Status of experimental studies and perspective options for nuclear isomer application in a mode of controlled source of energy or/and g radiation are reviewed. Properties of isomeric states, methods of their production and approaches to effective stimulation by different kind radiation are analyzed. A list of long-lived isomers which can be accumulated in reactor irradiations, or in other nuclear reactions with abundant yield, is reduced. The isomers are estimated by two parameters: the specific energy accumulated by isomer per one nucleus, and the neutron cross-section for production of the isomer. Nuclear species are selected being the most promising for: creation of controlled pulses of g radiation, or for accelerated release of the radioactive decay energy, or for test experiments on detection in hindered electromagnetic transitions from ground to isomeric states in stable targets. In all cases, a possibility of external stimulus impact to nuclear transition would be of major significance, and this process must be explored. Experimental results are described on successful isomer stimulation at excitation energy E^*1 MeV in reactions with bremsstrahlung and in the Coulomb excitation by ion beam. Deterioration of K hindrance with growth of E^*, and K mixing at high frequency of rotation for high-spin levels are discussed. Attempts to detect triggering induced by radiation in X-ray range are under scope, in particular, the experiments aimed at the ^178m2Hf isomer triggering using the X-ray sources and radiation at synchrotrons. Excitation of nuclei by transitions in ionized atomic shells may be promising for isomer stimulation, and different schemes of such experiments are considered. Atomic cross sections are higher than nuclear ones by 8 orders of magnitude, and the isomer stimulation may happen even at low probability of the conversion of atomic to nuclear excitation.

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