A comparative analysis of various schemes of a gamma-ray laser is presented. All of the schemes employ a hidden inversion of nuclear states, which appears in cooled nuclear ensembles due to nuclear recoil in the emission and absorption of gamma photons.
The concept of a γ-ray laser which makes use of the kinematic shift and the splitting of the spectral lines of radiative γ-transitions observed in an ensemble of free nuclei owing to recoil is considered. These effects form a prerequisite for the formation of hidden population inversion with no excess of the number of excited emitters over the number of unexcited ones, for implementation of a broadband unperturbing x-ray pump in an unconventional 'two-level' scheme, for the manifestation of anisotropic quantum amplification without any reflecting structures, etc. These attractive properties offer strong possibilities of staging an initial demonstration nuclear γ-ray laser experiment.
The evolution of the foreign and native search for solving the problem of a nuclear gamma-ray laser (NGL), which has been attracting attention for almost half a century despite the absence at present of any convincing data about its experimental solution, is considered. It is shown that the key conflict inherent in any conception of the NGL is the antagonism between the necessity to accumulate a sufficient amount of excited nuclei and the requirement to narrow down the emission gamma-ray line to its natural radiative width. The critical analysis of different approaches for solving this conflict (Mössbauer scheme, deeply cooled ensembles of free nuclei with the hidden inversion, nuclear inversionless amplification, two-quantum gamma emission in counterpropagating photon beams, hypothetical amplifying medium of long-lived isomers in a Bose—Einstein condensate) shows that this search is important not only due to the expected result, which could stimulate the development of quantum nucleonics as a new branch in physics, but also is of interest due to a variety of physical disciplines and experimental approaches used in this search.
We discuss a possibility for obtaining the self-amplified spontaneous emission of gamma radiation in an active medium with hidden population inversion of nuclear states. Hidden inversion appears in cooled nuclear ensembles due to assistance of the nuclear recoil in emission and absorption of gamma quanta. Establishing a hidden inversion allows an amplification of gamma radiation to occur in a particular frequency range without excess of the number of excited nuclei over the number of unexcited nuclei. We formulate a set of requirements to the pumping radiation and to the amplifying nuclear medium crucial to the development of gamma-ray lasing.