Accelerated Emission of Gamma Rays from the 31-yr Isomer of 178 Hf Induced by X-Ray Irradiation

A sample of 6.3{times}10{sup 14} nuclei of the 4-quasiparticle isomer of {sup 178}Hf having a half-life of 31thinspthinspyr and excitation energy of 2.446thinspthinspMeV was irradiated with x-ray pulses from a device typically used in dental medicine. It was operated at 15thinspthinspmA to produce bremsstrahlung radiation with an end point energy set to be 70 or 90thinspthinspkeV. Spectra of the isomeric target were taken with a high purity Ge detector. Intensities of selected transitions in the normal decay cascade of the {sup 178}Hf isomer were found to increase by about 4{percent} . Such an accelerated decay is consistent with an integrated cross section of 1{times}10{sup {minus}21}thinspcm{sup 2}thinspkeV for the resonant absorption of x rays to induce gamma decay. {copyright} {ital 1999} {ital The American Physical Society }