What is a core isomer

Core isomers as energy storage

The ability to store and extract energy in a controlled manner is one of the most important challenges of our time. One possibility for this is long-lived, excited states of atomic nuclei, so-called nuclear isomers. A research team led by Jonas Gunst from the Max Planck Institute for Nuclear Physics in Heidelberg has now theoretically investigated what happens when the energy of such nuclear isomers is released with the help of an X-ray laser. To their surprise, the scientists discovered that an electronic process previously viewed as a side effect has a stronger effect on the excited nuclei than the X-ray radiation itself.

Molybdenum-93 decay scheme

Gunst and his colleagues considered a hypothetical scenario in which a foil made of metallic niobium is bombarded with hydrogen nuclei. The niobium nuclei produce excited molybdenum nuclei containing 93 protons and 93 neutrons. Such a sample could be stored for a few hours and then irradiated with X-rays to release the stored energy. However, the decomposition of the core molybdenum isomers cannot be influenced or controlled without further ado. The researchers therefore took a detour: In their simulation, they continued to excite the core isomers with external fields until an even higher, short-lived excited state was reached. With a relatively small amount of energy, the entire excitation energy of the core can then be released in one fell swoop.

The researchers have calculated that in the case of molybdenum nuclei, an energy of five kiloelectron volts is sufficient for the nuclei to emit gamma radiation with an energy of 2.4 megaelectron volts. Photons with five kiloelectron volts can already be generated by so-called free-electron lasers. If the rays of such a high-energy X-ray laser hit a material like molybdenum, a plasma is created: the electrons are separated from their atoms and swing freely around the remaining ions. Under certain conditions, some ions can recapture the free electrons, so that their atomic nucleus is excited at the same time. The theoretical calculations suggest that the photons from the X-ray light supply exactly the right amount of energy. To their astonishment, the researchers discovered that electron capture - a previously neglected side effect - stimulates orders of magnitude more nuclei than X-rays alone.

“This conclusion is by no means relevant only to the case of molybdenum-93. In almost all nuclear excitations that could be achieved with the X-ray laser nowadays, the nuclear excitation through electron capture instead of side effects will make the main contribution, ”explains first author Jonas Gunst from the Max Planck Institute for Nuclear Physics. “That can be of great importance for future nuclear physics experiments with X-ray lasers.” The probability with which the energy of the core isomer is released is, however, still too small to speak of efficient control. "Unfortunately, we are still a long way from the nuclear battery of the future," admits co-author Adriana Pálffy from the Max Planck Institute for Nuclear Physics, "but our results show that positive, in our case intensifying, surprises can also occur."