Nuclear excitation by electronic transition in ¹⁸⁹Os

Monochromatic x rays have been used to explore the phenomenon of nuclear excitation by electronic transition (NEET) in the ¹⁸⁹Os atomic/nuclear system. A new theoretical approach to calculating this process has also been developed and predicts a value for the “NEET probability,” P_NEET, of 1.3×10^-10. P_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 ¹⁸⁹Os). This value is much lower than most of the calculated values given in the literature for this system. Our measurement gives the result P_NEET<9×10^-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.