Prospects for isomeric energy release

Abstract

The state of experimental studies and promising proposals for the application of nuclear isomers presented as controlled energy or γ-ray sources are reviewed. The properties of isomeric states, methods of their production, and approaches to their efficient stimulation using various types of radiation are analyzed. The long-lived isomers, which can be accumulated in reactor irradiations or in other nuclear interactions with abundant yield, are listed. The isomers are estimated according to their specific energy accumulated per nucleus and the level of the cross section for their formation in reactions with neutrons. The nuclei are classified as promising either for obtaining controlled γ-ray pulses, for the enhanced release of the radioactive decay energy, or for experimental studies on detecting forbidden electromagnetic transitions from the ground to isomeric state. In all cases, the possibility of external-stimulus action on nuclear transitions has key significance, which should become the subject of investigations. The results of successful observation of stimulation of isomers are described at excitation energy E* > 1 MeV in the reactions with bremsstrahlung photons and Coulomb excitation in the ion beam. The essential increase in the K-hindered transitions with increasing energy and also the K-mixing at high rotational frequency for high-spin levels are discussed. The attention is focused on attempts to detect the triggering induced by the radiation in the x-ray range, in particular, that of the 178m2Hf isomer with the help of x-ray sources and the synchrotron radiation. Proposals for experiments with other isomers are considered. The possibility of affecting the nuclear states by means of ionization of electron shells of a corresponding atom is discussed as promising, and various schemes of similar experiments are proposed. The atomic cross sections are eight orders of magnitude higher than the nuclear ones; therefore, the stimulation of an isomer can occur even if the conversion from atomic excitations to nuclear ones has a low probability.