Accelerated emission of gamma rays from the 31-yr isomer of Hf-178 inducedby X-ray irradiation

A sample of 6.3 X 10(14) nuclei of the 4-quasiparticle isomer of Hf-178 hav ing a half-life of 31 yr and excitation energy of 2.446 MeV was irradiated with x-ray pulses from a device typically used in dental medicine. It was o perated at 15 mA to produce bremsstrahlung radiation with an end point ener gy set to be 70 or 90 keV. Spectra of the isomeric target were taken with a high purity Ge detector. Intensities of selected transitions in the normal decay cascade of the Hf-178 isomer were found to increase by about 4%. Suc h an accelerated decay is consistent with an integrated cross section of 1 x 10(-21) cm(2) keV for the resonant absorption of x rays to induce gamma d ecay.