Nuclear Antenna

A new calculation shows that x rays, produced at machines planned or under construction, might be convertible into gamma rays or other particles using a process that is analogous to what happens when radio waves strike an antenna. In a conventional antenna, radio waves excite the motion of electrons over an extended region; in a rooftop antenna, for instance, electron interactions with incoming radio waves are spread out over the size of the antenna, typically a meter or more.

The concerted electron excitation is then amplified into a more energetic signal in a tuned circuit in a radio receiver set. Michael Kuchiev, a physicist at the University of New South Wales (kmy@phys.unsw.edu.au, 61-2-9385-4634) has contrived a nuclear equivalent of all this, a process in which x rays can interact in an “antenna” and be “amplified” into a new form of energy---in the form of particles. Doing this requires just the right circumstances.

The theory of quantum electrodynamics (QED) suggests that such conversion of laser light, in the vicinity of an atomic nucleus, can occur in strong electric fields on the order of 10^18 V/m and at laser power densities of more than 10^29 W/cm^2. Such conditions might be reached in upcoming x-ray free electron laser (FEL) facilities.

In this kind of environment, with intense laser electric fields interacting with the static electric field of an atomic nucleus at a distance of 10^-4 nm, the successive absorption of hundreds or thousands of laser photons could be turned into an electron-positron pair. This kind of pair production is fully expected at the next generation of x-ray sources.

But according to Kuchiev, further interesting things can be expected. The electron-positron pair is still immersed in the potent electric field, he says, and prone to soak up still more energy (in analogy to the concerted electron motion being amplified into a usable signal in a radio), so much energy that the electron-positron pair might even be turned into a pair of muons, the heavier cousins of electrons (see figure at http://www.aip.org/png/2007/287.htm). In effect the nuclear-antenna phenomenon would be like having an electron-positron collider the size of an atom. (Physical Review Letters, upcoming article