchers better insight into the properties of atoms and subatomic particles.
”Finding out the charge radius of silver-96 is like another piece in the jigsaw puzzle that helps us understand the nuclear structure better. It gives supplementary information on how protons and neutrons interact in the nucleus”, Kortelainen depicts.
Atoms or nuclei do not have any solid surface or boundary, but the nuclear charge radius provides fundamental information about the size of the nucleus. More specifically, the measured radius provides direct information how the protons are distributed within the nucleus.
The nuclear chart is constantly updated. At the moment, it contains more than 3000 nuclei, of which 288 form the so-called valley of stability.
Numerical methods have predicted that there would be approximately 7000 different nuclei.
“We have thus still much to discover. The yet unknown part of the nuclear chart is generally called as Terra Incognita. New isotopes are added to the chart annually, with one of the most recent discoveries being 149- Lutetium made in JYFL-ACCLAB by Kalle Auranen and occasionally new elements as well, the latest of these, with the proton number 113, was named in 2016 as nihonium”, Mikael Reponen tells.
The nuclear chart has had an area difficult to approach
Like Mikael Reponen, also many other physicists have looked at the area below tin-100 (100Sn) on the nuclear chart, but only few have reached there as yet. Reponen’s 96Ag measurement was the first successful optical measurement below the neutron number of 50.
”The nuclei in this area have been hard to approach. Technical difficulties in producing these nuclei have prevented the measurements of mass, size, and form. However, many people are interested in this area”, Reponen describes.
The 96Ag measurements – and after these, also 95Ag measurements – were successful, since Reponen and colleagues used a variety of state-of-the-art technologies in the Accelerator Laboratory of the Ģֱ. The necessary equipment included a Penning trap mass spectrometeter, a hot cavity catcher laser ion source, and a laser resonance ionisation spectroscopy system.
”A Penning trap is an efficient mass separator, and it purifies the specific isotopes we wish to measure from contaminants. We combined it with laser spectroscopy so that we could measure the properties of thethe desired isotopes with optical means without any background interference, Reponen explains.
The time span in physicist’s work is exemplified by the time spent on constructing the measurement facilities: Mikael Reponen started building the hot cavity catcher laser ion supply apparatus already as a part of his PhD research in 2007.
The number of collaborative articles is increasing
The above-mentioned article is an example of the growing trend in the field of physics: Dialogue between experimental and theoretical research has increased, Reponen and Kortelainen consider.
”The number of collaborative publications has grown. Over the years, they will also gain plenty of citations”, Reponen says.
The Ģֱ rewarded Reponen and Kortelainen with the Scientific Breakthrough Award in March.
The researcher duo thinks that discussion about their discovery will become more active along with improved researcher mobility after the removal of COVID-19 restrictions. Their colleagues have already noticed the results, and over time the word will spread also more widely within the research community.
The lack of international events has been a considerable weakness in researchers’ work during the COVID-19 era, the physicists consider. Conference and research trips have become scarce, and online events cannot fully compensate them.
For researchers, international experience is important in all stages of their careers. Kortelainen has worked in the United States, and Reponen, for his part, has worked in Japan and also in the payroll of the University of Manchester here in Jyväskylä.
Both men warmly recommend physicist’s profession.
”An experimental physicist needs to be ready to do a lot of work when the opportunity is offered. The time allotted to your measurements on accelerators must be used efficiently. In counterbalance, we have academic freedom, which I appreciate a lot”, says Mikael Reponen.
”The work of a theorist is perhaps slightly more evenly paced. Sometimes I think about these matters in my free time as well. Usually it goes so that if I come across with an idea in the evening, I decide to consider it more closely at work next morning”, Kortelainen tells smiling.
