Prospects for a gamma-ray laser—commonly thought to be impossible due to the cubic dependence of subthreshold spontaneous emission power on frequency—have brightened. Physicists at the University of Texas at Dallas (UTD) describe development of an isomer of hafnium as a potential gamma-ray laser gain medium. With four of its nucleons in an excited metastable state, the isomer has a 31-year half-life and emits cascades of gamma rays when perturbed by soft x-rays. A single 40-keV photon triggers a gamma cascade with energy totaling 2.5 MeV, a 60X enhancement in energy. Produced by a process called proton spallation, the isomeric hafnium is capable of storing 1.3 GJ of releasable energy per gram.
Obtaining a sample of the material from Los Alamos Scientific Laboratory (Los Alamos, NM), the UTD physicists teamed with researchers from five countries to experimentally trigger gamma-ray production. According to Carl Collins at UTD, the cross section of the hafnium is so large that the team was able to use an ordinary dental x-ray machine as the triggering source. As of yet, the emitted gamma rays are not coherent. Contact Carl Collins at cbc@utdallas.gifdu.