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AN UNUSUAL exchange of energy between an atom鈥檚 nucleus and its electrons has
been seen for the first time, some 25 years after physicists predicted it. The
phenomenon could change our understanding of how the Universe鈥檚 heavy elements
are formed inside supernovae.

Normally there is an unbridgeable gap between the low-energy world of an
atom鈥檚 electron cloud and the high-energy world of its nucleus. The electrons
produce or absorb the low-energy photons in infrared, visible and ultraviolet
light. By contrast, the nucleus spits out bits of matter, X-rays and gamma rays
which are thousands of times more energetic.

But in some nuclei, under just the right conditions, the energy levels of
electrons and nuclei exactly match. Physicists predicted that when this happens,
excited electrons should occasionally shuttle their energy to the nucleus in a
process known as nuclear excitation by electronic transmission, or NEET. Now a
team led by Shunji Kishimoto of Japan鈥檚 Institute of Materials Structure Science
in Tsukuba has spotted this rare cooperation for the first time.

The researchers knew that shining X-rays onto gold foil would knock an
electron out of a gold atom鈥檚 lowest energy shell. When this happens, a second
electron from a higher energy shell falls into the empty space, giving off
energy.

The researchers found that roughly once every 20 million times, this energy
passes to the nucleus. The nucleus holds onto the energy for just a few
nanoseconds before dumping it back into the electron cloud and knocking another
electron out of the atom. Using a sophisticated particle detector Kishimoto and
his colleagues were able to observe this electron.

鈥淭his is the first real demonstration of NEET,鈥� says Donald Gemmell, a
physicist at Argonne National Laboratory in Illinois. 鈥淭hat鈥檚 quite a
迟谤颈耻尘辫丑.鈥�

NEET only works if the nucleus is able to absorb precisely the same amount of
energy as the infalling electron has to lose, says Jean-Fran莽ois Chemin,
a physicist at France鈥檚 Centre for Nuclear Studies in Bordeaux. His team at the
country鈥檚 National Large Heavy Ion Accelerator in Caen recently observed excited
tellurium nuclei releasing energy that sent an electron from a low energy shell
to a higher one. This process, called bound-state internal conversion (BIC), is
NEET in reverse.

Because NEET and BIC affect how long nuclei remain excited, they could
ultimately affect the amounts of different isotopes produced inside supernovae,
Chemin says. But Stan Woosley, an astrophysicist at the University of
California, Santa Cruz, is sceptical of this idea because the processes are
known to work only for a few rare nuclei. All the same, 鈥渋t鈥檚 an interesting
piece of physics鈥�, Woosley says, 鈥渨hether it changes astrophysics or not.鈥�