Upper limits for the energy-integrated cross section for the decay of the 31-yr ${}^{178}\mathrm{Hf}$ isomer, induced by x-ray irradiation, are extended to lower x-ray energies than we reported previously. Over the range of incident x-ray energies $6<\mathrm{}{E}_x$ (keV)<20, we report the energy-integrated cross section as less than $1\times {10}^{-26}{\mathrm{cm}}^2\mathrm{keV},$ with less stringent limits set down to 4 keV. These limits are at least 3 orders of magnitude below values for which enhancements to the isomer decay rate have been reported by other workers.

• Received 18 November 2002

DOI:https://doi.org/10.1103/PhysRevC.67.041305