Like a stoic cow in a fly-ridden pasture, the nucleus of an atom is rarely perturbed by the electrons that buzz around it. Now two research teams have shown that, under certain circumstances, the nucleus and its electrons do interact. This discovery opens up the possibility of someday storing and retrieving energy in the nucleus--a kind of atomic battery.
Researchers had assumed that electrons and nucleus couldn't exchange energy. The reason has to do with the relative difficulty of emitting photons that match in energy. Frenetic electrons throw off photons every time they jump between energy states. But it takes the massive nucleus--think again of a cow, if you like--up to 1 million times more oomph to change its energy state and emit a photon that has a shorter wavelength and much higher energy. To most physicists, that meant it was highly unlikely that the tiny amounts of energy thrown off by electrons would ever goad the nucleus into a higher energy state. And, similarly, that a nucleus wouldn't have much effect on the electrons surrounding it.
Two teams have now dispelled this myth. The first, led by Shunji Kishimoto of KEK, Japan's High-Energy Accelerator Research Organization in Tsukuba, fired short, intense x-ray pulses at gold atoms. The trick was calculating the precise amount of energy to pump into the atom; just the right amount would excite a specific electron. When the electron released its new energy, it was exactly the right amount needed to bump the nucleus to a higher energy state for about 1.9 nanoseconds, they report in the 28 August Physical Review Letters.
The success raises the prospect of using excited electrons to pass energy to the nucleus, says Jean-François Chemin of the University of Bordeaux. "This would allow us to store energy in long-lasting nuclei and then retrieve this energy by deexcitating these nuclei," he says. Chemin and his colleagues, meanwhile, have shown that the interaction can go both ways: In the 28 August Physical Review Letters, they report that tellurium nuclei primed with the right amount of energy can transfer it to electrons, which in turn give up photons.
Another benefit from these kinds of studies might be more precise measurements of transition energies in nuclei, a value that's tricky to measure, says physicist Fritz Bosch of the Institute for Heavy Ion Research in Darmstadt, Germany. Measuring the energies needed for electron transitions, a much easier task, would provide a yardstick of the nuclear transitions if, as in these experiments, the two energy levels are matched.