The most general conditions required for the observation of the quantum amplification of a gamma-quantum flux by excited nuclei are formulated. The region of admissible parameters for two alternative variants of the aggregate state of an amplifying medium (atoms in solids and free atoms) is found.
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.
This report results from a contract tasking Moscow State University as follows: Exploration of methods to control the radiative gamma- processes in nuclei is currently one of the most promising investigation directions of the modern Quantum Nucleonics - a new realm of contemporary Physics which spreads the basic conceptual ideas and methods of optical Quantum Electronics of atoms and molecules to the gamma-ray range and atomic nuclei and even anti-particles. One of the most important problems in this field is the generation of coherent gamma-radiation by a Gamma-ray Lasing (GRL) process which makes use of excited nuclei as an amplifying medium. The most likely medium for gamma-ray lasers is an ensemble of metastable nuclear isomers, Direct excitation of these isomeric states by electrons or X-ray photons will also create a deeper understanding of nuclear structure and properties.
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.