Enhanced nuclear level decay in hot dense plasmas

Abstract

A model of nuclear level decay in a plasma environment is described. Nuclear excitation and decay by photon processes, nuclear excitation by electron capture, and decay by internal conversion are taken into account. The electrons in the plasma are described by a relativistic average atom model for the bound electrons and by a relativistic Thomas-Fermi-Dirac model for the free electrons. Nuclear decay of isomeric level may be enhanced through an intermediate level lying above the isomer. An enhanced nuclear decay rate may occur for temperatures far below the excitation energy of the transition to the intermediate level. In most cases, the enhancement factor may reach several decades. PACS number(s): 23.20.NxIn a hot dense plasma, either laser heated or of astrophysical interest [1-3], nuclei in an isomeric state may have a decay rate different from the laboratory value. The large number of photons and free electrons modifies the environment in which the nucleus naturally decays under laboratory conditions through spontaneous emission and internal conversion (IC). In the plasma, the IC decay rate can be modified by the lower number of available electrons in the partially ionized atom. Moreover, new decay modes also appear such as induced photon emission, free electron scattering, or bound internal conversion.Furthermore, indirect decay channels may be opened, if there exists a nuclear level lying above the isomeric level that may be excited from the isomeric state, and then decay down to the ground state. At first glance, one may expect that this indirect decay mode may become significant when the temperature is around the energy difference between the isomeric level and the upper level. However, if the multipolarities of the excitation transition of the intermediate level and of its decay are more favorable than that of the isomeric transition, one may expect that the indirect decay mode becomes predominant at a lower temperature.This indirect process would be a valuable tool for investigating nuclear excitation in plasmas. In a laser heated plasma, it could provide experimental conditions allowing tests of the nuclear transition rate model as well as the ability to reach significantly populated higher energy nuclear levels. These studies are critical for nuclear energy storage on an isomeric level and its release mechanism.If these hot plasmas are maintained for a long enough duration at the thermodynamic equilibrium at a temperature T, the different nuclear level populations reach an equilibrium given by the Boltzmann law where the ratio between any two populations N i and N f is easily calculated:where k B is the Boltzmann constant, E i and E f the excitation energies, and J i and J f the spins of lower initial state i and the upper final state f.One remarkable feature of the thermodynamic equilibrium is that the populations can be calculated without any explicit knowledge of the detailed excitation and decay processes. However, in the case of plasmas where only the nuclear populations are not at equilibrium, i...

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