Professor Yury Litvinov is head of the “SPARC Detectors” group within the GSI research department “Atomic Physics”. The group conducts experiments with exotic atomic nuclei in the experimental storage ring ESR. His team is responsible for the construction of novel instrumentation for the future SPARC experiments at the FAIR storage rings. Since 2016, Litvinov is leading scientist for the EU-funded ERC Consolidator Grant “ASTRUm” and since 2017 he is an adjunct professor at the Ruprecht-Karls-University in Heidelberg. Interview by Carola Pomplun during the experimental period in March/April 2020.
What is your experiment about?
We are presently carrying out a very complex experiment that we have been planning for a long time. It uses our entire accelerator chain, from the ion source, the linear accelerator UNILAC to the ring accelerator SIS18, the fragment separator FRS and the storage ring ESR with its full arsenal of capabilities: beam accumulation, fast stochastic and electron cooling, internal supersonic gas-jet, non-destructive particle detection, etc. We accelerate lead ions and then knock out a proton in a production target to create the isotope 205-thallium. To filter it out of the multitude of other nuclear reaction products in the target, we use the FRS. Then we feed it into the ESR, where it is accumulated and cooled, and we wait for the ultra-slow bound-state beta decay, where one of the neutrons of the thallium is converted into a proton. The resulting electron is captured directly by the daughter nucleus 205-lead, therefore the name bound-state decay. But 205-thallium and 205-lead are so similar in mass that we can’t bring them apart while they are both circulating in the ESR. Therefore, after some time we strip the produced electron off the lead ion in the gas jet, then the orbit of the lead in the ring changes sufficiently so that we can distinguish and measure both nuclei. As of today, such exotic decays can only be studied at GSI where we have these unique experimental facilities and instrumentation.
What insights can be gained from the experiment?
The decay processes play a major role in understanding the formation of elements in stars, the so-called nucleosynthesis. Thallium is located at the very end of the slow neutron capture, the so-called s-process. This is one of the processes by which heavy elements are formed in giant stars. It is believed that the s-process is responsible for about one half of all elements we see around us on earth, in our solar system, or in the galaxy. What makes 205-thallium special, is that it is normally stable and only becomes unstable when it has lost its electrons, as can often happen in a star interior. Furthermore, the daughter 205-lead has a very long half-life of 17 million years and decays to our 205-thallium. Whether and when lead decays to thallium and thallium decays back to lead then of course influences the sensitive balance between the final amount of produced thallium and lead. It is therefore an important building block for understanding the abundance of chemical elements in our universe. From the balance we can learn about the origin of the material from which our solar system was formed.
How has the corona pandemic changed your everyday life?
It was an extreme cut that brought us to the limit. A large number of colleagues from Germany and abroad — from Europe, Asia and North America — were supposed to come and contribute to the experiment, but now they can no longer travel. It is the hands of the external experts, for example for special equipment, that we lack on site. But they try to support us as much as they can from a distance via all available communication channels. Personally, I particularly miss the exciting and lively atmosphere with many colleagues present in the main control room. Normally we uninterruptedly discuss with people involved, we develop ideas and often controversial solutions to problems and give each other feedback. And of course we also instruct young researchers who look over our shoulders and learn. None of this is possible right now, the people on site have to just make a lot of decisions on their own since an effective brainstorming does not always work via an ad hoc video conference. Because of the reduced staff, many activities take longer. Normally we work three shifts at the experiment, but we have currently reduced this to two shifts and take breaks at night.
But I would like to express my great praise and many thanks to my colleagues. The commitment and willingness is enormous! Everyone is contributing their best to overcome any difficulties that arise. Also I would like to thank our management for a constant support. Our Scientific Managing Director Professor Paolo Giubellino is visiting us every evening in the main control room, which we appreciate very much.
What protective measures have you taken for yourself and those involved?
The rule of distance naturally also applies to us. We try to work with a maximum of two people at the same place, who then also wear protective masks where necessary. Our colleagues at the accelerator laboratory in Lanzhou in China, with whom we have had a long-standing friendship, were very kind to send us many protective masks that we can use. We owe them a great deal of thanks for this.
What was a particularly difficult problem to solve?
We had to fight a little bit to adjust the whole machine so that our experiment could run and now successfully take data. Many of these challenges would certainly have existed without the corona pandemic, but there would have been more people to help mastering them. Without corona, there would have been around 30 people on site at all times, along with the externals, but now we have to manage with less than ten. For me it is also psychologically difficult that the many externals who have contributed so much to the experiment in the preparation phase can’t be here right now to see and harvest the fruits of their work.
What do you wish for the near future?
A little sleep. And I’ve heard rumors there’s been some sunshine during these past few days! But jokes aside, the whole team here has really given all they’ve got over the past few days. First of all, we are happy that our experiment is going well. I hope that we will be able to take good data for our doctoral students to successfully write their theses, and preferably go home with a “summa cum laude”. And I very much hope that my mentor and friend Fritz Bosch, who unfortunately passed away some time ago and who was very much missed in the main control room, would also be satisfied with the experiment.