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The time delays in fission induced by bombardment of W with 180 MeV $^{32}\mathrm{S}$, $240\text{\ensuremath{-}}255$ MeV $^{48}\mathrm{Ti}$, $330\text{\ensuremath{-}}375$ MeV $^{58}\mathrm{Ni}$, and 390 MeV $^{74}\mathrm{Ge}$ have been measured by observation of crystal blocking. Nearly all results are consistent with exponential decay with lifetimes of order ${10}^{\ensuremath{-}18}$ 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.

A sample of $6.3\ifmmode\times\else\texttimes\fi{}{10}^{14}$ nuclei of the 4-quasiparticle isomer of ${}^{178}\mathrm{Hf}$ 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 ${}^{178}\mathrm{Hf}$ isomer were found to increase by about $4%$. Such an accelerated decay is consistent with an integrated cross section of $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21}\mathrm{cm}{}^{2}\mathrm{keV}$ for the resonant absorption of x rays to induce gamma decay.

In inelastic scattering of 22 MeV deuterons from an isotope-separated target of $^{178}\mathrm{Hf}$ containing about 2\ifmmode\times\else\texttimes\fi{}${10}^{13}$ nuclei in the isomeric ${16}^{+}$ state we observed rotational excitation to the ${17}^{+}$ state at an excitation energy with respect to the isomeric state of 356.5\ifmmode\pm\else\textpm\fi{}0.4 keV and weak evidence for the ${18}^{+}$ state at 737\ifmmode\pm\else\textpm\fi{}2 keV. We compared the differential cross sections with coupled-channel calculations and with scattering from $^{178}\mathrm{Hf}$ (${0}^{+}$) and $^{177}\mathrm{Hf}$ (7/${2}^{\mathrm{\ensuremath{-}}}$). \textcopyright{} 1996 The American Physical Society.

Time delays in fission induced by bombardment of Mo with 170- and 180-MeV ${}^{32}$S, 225- and 240-MeV ${}^{48}$Ti, and 300-MeV ${}^{58}$Ni have been measured by observation of crystal blocking of fission fragments. In contrast to earlier measurements with a W target, the results are consistent with fission of a compound nucleus in competition with mainly neutron emission. Most of the fissions happen on a time scale much shorter than attoseconds but there is a significant component of fission with much longer lifetimes. The measurements are reproduced with a standard statistical model, including a Kramers correction to fission widths from the viscosity of hot nuclear matter. These new results support the interpretation of our earlier measurements with a W target, which indicate that there is a transition in heavy-ion-induced fission at large atomic number and mass, from multichance fission in the standard Bohr-Wheeler picture to fission without formation of a compound nucleus. The process is slowed down by nuclear viscosity, with measured delays of order attoseconds.

The scattering of thermal neutrons from isomeric nuclei may include events in which the outgoing neutrons have increased kinetic energy. This process has been called inelastic neutron acceleration, or INNA, and occurs when the final nucleus, after emission of the neutron, is left in a state with lower energy than that of the isomer. The result, therefore, is an induced depletion of the isomer to the ground state. A cascade of several $\ensuremath{\gamma}$'s must accompany the neutron emission to release the high angular momentum of the initial isomeric state. INNA was previously observed in a few cases, and the measured cross sections were only in modest agreement with theoretical estimates. The most recent measurement of an INNA cross section was ${\ensuremath{\sigma}}_{\mathrm{INNA}}=258\ifmmode\pm\else\textpm\fi{}58 \mathrm{b}$ for neutron scattering by $^{177}\mathrm{Lu}$${}^{m}$. In the present work, an INNA cross section of ${\ensuremath{\sigma}}_{\mathrm{INNA}}=168 \ifmmode\pm\else\textpm\fi{} 33 \mathrm{b}$ was deduced from measurements of the total burnup of the high-spin, four-quasiparticle isomer $^{178}\mathrm{Hf}$${}^{m2}$ during irradiation by thermal neutrons. Statistical estimates for the probability of different reaction channels past neutron absorption were used in the analysis, and the deduced ${\ensuremath{\sigma}}_{\mathrm{INNA}}$ was compared to the theoretically predicted cross section.

A 98% pure Am-242m ( K = 5(-), t(1/2) = 141 years) isomeric target was Coulomb excited with a 170.5-MeV Ar-40 beam. The selectivity of Coulomb excitation, coupled with the sensitivity of Gammasphere plus CHICO, was sufficient to identify 46 new states up to spin 18h in at least four rotational bands; 11 of these new states lie in the isomer band, 13 in a previously unknown yrast K-pi = 6(-) rotational band, and 13 in a band tentatively identified as the predicted yrast K-pi = 5(+) band. The rotational bands based on the K-pi = 5- isomer and the 6(-) bandhead were populated by Coulomb excitation with unexpectedly equal cross sections. The gamma-ray yields are reproduced by Coulomb excitation calculations using a two-particle plus rotor model (PRM), implying nearly complete Delta K = 1 mixing of the two almost-degenerate rotational bands, but recovering the Alaga rule for the unperturbed states. The degeneracy of the 5(-) and 6(-) bands allows for precise determination of the mixing interaction strength V, which approaches the strong-mixing limit; this agrees with the 50% attenuation of the Coriolis matrix element assumed in the model calculations. The fractional admixture of the I-K(pi) = 6(6)(-) state in the nominal 6(5)(-) isomer band state is measured within the PRM as 45.6(-1.1)(+0.3)%. The E2 and M1 strengths coupling the 5(-) and 6(-) bands are enhanced significantly by the mixing, while E1 and E2 couplings to other low-K bands are not measurably enhanced. The yields of the 5(+) band are reproduced by an E3 strength of approximate to 15 W.u., competitive with the interband E2 strength. Alignments of the identified two-particle Nilsson states in Am-242 are compared with the single-particle alignments in Am-241. (Less)

The time delay in fission induced by bombardment of W with 180 MeV $^{32}\mathrm{S}$, 240--255 MeV $^{48}\mathrm{Ti}$, and 315--375 MeV $^{58}\mathrm{Ni}$ has been measured by observation of crystal blocking. There is a clear narrowing and a small increase in the minimum yield of the angular dips for fission compared with scaled dips for elastically scattered ions. This is interpreted as a fission delay of about 2 as, only weakly dependent on energy and atomic number. The delay is longer by 1 to 2 orders of magnitude than obtained from standard interpretations of measurements of prescission neutrons and giant-dipole-resonance gamma rays and from calculations of the nuclear dynamics in heavy-ion reactions.

An experiment has been performed to detect the $\ensuremath{\alpha}$ emission mode in ${}^{178}\mathrm{Hf}{}^{m2}$ isomer decay and a partial half-life of $(2.5\ifmmode\pm\else\textpm\fi{}0.5)\ifmmode\times\else\texttimes\fi{}{10}^{10}$y was measured. It was concluded that $\ensuremath{\alpha}$ decay is strongly retarded by the centrifugal barrier arising due to the high spin of this isomeric state. Additional analysis shows, however, that the $K$-hindrance in this $\ensuremath{\alpha}$ decay is relatively weak, despite the strong manifestation of spin-hindrance.

The availability of a target of $^{178}\mathrm{Hf}$ in its aligned four-quasiparticle, ${\mathit{J}}^{\mathrm{\ensuremath{\pi}}}$=${16}^{+}$ isomeric state, has allowed to measure the effect of blocking of the aligned-quasineutron orbitals in the (p,t) reaction. The experimental ratio of cross sections R=${\mathrm{\ensuremath{\sigma}}}_{(16}^{+}$\ensuremath{\rightarrow}${16}^{+}$)/${\mathrm{\ensuremath{\sigma}}}_{(0}^{+}$\ensuremath{\rightarrow}${0}^{+}$) agrees with a predicted value deduced from the known (p,t) cross section on odd Hf isotopes.

Fission-fragment mass distributions in the ${}^{232}\mathrm{Th}(\ensuremath{\gamma},f)$ reaction have been measured from the cumulative yields of radionuclides following activation at bremsstrahlung endpoint energies ${E}_{e}=7.5,$ 12.0, and 24.0 MeV. The yields of the light nuclei ${}^{24}\mathrm{Na}$ and ${}^{28}\mathrm{Mg}$ were detected following activation at ${E}_{e}=16.5$ and 24.0 MeV. Energy dependences of the symmetric mode and light nuclide yields are discussed. Fine-structure manifestations near asymmetric and symmetric masses were observed.

A sample containing 6.3x10{sup 14} nuclei of the 16{sup +} isomer of {sup 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{sup +}{yields}2{sup +}) and 325.5 keV (6{sup +}{yields}4{sup +}) transitions in the ground state band of {sup 178}Hf 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 {sup 178}Hf isomer is consistent with an integrated cross section exceeding 2.2x10{sup -22} cm{sup 2} keV if the resonant absorption takes place below 20 keV as indicated by the use of selective absorption filters in the irradiating beam. (c) 2000 The American Physical Society.

A Reply to the Comment by Silviu Olariu and Agata Olariu. Also D. P. McNabb, et al. also P. von Neumann-Cosel and A. Richter. (c) 2000 The American Physical Society.