We have studied the atomic charge state dependence of the nuclear lifetime of the 35.5-keV first excited state of ${}^{125}$Te. For the 4$7^{+}$ and 4$8^{+}$ ions, 300% and 640% increases, respectively, of the half-life were found with respect to the neutral-atom value (1.49 ns). These unusually large effects are due to the energetic blocking of $K$-shell internal conversion as the charge state increases past 4$7^{+}$.

Reactions of the halo systems ${}^{11}\mathrm{Be}$ and ${}^{11}\mathrm{Li}$ (at 460 and 280 MeV/nucleon) with a carbon target demonstrate that ( ${n+}^9\mathrm{Li}$) has an (unbound) $l=0\mathrm{}$ ground state very close to the threshold. The neutron halo of ${}^{11}\mathrm{Li}$ has appreciable $({1s}_{1/2}{)}^2$ and $({0p}_{1/2}{)}^2$ components.

High-spin states in ${}^{84}\mathrm{Zr}$ were studied using the early implementation phase of the Gammasphere array and the “Microball” charged-particle detector system. A cascade of nine $\gamma$ rays with a dynamic moment of inertia which is characteristic of superdeformed rotational bands in the $A=80\mathrm{}$ region has been identified and assigned to ${}^{84}\mathrm{Zr}$. The measured transition quadrupole moment of the band corresponds to a prolate quadrupole deformation of ${\beta }_2=0.53\mathrm{}$ and confirms the superdeformed nature of this band. This is the first direct experimental confirmation of the existence of the predicted superdeformed shell gap at $N\simeq 44$ particle number.

Multifragment decays of central collisions in ${}^{84}\mathrm{Kr}{+}^{197}\mathrm{Au}$ at $E/A=35\mathrm{}$, 55, and 70 MeV are studied. The dependence of the extracted emission time on the velocity of the fragment pair is investigated. More energetic pairs manifest a stronger Coulomb interaction, indicating emission from a source of smaller spatial-temporal extent than less energetic pairs. This trend can be understood in the context of a statistical model that allows the source characteristics to evolve as the fragments are emitted.

We report on measurements of low-mass electron pairs in 450 GeV $p$-Be, $p$-Au, and 200 GeV/nucleon S-Au collisions at central rapidities. For the proton induced interactions, the low-mass spectra are, within the systematic errors, satisfactorily explained by electron pairs from hadron decays, whereas in the S-Au system an enhancement over the hadronic contributions by a factor of $5.0\pm 0.7\left(\mathrm{stat}\right)\pm 2.0\left(\mathrm{syst}\right)$ in the invariant mass range $0.2<m<\mathrm{}1.5\mathrm{GeV}{/c}^2$ is observed. The properties of the excess suggest that it arises from two-pion annihilation $\pi \pi \to e^{+}{e}^{-}$.

Statistical $\gamma$-decay spectra from excited nuclear states are calculated. The spectra reflect the perturbation of the level density by pair correlations and, hence, the stepwise weakening of the pair correlations with increasing quasiparticle number. The level density is obtained by counting many-quasiparticle states from a self-consistent BCS calculation or from a diagonalization of particle number-projected states. The calculated spectra resemble measured spectra from the decay out of superdeformed bands, but have only $\sim 70$% of the measured intensity.

It is shown that the anharmonic vibrator reproduces the ground band energies and $B\left(E2\mathrm{}\right)$ values, not only of vibrational and transitional nuclei, but of good rotor nuclei, at least as well as other models, including those designed specifically for rotational nuclei.

We propose a Monte Carlo method for solving the quantum many-body interacting systems. Mean fields dominating the structure of low-lying states are selected by a Monte Carlo method, which generates optimum many-body basis states for diagonalizing the Hamiltonian consisting of one- and two-body terms. Not only the ground state but also low-lying excited states are obtained with their wave functions. Results are examined by comparison to exact values.

A breakdown of the impulse approximation is studied in pion photoproduction on ${}^3\mathrm{He}$ at high momentum transfers. The usual distorted wave impulse approximation formalism with Faddeev wave functions that works well for small momentum transfers deviates from experimental measurements by up to 2 orders of magnitude for $Q^2>6\mathrm{}{\mathrm{fm}}^{-2\mathrm{}}$. It is found that the explicit inclusion of two-body mechanisms, where the photon is absorbed on one nucleon and the pion is emitted from another nucleon, removes most of the disagreement with the data.

Fragment distributions resulting from $\mathrm{Au}+\mathrm{Au}$ collisions at an incident energy of $E/A=600\mathrm{MeV}$ are studied. From the measured fragment and neutron distributions the mass and the excitation energy of the decaying prefragments were determined. A temperature scale was derived from observed yield ratios of He and Li isotopes. The relation between this isotope temperature and the excitation energy of the system exhibits a behavior which is expected for a phase transition. The nuclear vapor regime takes over at an excitation energy of 10 MeV per nucleon, a temperature of 5 MeV, and may be characterized by a density of 0.15–0.3 normal nuclear density.

We explain how, in the thermodynamic interpretation of multiparticle reactions, it may be possible to experimentally determine a basic quantity characterizing the presumed thermodynamic system: the heat capacity. We exploit the fact that new heavy-ion experiments have such a high multiplicity that a “temperature” can be assigned on an event-by-event basis. This permits temperature fluctuations also to be defined. In a thermodynamic system, the latter are related to the heat capacity. The heat capacity may then be used to search for phase transitions and to study other thermodynamic aspects of the system.

Differential $\mathrm{np}$ cross sections for unpolarized neutrons of 162 MeV have been measured to high precision with particular attention to the absolute normalization. These data can be extrapolated precisely and model independently to the pion pole and give a $\pi \mathrm{NN}$ coupling constant $g^2=14.62\mathrm{}\pm 0.06\pm 0.29$ or $f^2=0.0808\mathrm{}\pm 0.0003\pm 0.0017$. This is higher than recently suggested values.

Alternating-parity states connected by strong $E1\mathrm{}$ transitions, characteristic of a reflection-asymmetric rotor, have been observed to high spins in the isotones ${}^{220}$Ra and ${}^{222}$Th. This level structure is observed up to $J^{\pi }{=29\mathrm{}}^{-}\left({31}^{-}\right)$ in ${}^{220}$Ra while it cannot be seen beyond $J^{+}{=24\mathrm{}}^{+}\left({25}^{-}\right)$ in ${}^{222}$Th. These observations are consistent with Woods-Saxon-Bogolyubov cranking calculations which predict that the yrast band of ${}^{222}$Th will undergo a shape transition at $J=24\mathrm{}ħ$, in contrast to that of ${}^{220}$Ra which maintains its reflection asymmetry to higher spins.

Nuclear states in ${}^{28}\mathrm{Si}$ were populated at an initial excitation energy $E^{*}=50\mathrm{}$ MeV via the isospin $T=0\mathrm{}$ reaction ${}^{16}\mathrm{O}{+}^{12}\mathrm{C}$. The dilepton- and photon-decay yields were measured simultaneously for the first time up to transition energies of 30 MeV. Statistical decay calculations describe well both measurements on an absolute scale. The measured absolute dilepton cross section, integrated over the transition energy region 15–30 MeV, amounts to $\sigma \left(e^{+}{e}^{-}\right)=84\mathrm{}\pm 6$ nb. Implications for measuring a possible monopole strength are discussed.

A Comment on the Letter by M. Thoennessen and G. F. Bertsch, Phys. Rev. Lett. 71, 4303 (1993). The authors of the Letter offer a Reply.

A Reply to the Comment by H. van der Ploeg et al.

A Comment on the Letter by M. L. Gilkes, et al., Phys. Rev. Lett. 73, 1590 (1994). The authors of the Letter offer a Reply.

A Reply to the Comment by Wolfgang Bauer and William A. Friedman.

A $T_{1/2}=220\mathrm{}\mathrm{ns}$, eight-quasiparticle isomer, with four unpaired neutrons and four unpaired protons, has been established at an excitation energy of 6576 keV in the prolate deformed nucleus, ${}^{178}\mathrm{W}$. The associated rotational band has also been identified, revealing the collective properties in the presence of blocked pairing correlations, with expected quenching of the nuclear superfluidity. The band retains a small degree of rotational alignment, and has a less-than-rigid dynamic moment of inertia.

A self-consistent method of treating excitations of the proton-neutron quasiparticle random-phase approximation is presented. The non-self-consistent methods violate the Pauli exclusion principle and lead to an eventual collapse of the ground state. This behavior renders a reliable calculation of the nuclear matrix elements, relevant for the prediction of double-beta-decay half-lives, difficult. The present formalism promotes the Pauli exclusion principle and avoids the collapse of the double-beta-decay matrix elements. We have applied this formalism to the double beta decay of ${}^{100}$Mo.