$\gamma$ emission from the 31-yr isomer of ${}^{178}\mathrm{Hf}$ induced by x-ray irradiation

A sample containing $6.3\times {10}^{14}$ nuclei of the ${16}^{+}$ isomer of ${}^{178}\mathrm{Hf}$ having a half life of 31 years and excitation energy of 2.446 MeV was irradiated with x-ray pulses derived from a device operated at 15 mA to produce bremsstrahlung radiation with an end point energy set to be 63 keV. Gamma spectra of the isomeric target were taken with two independent Ge detectors. Intensities of the 213.4 keV ${(4\mathrm{}}^{+}\to 2^{+})$ and 325.5 keV ${(6\mathrm{}}^{+}\to 4^{+})$ transitions in the ground state band of ${}^{178}\mathrm{Hf}$ were found to increase when irradiated. The largest enhancement was $1.6\pm 0.3\%$ measured in the 213.4 keV transition. Such an accelerated decay of the ${}^{178}\mathrm{Hf}$ isomer is consistent with an integrated cross section exceeding $2.2\times {10}^{-22}{\mathrm{cm}}^2\mathrm{keV}$ if the resonant absorption takes place below 20 keV as indicated by the use of selective absorption filters in the irradiating beam.