Recent studies of the gamma-ray laser problem

The possibility of extending the laser principle into the hard X-ray region above a few keV depends upon the ability of a pump to create the critical density of population inversion for which gain overcomes loss by absorption. Although this critical density decreases with the wavelength of the radiation to be stimulated, the power required to generate it depends upon the lifetime of the state being pumped. Nuclear states have lifetimes ranging from fractions of picoseconds to millennia. In the so called recoilless or Mossbauer transitions of nuclear isomers, it was observed that the resonance cross section often exceeds the nonresonant absorption cross section by several orders of magnetide: just the condition for lasing in an inverted population. If, other things being equal, the absorber foil of a Mossbauer experiment contained an excess of excited states, then, instead of a Mossbauer experiment contained an excess of resonance, there would be an increase of intensity, and amplification by stimulated emission would be achieved.