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Unsettled – Gaining energy from excited nuclei



Is it possible to store energy in an atomic nucleus? In prinicipal yes, if you externally deliver the energy so that the nucleus will be lifted to an exited state. Scientists from the Max-Planck-Institute for Nuclear Physics and from the GSI Helmholtzzentrum für Schwerionenforschung GmbH have now calculated how to gain the energy back from this "nuclear battery". They found a surprising result: an effect considered secondary plays a dominant role.

Atomic nuclei can exist on different energy levels. Most possess a stable ground state the nucleus preferably resides in. By adding external energy one can "unsettle" the nucleus and lift it into an exited state. These states are usually unstable. The nucleus wants to get rid of the additional energy and decays into its ground state by emission of light. But some nuclei have a so called metastable state. They can be exited and stay on a higher energy level for quite some time without decaying. By this means energy can be stored in them similiar to a battery.

But how can you insert energy into this "nuclear battery", and how can you extract it again? "The addition of the energy could be done by an accelerator", explains Dr. Yuri Litvinov, who investigates the phenomenon at the GSI accelerator facility. "If you irradiate the metal niobium with protons, e.g. from the GSI accelerator UNILAC, the niobium nucleus can capture a proton and become molybdenum." Many molybdenum nuclei are then in an exited metastable state—the battery is charged. To make them jump into their ground state and emit their energy they can be irradiated with intense x-rays, e.g. from an x-ray free electron laser. The x-rays will lift the nucleus into an even higher, but unstable state. The nucleus immediately decays into its ground state and emits the total amount of previously fed energy by emission of light—the battery is discharged.

This second excitation of the nucleus by x-rays is very unlikely, because another effect occurs: instead of exciting the nuclei the x-rays ionize the atoms. That means they rip the electrons off of their shells. In a material sample like in the molybdenum this leads to the generation of a plasma of unbound electrons. Litvinov and his colleagues around project leader Adriana Pàlffy from the Max-Planck-Institute for Nuclear Physics have now calculated that this side effect might even be beneficial. "The nuclei recombine with the electrons", says Litvinov. "The electrons emit their surplus energy in form of light. If this energy fits the excitation energy to lift the nucleus into the unstable state, then a direct transfer of energy to the nucleus can succeed." This can serve as the trigger for the discharge. The calculations show that this effect might even be dominant, meaning its occurence is more likely then the direct excitation of the nucleus by the x-rays.

In experiments the scientists now want to verify their calculations. "We are looking for this phenomenon at the GSI facility but couldn't observe it so far", says Litvinov. "Experiments with our GSI storage ring and our laser system PHELIX are planned. Once we have understood the physics better, it might even be possible to build real batteries for our everyday life with this technology."

"At the x-ray free electron laser XFEL currently under construction in Hamburg we could learn more about nuclear excitations with electrons and photons in dense plasmas", says Pàlffy. "It is still a long way to the nuclear battery of the future, but our results show that positive and in our case reinforcing surprises may occur. The physics of metastable nuclear states stays thrilling."

Function of the "nuclear battery"
a) A niobium nucleus becomes molybdenum in a metastable state by irradiation with a proton from the accelerator; b) intense x-rays rip an electron out of the shell; c) in the process of recombination energy can be transferred directly to the nucleus; d) the additional energy lifts the molybdenum into an unstable state. It consequently decays to its ground state by emission of energy.
Picture: GSI Helmholtzzentrum für Schwerionenforschung