Study of the gamma emission from the 31-year isomer of 178Hf induced by X-ray irradiation

A sample containing 6.3x1014 nuclei of the 16+ isomer of 178Hf 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 15mA to produce Bremsstrahlung radiation with an end point energy set to be 90 keV. Gamma-spectra of the isomeric target were taken with a Ge-detector. Intensity of the 325.5 keV (6+→4+ transition in the ground state band of 178Hf was found to increase by about 2%. Such an accelerated decay of the 178Hf isomer is consistent with an integrated cross section of 3x10-23 cm2·keV if the resonant absorption takes place within the energy ranges corresponding to the maxima of the x-ray flux, either near 20 keV, or at the K emission lines of W