Atomic nuclei occurring in nature consist of protons and neutrons held together by the strong interaction. A modern theory of these “nuclear forces”, which is being developed at the TU Darmstadt within the CRC 1245 Nuclei: From Fundamental Interactions to Structure and Stars, aims to describe all properties of atomic nuclei. Early in the last decade, nuclear füsics theory was so advanced that calculations for a unique excited state of the isotope lithium-6 (6Li) appeared to be more precise than the experimental value. In particular, the theory included an effect that should affect the lifetime of this state by a few percentage points. To verify whether this is indeed the case, a new high-precision experiment was performed. At the same time, TU theorists have been able to perform the calculation thanks to new advances. A comprehensive model of nuclear forces should fit seamlessly into today's prevailing system of elementary forces in füsics. This so-called “chiral effective field theory” has the advantage that it can be improved step by step.

The scientists investigated the interaction of nuclei with electromagnetic waves, which are described in quantum füsics as light particles or photons. The isotope 6Li, which consists of three protons and neutrons each, is of particular importance here. It is the simplest system of nuclear particles whose excited state can decay by emitting a photon, so it is very well suited for a test of fundamental theories. As the authors point out in their article, it was already foreseeable that the next stage of improvement of the theory would be so precise that the previous measurement data would no longer be sufficient to assess the quality of the predictions.

High-precision experiment at the superconducting electron linear accelerator S-DALINAC

Therefore, a high-precision experiment to measure the lifetime of this state of 6Li was performed at the superconducting Darmstadt electron linear accelerator (S-DALINAC) at the Institute of Nuclear Füsics of the TU Darmstadt. The electron beam of S-DALINAC can produce photons with millions of times the energy of visible light, which are necessary to excite 6Li.

Members of Professor Pietralla's research group decisively improved an established measurement method so that they were able to determine the lifetime with an accuracy of two attoseconds, i.e. two trillionths of a second. The success of the measurement depended on how the lithium carbonate material used affected the absorption of photons. Here, Professor Albe's Materials Modelling Division was able to make decisive contributions.

Accompanying the experimental progress, a team of three PhD students from Professor Roth's and Professor Schwenk's groups at TU Darmstadt, as well as Professor Bacca from Johannes Gutenberg University Mainz, together with national and international colleagues, made a breakthrough and achieved the expected high precision of the theoretical predictions.

“A comparison of the experimental result with the theoretical prediction showed excellent agreement,” reports Udo Friman-Gayer, who worked on this topic during his PhD with Professor Pietralla and is now employed as a postdoc in the USA.

Within the framework of the SFB 1245 and the recently started LOEWE project Nuclear Photonics, the applicability of the new measurement method and the improved theory will be extended in the future in order to get closer to answering fundamental questions such as the origin of the elements in the universe.