No abstract

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

A sample containing 6.3ϫ10 14 nuclei of the 16 ϩ isomer of 178 Hf 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 ϩ →2 ϩ ) and 325.5 keV (6 ϩ →4 ϩ ) transitions in the ground state band of 178 Hf were found to increase when irradiated. The largest enhancement was 1.6Ϯ0.3% measured in the 213.4 keV transition. Such an accelerated decay of the 178 Hf isomer is consistent with an integrated cross section exceeding 2.2ϫ10 Ϫ22 cm 2 keV if the resonant absorption takes place below 20 keV as indicated by the use of selective absorption filters in the irradiating beam.

A process for transferring energy from electron shells into nuclear excitation, NEET, has offered the promise for modulating nuclear properties at accessible levels of power. It had been proven recently by exciting a nuclear level of 197Au with synchrotron radiation, but measured couplings were far below theoretical objectives. Reported here is an extension of that approach for excitation to 178Hf m2 isomeric nuclei. Isomeric targets were irradiated with X-rays in the beamline BL01B1 at the synchrotron radiation source SPring-8. Energies were tuned from 9 to 13 keV. In this range an excitation branch attributed to NEET was found to have a probability of 2 × 10−3 relative to L-shell photoionization. The resulting emission of exoergic γ-photons was observed from the target at a rate approaching the theoretical maximum.