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Advancements in Understanding Atomic Nuclei Magnetism

Many atomic nuclei exhibit magnetic fields that are significantly more intense than that of Earth. For instance, at the surface of heavy nuclei like lead or bismuth, the magnetic field can be trillions of times stronger, drawing closer comparison to that found in a neutron star. The question of whether electrons can effectively function within such formidable fields remains an important area of research.

A research team from TU Darmstadt, collaborating with the GSI Helmholtz Center for Heavy Ion Research, has made a pivotal advancement in this domain. Their research, featured in Nature Physics, substantiates earlier theoretical predictions.

Hydrogen-like ions, which consist of atomic nuclei with only a single electron bound to them, provide a simplified model for study. For heavy nuclei with high proton counts—bismuth, for example, has 83 protons—the strong electrostatic attraction pulls the electron closer to the nucleus, placing it within this extreme magnetic environment. In such settings, the electron’s magnetic field aligns with that of the nucleus, akin to a compass needle finding true north.

By supplying the precise amount of energy, researchers were able to invert this alignment, effectively flipping the compass needle. This novel approach was successfully applied to a radioactive isotope where the energy required for the flip was theoretically well-defined.

The energy thresholds can be calculated through the principles of quantum electrodynamics (QED), the quantum mechanics branch dealing with electromagnetism. However, accurately determining these values has been hampered by a limited understanding of complex heavy atomic nuclei, which has hindered experimental validation of QED predictions.

Previous measurements on the stable isotope Bi-209 matched theoretical expectations. Nonetheless, doubts lingered about whether factors related to nuclear structure could truly be disregarded as presumed. To bridge this gap, it was recommended to examine a different bismuth isotope with alternate nuclear characteristics. Given that bismuth lacks additional stable isotopes, the research team, led by Professor Wilfried Nörtershäuser, utilized the radioactive isotope Bi-208. This isotope, having one less neutron than Bi-209, exhibits an even stronger magnetic field.

Dr. Max Horst, the study’s lead author, pointed out the initial challenge: “The key to this experiment was generating and isolating hydrogen-like ions from the Bi-208 isotope.” This process involved knocking out a neutron from stable Bi-209 during a nuclear reaction, after which the fragments were captured in the ESR experimental storage ring.

In addition, to form the hydrogen-like system, all but one of Bi-208’s 83 electrons needed to be stripped away. The resulting fragments traveled around the storage ring at an impressive 72% of light speed, approximately 200,000 kilometers per second. The research team successfully identified the hydrogen-like ions of Bi-208 and eliminated unwanted reaction by-products.

“In our earlier experiments with Bi-209, we had about 1,000 times more ions available,” Horst noted, explaining the necessity for heightened efficiency and sensitivity in this study.

The experimental methodology involved the utilization of a laser beam aimed at flipping the magnetic orientation of the electron. The ion absorbs a photon from the laser, transferring energy to the electron, which subsequently shifts its magnetic orientation to align unfavorably with the nuclear magnetic field.

After about half a millisecond on average, the electron reverts to its original state, releasing a photon. By this stage, the ion has circled the storage ring numerous times, and the emitted photons are detected by highly sensitive apparatus positioned to minimize background noise.

Given the limited number of ions, precise prediction of the necessary photon energy for the process was crucial. “Exploring a wide wavelength range at such low signal rates would have been time-intensive,” Nörtershäuser explained.

To enhance prediction accuracy, measurements were conducted at CERN on neutral atoms of both Bi isotopes, allowing estimation of the disparate influences of their nuclear structures. This data was combined with prior findings related to the stable isotope’s hydrogen-like ion, resulting in a precise energy transition prediction for hydrogen-like Bi-208.

This predicted value aligned closely with both full quantum mechanical calculations and the experimental results obtained in the storage ring, validating the methodology. These findings pave the way for further exploration of nuclear structure influences on other charge states of Bi-208 and can be adapted for studies involving different isotopes of bismuth or additional elements.

More information: Max Horst et al, Storage-ring laser spectroscopy of accelerator-produced hydrogen-like 208Bi82+, Nature Physics (2025). DOI: 10.1038/s41567-025-02885-x

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phys.org