Prospects for triggering gamma emission as a precursor to a gamma-ray laser

A gamma-ray laser would provide coherent radiation at sub-Angstrom wavelengths from electromagnetic transitions between excited nuclear states. Even if the output radiation was essentially isotropic, more of a gamma flash, simply the ability to trigger the emission of gamma radiation in a controllable manner could have important consequences. In addition, since the radiation would serve to de-excite isomeric nuclei to their ground states, there would be no radioactive by-products: an isomeric material could provide a 'clean' source of nuclear energy, much like a nuclear battery. These possibilities are suggested by unique advantages inherent in nuclear electromagnetic transitions that can provide exceptional energy storage. The basic concept for the gamma-ray laser was introduced in 1961, yet the interdisciplinary nature of the gamma-ray laser problem has frustrated efforts for decades. In fact, several reviews concluded that there was no known effective approach to achieve a gamma-ray laser, largely due to technological restrictions. Fortunately, new ideas have continued to be introduced and among those 'optical' pumping, analogous to the approach taken in the ruby laser, has provided sustained positive results over the past decay. Experiments along this line have shown that isomeric states can be populated from their ground states by photons and that the inverse process, induced gamma radiation (IGE), can be triggered in a test material. A gamma flash of even a gamma-ray laser based upon these results is feasible if an appropriate isotope can be chosen. This paper reviews the optical pumping approach, the experimental demonstration of triggered gamma-emission and the systematics that identify the most promising nuclide. (author)