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The atomic nucleus and its electrons are often thought of as independent systems that are held together in the atom by their mutual attraction. Their interaction, however, leads to other important effects, such as providing an additional decay mode for excited nuclear states, whereby the nucleus releases energy by ejecting an atomic electron instead of by emitting a γ-ray. This 'internal conversion' has been known for about a hundred years and can be used to study nuclei and their interaction with their electrons. In the inverse process-nuclear excitation by electron capture (NEEC)-a free electron is captured into an atomic vacancy and can excite the nucleus to a higher-energy state, provided that the kinetic energy of the free electron plus the magnitude of its binding energy once captured matches the nuclear energy difference between the two states. NEEC was predicted in 1976 and has not hitherto been observed. Here we report evidence of NEEC in molybdenum-93 and determine the probability and cross-section for the process in a beam-based experimental scenario. Our results provide a standard for the assessment of theoretical models relevant to NEEC, which predict cross-sections that span many orders of magnitude. The greatest practical effect of the NEEC process may be on the survival of nuclei in stellar environments, in which it could excite isomers (that is, long-lived nuclear states) to shorter-lived states. Such excitations may reduce the abundance of the isotope after its production. This is an example of 'isomer depletion', which has been investigated previously through other reactions, but is used here to obtain evidence for NEEC.

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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

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.

The time delays in fission induced by bombardment of W with 180 MeV 32 S, 240-255 MeV 48 Ti, 330-375 MeV 58 Ni, and 390 MeV 74 Ge have been measured by observation of crystal blocking. Nearly all results are consistent with exponential decay with lifetimes of order 10 −18 s which depend weakly on the atomic number of the composite nucleus. This is inconsistent with the Bohr-Wheeler model of fission from a compound nucleus in statistical equilibrium at each stage in a neutron evaporation cascade and supports a picture of strongly damped quasifission. A simple diffusion model with one-body dissipation reproduces roughly the observed time scale and the exponential decay. It suggests that the outer fission barrier could play a significant role in the observed, very slow decays.