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A sample of 6.3 3 10 14 nuclei of the 4-quasiparticle isomer of 178 Hf having 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 operated at 15 mA to produce bremsstrahlung radiation with an end point energy 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 178 Hf isomer were found to increase by about 4%. Such an accelerated decay is consistent with an integrated cross section of 1 3 10 221 cm 2 keV for the resonant absorption of x rays to induce gamma decay. [S0031-9007(98)08333-1] PACS numbers: 25.20.Dc, 21.10.Ma, 21.10.Tg, 27.70. + qThe 4-and 5-quasiparticle isomers of Lu, Hf, and Ta are interesting because they have relatively long lifetimes for states with 2 to 3 MeV excitation energies. They are termed K isomers because spontaneous radiative decay is hindered by structural changes forbidden by K quantum numbers. In this mass region the nuclei are deformed, and the projection of the total angular momentum upon the symmetry axis contributes this quantum number K which should change during a radiative transition by no more than the multipolarity of the mediating moment. Transitions from the high-K isomer to the rotational states of a low-K band are "forbidden," and so relatively long lifetimes are inevitable. The most interesting example may be the 31-yr, 4-quasiparticle isomer 178 Hf having a 2.446 MeV excitation energy.Proposals to trigger the release of the energy of a nuclear isomer by exciting it to some higher level associated with freely radiating states have been known for over a decade [1]. To be efficient such schemes require the existence at an energy near that of the isomer of a state of mixed K. It was proposed [1] to use the resonant absorption of x rays from a bremsstrahlung source to excite some fraction of a high-K isomeric population to the K-mixing level. From there some decay to one or more heads of low-K cascades could subsequently release the total energy of the isomer plus that of the absorbed trigger photon.The types of K-mixing states needed in such schemes to induce the decay of nuclear isomers have been reported [2] in 180 Ta and described in 174 Hf and other isomers [3]. In the case of the Ta, the resonant absorption of x rays excited the 2-quasiparticle isomer of 180 Ta to a K-mixing level [4] at 2.8 MeV which then spontaneously decayed in part to the ground state through a gamma cascade. The integrated cross section for the resulting deexcitation of the isomer was 1.2 3 10 225 cm 2 keV. Studies of systematics have shown [5] that a similar K-mixing level could be reasonably expected in 178 Hf not more than 300 keV above the 2.446 MeV, 16 1 level of the 4-quasiparticle isomer. However, quantitative 0031-9007͞99͞82(4)͞695(4)$15.00

Induced release of the high energy densities stored in isomeric nuclear states may be important in the development of ultrashort wavelength lasers. Such a release could compensate the spontaneous power density radiated from the laser medium at threshold. The most promising candidate for such a role seems to be the 31-yr isomeric nucleus of Hf-178 that stores 1.3 GJ/g in the electromagnetic excitation of its constituent protons and neutrons. Successful studies of the induced release of energies from such isomeric states have required an extension of techniques for nuclear resonance spectroscopy using synchrotron radiation (SR) that had previously been applied only to ground state materials. In 2004, monochromatic X-rays from the SPring-8 SR source were used to identify one of the excited nuclear states that mediates the induced decay of the 31-yr isomer of Hf-178. That trigger level was found to lie at 2457.20(22) keV. It was excited when an isomeric nucleus absorbed an incident X-ray photon. We found that one branch of its subsequent decay consisted of a strong electromagnetic transition to the ground state of the nucleus. The energy of the γ-photon emitted was equal to the energy of the trigger level. Proximity in energy of that level to the energy of 2446.06 keV stored by the isomer makes it easy to induce a release of the stored energy and encourages prospects for the development of a gamma ray laser. Spectrum of γ-photons emitted from a sample of the 31-yr isomeric (16+) state of Hf-178 being irradiated with monochromatic X-rays from the SPring-8 synchrotron source.