“Magic” element challenges current model of nucleosynthesis

It may just be a small dotted line on a graph, but to physicists it is a significant one, because it represents an unexpected result. “This dotted line is based on the measurements made by the n_TOF team,” says Emilio Andrea Maugeri, a chemist at PSI’s Laboratory for Radiochemistry. He moves the mouse cursor up to a second, solid line on the diagram: “And this is the previous theoretical model”,  he explains. Even higher up, a red dot represents astronomical observations of stars. The researchers were hoping to close the gap at the top. Instead, they increased it further – and the consequences for the theory of nucleosynthesis in stars cannot yet be foreseen.

The details are as follows: the synthesis of heavy elements is governed by two processes, known as the s(slow)- and r(rapid)-processes. The first takes place in the outer layers of stars, where neutrons are captured by atomic nuclei, which then decay into protons and electrons, as well as antineutrinos. This is how the elements with higher atomic numbers in the periodic table are produced. Theoretical models predict the s-process very well, reproducing the abundance of these elements in certain stars in our home galaxy, the Milky Way.

A theoretical model with difficulties

However, although cerium is primarily produced by the s-process, theoretical models struggle to reproduce the observed quantities. Cerium is particularly interesting because its isotope Ce-140 is a “neutron magic” nucleus, meaning that its neutron shell is complete and therefore has little “desire” to absorb further neutrons. Physicists refer to this phenomenon as having a low effective neutron cross-section. In certain stars, where cerium-140 is produced via the s-process, astrophysicists have measured a much higher abundance of this isotope than predicted. This discrepancy cannot be explained by current theoretical models, which are otherwise reliable in that their predictions are correct for all elements close to cerium.

A possible explanation might be that the calculations used incorrect input values for the effective cross-sections. If the neutron cross-section of cerium-140 were found to be lower, the discrepancy would be smaller. Indeed, its cross-section has only been measured with low accuracy until now. Researchers around the world are therefore very interested in measuring its value with greater precision.

This is what the n_TOF research team has done. Over a period of four years, the measurements were prepared, carried out at CERN, and finally analysed. The n_TOF neutron source in Geneva is currently the best in the world for so-called time-of-flight measurements using neutrons at energies comparable to those encountered during stellar nucleosynthesis with the very high energy resolution required. The PSI team played an important part in the experiment, producing a high-purity cerium-140 target for neutron bombardment. It took many months and tests to achieve a target with the right properties, which were essential to perform a measurement with unprecedented precision.

Excellent work

As mentioned to begin with, the result was not what the researchers were expecting. They thought they would measure a lower cross-section for the capture of a neutron by cerium-140, closing the gap between the measured values and observations. However, instead of moving up, the line on the graph actually dropped to the level of the dashed line. This means that the cross-section is in fact even larger than previously assumed. The experiment at CERN is so precise that there is no doubt about the effect. This was also confirmed by the reviewers of the publication in the renowned journal Physical Review Letters: they awarded it the distinction of being “Best Paper” for exceptional research.

What now? If all the measurements are correct and the model is too, but they still don’t match, some previously unknown process must explain the discrepancy. This process is not entirely “mysterious”, however. In fact, physicists have long speculated that there could be a third process, in addition to the s-process and the r-process, which they call the i-  or intermediate process.

Emilio Andrea Maugeri is euphoric. “We may have just established a new field of research.” Next year, the experts plan to meet at a conference specifically dedicated to this topic, to discuss how to proceed. Maugeri: “It could be that we have to completely review the theoretical model of how elements that are heavier than iron are formed inside stars.” But one thing is already clear: “The black-and-white thinking of s- and r-processes could be over. In the future, we will also have to think in shades of grey.”

Text: Bernd Müller

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