Eyes on the Sun: Naked thallium-205 ion decay reveals history over millions of years

The Sun, the essential engine that sustains life on Earth, generates its tremendous energy through the process of nuclear fusion. At the same time it releases a continuous stream of neutrinos— particles that serve as messengers of its internal dynamics. Although modern neutrino detectors unveil the Sun’s present behavior, significant questions linger about its stability over periods of millions of years—a timeframe that spans human evolution and significant climate changes. Finding answers to this is the goal of the LORandite EXperiment (LOREX) that requires a precise knowledge of the solar neutrino cross section on thallium. This information has now been provided by an international collaboration of scientists using the unique facilities at GSI/FAIR's Experimental Storage Ring ESR in Darmstadt to obtain an essential measurement that will help to understand the long-term stability of the Sun. The results of the measurements have been published in the scientific journal Physical Review Letters.

LOREX is the only long-time geochemical solar neutrino experiment still actively pursued. Proposed in the 1980s, it aims to measure solar neutrino flux averaged over a remarkable four million years, corresponding to the geological age of the lorandite ore.

Neutrinos produced in our Sun interact with thallium (Tl) atoms, present in the lorandite mineral (TlAsS₂), and convert them into lead (Pb) atoms. The isotope ²⁰⁵Pb is particularly interesting due to its long half-life time of 17 million years, making it essentially stable over the four million years timescale of the lorandite ore. As it is currently not feasible to directly measure the neutrino cross-section on ²⁰⁵Tl, researchers at GSI/FAIR in Darmstadt, Germany, came up with a clever method to measure the relevant nuclear physics quantity needed for the determination of the neutrino cross section. They exploited the fact that this quantity, the nuclear matrix element, also determines the bound-state beta decay rate of fully ionized ²⁰⁵Tl⁸¹⁺ to ²⁰⁵Pb⁸¹⁺.

The experimental measurement of the half-life of the bound-state beta decay of fully ionized ²⁰⁵Tl⁸¹⁺ ions was only possible thanks to the unique capabilities of the Experimental Storage Ring (ESR) at GSI/FAIR. The ESR is presently the only facility where such measurements are feasible. The ²⁰⁵Tl⁸¹⁺ ions were produced using nuclear reactions in GSI/FAIR’s Fragment Separator (FRS) and then stored long enough for its decay to be observed and successfully measured in the storage ring. “Decades of continuous advancements in accelerator technology made it possible to generate an intense and pure ²⁰⁵Tl⁸¹⁺ ion beam and measure its decay with high precision,” said Professor Yuri A. Litvinov, spokesperson for the experiment and principal investigator of the European Research Council (ERC) Consolidator Grant ASTRUm.

“The team measured the half-life of ²⁰⁵Tl⁸¹⁺ beta decay to be 291 (+33/-27) days, a key measurement which allows to determine the Solar neutrino capture cross-section”, explained Dr. Rui-Jiu Chen, a postdoctoral research associate involved in the project. Once the concentration of ²⁰⁵Pb atoms in the lorandite minerals is determined by the LOREX project, it will be possible to provide insights into the Sun’s evolutionary history and its connection to Earth’s climate over millennia.

“This milestone experiment highlights the power of nuclear astrophysics in answering fundamental questions about the universe,” said Professor Gabriel Martínez-Pinedo and Dr. Thomas Neff, who led the theoretical work to convert the measurement into the neutrino cross section.

Dr. Ragandeep Singh Sidhu, the first author of the publication, emphasized its broader significance: “This experiment highlights how a single, albeit challenging, measurement can play a pivotal role in addressing significant scientific questions related to the evolution of our Sun.”

The publication is dedicated to the memory of late colleagues Fritz Bosch, Hans Geissel, Paul Kienle, and Fritz Nolden, whose contributions were integral to the success of this project.