γ emission from the 31-yr isomer of 178Hf induced by x-ray irradiation

A sample containing 6.3×10¹⁴ 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 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⁺-->2⁺) and 325.5 keV (6⁺-->4⁺) transitions in the ground state band of ¹⁷⁸Hf were found to increase when irradiated. The largest enhancement was 1.6+/-0.3% measured in the 213.4 keV transition. Such an accelerated decay of the ¹⁷⁸Hf isomer is consistent with an integrated cross section exceeding 2.2×10^-22 cm² keV if the resonant absorption takes place below 20 keV as indicated by the use of selective absorption filters in the irradiating beam.