Nuclear excitation by electronic transition in189Os

Monochromatic x rays have been used to explore the phenomenon of nuclear excitation by electronic transition (NEET) in the ${}^{189}\mathrm{Os}$ atomic/nuclear system. A new theoretical approach to calculating this process has also been developed and predicts a value for the “NEET probability,” $P_{\mathit{NEET}},$ of $1.3\times {10}^{-10}.$ $P_{\mathit{NEET}}$ is the probability that a given atomic excitation (in this case a K vacancy), will result in the excitation of a specific nuclear state (in this case the 69.5-keV level in ${}^{189}\mathrm{Os}).\mathrm{}$ This value is much lower than most of the calculated values given in the literature for this system. Our measurement gives the result $P_{\mathit{NEET}}<9\mathrm{}\times {10}^{-10},$ an upper limit which is several orders of magnitude lower than the values found in previous measurements, but which is consistent with the new calculation.