The relativistic excitation of the double giant dipole resonance in ${}_{}{}^{136}{}_{}{}^{}\mathrm{Xe}$ has been calculated, describing the resonance as a harmonic vibration of the mean field, damped through its coupling to doorway states. While the predictions associated with the one-phonon states provide an overall account of the experimental findings, the calculated cross section for the two-phonon states is much smaller than that extracted from the involved analysis of the data. The experimental elucidation of this discrepancy seems essential for assessing the validity of the standard picture of giant resonances.

Transverse momentum spectra of η mesons have been measured near the free nucleon-nucleon production threshold in the heavy ion reactions ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$+ ${}_{}{}^{\mathrm{nat}}{}_{}{}^{}\mathrm{Ca}$, ${}_{}{}^{86}{}_{}{}^{}\mathrm{Kr}$ + ${}_{}{}^{\mathrm{nat}}{}_{}{}^{}\mathrm{Zr}$, and ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$+ ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$ at 1.0A GeV and also in ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$+ ${}_{}{}^{\mathrm{nat}}{}_{}{}^{}\mathrm{Ca}$ at 1.5A GeV. The measured transverse momentum distributions are compared to model calculations. The relative abundance of Δ(1232) and N(1535) resonances excited in the collision is deduced. A comparison to pion data reveals scaling with the transverse mass of the emitted meson.

The shell structure of exotic nuclei near proton and neutron drip lines is discussed in terms of the self-consistent mean-field theory. It is demonstrated that when approaching the neutron drip line, the neutron density becomes very diffused and the single-particle spectrum shows similarities to that of the harmonic oscillator with spin-orbit term. Interaction between bound orbitals and continuum is shown to result in quenching of shell effects in light and medium systems.

Peculiar cosmic ray events (Centauros) with anomalously large fluctuations in their isosopin composition have been observed during the past twenty years. Multiparticle symmetrization has been put forward as an explanation of the effect. In this Letter we show how the formation of pions through the sigma channel when combined with symmetrization can lead to a scenario where the isospin distribution is not only broadened but inverted. For reasonable choices of parameters we find that events can actually prefer consisting of either nearly all neutral or all charged pions.

The problem of cluster formation and growth in the first-order quark-hadron phase transition in heavy-ion collisions is considered. Behaving as Brownian particles, the clusters carry out random walks and can encounter one another, leading to coalescence and breakup. A simulation of the process in a cellular automaton suggests the possibility of a scaling distribution in the cluster sizes. The experimental determination of the cluster-size distribution is urged as a means to find a clear signature of the phase transition.

Nuclear level lifetimes have been measured in two superdeformed bands in ${}_{}{}^{194}{}_{}{}^{}\mathrm{Hg}$ using the Doppler-shift attenuation method. Average transition quadrupole moments derived from the lifetimes of an excited and yrast superdeformed bands are ${\mathit{Q}}_{\mathit{t}}$=17.6(30) and 17.2(20)eb, respectively. The Doppler shifts of the excited band relative to the yrast band indicate a slight difference in quadrupole moments [+4(5)%], assuming similar side feeding..ul2 These results imply that the second well is stable and rigid with respect to the particle excitation responsible for this excited band.

We present a practical solution to the ‘‘sign problem’’ in the auxiliary field Monte Carlo approach to the nuclear shell model. The method is based on extrapolation from a continuous family of problem-free Hamiltonians. To demonstrate the resultant ability to treat large shell-model problems, we present results for the ${}_{}{}^{54}{}_{}{}^{}\mathrm{Fe}$ in the full fp-shell basis using the Brown-Richter interaction. We find the Gamow-Teller ${\mathrm{\beta }}^{+}$ strength to be quenched by 58% relative to the single-particle estimate, in better agreement with experiment than previous estimates based on truncated bases.

The first measurement of the reaction ${}_{}{}^3{}_{}{}^{}\mathrm{He}$(γ→,p)X using linearly polarized photons is reported. Cross sections and beam-polarization asymmetries for ${\mathrm{\theta }}_{\mathit{p}}^{\mathrm{lab}}$=60°–100° and ${\mathit{E}}_{\gamma }$=195–304 MeV are compared with a microscopic calculation which includes one-, two-, and three-nucleon absorption mechanisms. One- and two-nucleon absorption alone fails to describe the data at low proton momenta. The inclusion of three-nucleon absorption significantly improves the comparison with the measured cross sections. However, some features of the asymmetry distributions are not explained.

The giant dipole resonance γ decay of ${}_{}{}^{240}{}_{}{}^{}\mathrm{Cf}$ excited to 67 and 93 MeV was measured from the fusion of ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$${+}^{208}$Pb. The γ spectrum and γ-fission angular correlation are analyzed in terms of prefission and postfission γ-ray yields using a stattistical model with nuclear viscosity in the fission process. The results show a strong increase in the prescission γ yield from 67 to 93 MeV. A saddle to scission time ${\mathrm{\tau }}_{\mathrm{s}\mathrm{s}\mathrm{c}=}$30×${10}^{\mathrm{-}21}$ s is extracted from this γ yield, equivalent to a normalized viscosity γ≊5 and full one-body dissipation.

We study how much gluon shadowing can be perturbatively generated through the modified QCD evolution in heavy nuclei. We model in simultaneously fusions from independent constituents and from the same constituent, both in a proton and in a large loosely bound nucleus of A∼200. In addition to the actual distributions at small x, we study the ratios of the distributions at an initial scale ${\mathit{Q}}_0$=2 GeV, and show that a strong nuclear shadowing can follow from the modified QCD evolution.

The moment of inertia of the yrast superdeformed band in ${}_{}{}^{149}{}_{}{}^{}\mathrm{Gd}$ exhibits an unexpected bifurcation at high rotational frequency. States differing by four units of angular momentum show an energy shift of about 60 eV. This indicates the remnant of a new quantum number associated with the fourfold rotational symmetry.

We examine the empirical domain of validity of statistical theory, as applied to fission data on prefission neutron, charged particle, and γ-ray multiplicities. Systematics are found in the threshold excitation energy for the appearance of nonstatistical fission. From the data on systems with not too high fissility,, the relevant phenomenological parameter is the ratio of the threshold temperature ${\mathit{T}}_{\mathrm{thresh}}$ to the (temperature dependent) fission barrier height ${\mathit{E}}_{\mathrm{b}\mathrm{a}\mathrm{r}\left(\mathit{T}\right)}$. The statistical model reproduces the data for ${\mathit{T}}_{\mathrm{thresh}}$/${\mathit{E}}_{\mathrm{b}\mathrm{a}\mathrm{r}\left(\mathit{T}\right)<\mathrm{}0.26\mathrm{}}$±0.05 but underpredicts the multiplicities at higher ${\mathit{T}}_{\mathrm{thresh}}$/${\mathit{E}}_{\mathrm{b}\mathrm{a}\mathrm{r}\left(\mathit{T}\right)}$ independent of mass and fissility of the systems.

A search for invisible axions is carried out by looking for invisible M1 transitions in ${}_{}{}^{139}{}_{}{}^{}\mathrm{La}$(5/$2^{+}$→7/$2^{+}$) with a transition energy of 166 keV. A limit to the branching ratio of axion emission to that of γ emission is obtained to be ${\mathrm{\Gamma }}_{\mathit{a}}$/${\mathrm{\Gamma }}_{\gamma }$<1.21×${10}^{\mathrm{-}6}$ at the 95% confidence level. Hadronic axions heavier than 26.7 keV are excluded by this upper limit. It is also concluded that the branching ratio of the second forbidden electron capture decay of ${}_{}{}^{139}{}_{}{}^{}\mathrm{Ce}$ into the ground state of ${}_{}{}^{139}{}_{}{}^{}\mathrm{La}$(7/$2^{+}$) is less than 9.7×${10}^{\mathrm{-}7}$ at the 95% confidence level.

The mass of the nucleus ${}_{}{}^{11}{}_{}{}^{}\mathrm{Li}$ has been determined from a measurement of the $Q$ value of the reaction ${}_{}{}^{14}{}_{}{}^{}\mathrm{C}$(${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$,${}_{}{}^{11}{}_{}{}^{}\mathrm{Li}$)${}_{}{}^{14}{}_{}{}^{}\mathrm{O}$ at $\frac{E}{A}\approx 32$ MeV. The results, which indicate a two-neutron separation energy of $S_{2n}\left({}_{}{}^{11}{}_{}{}^{}\mathrm{Li}\right)=295\mathrm{}\pm 35$ keV, put this basic quantity on a firm basis for use in theoretical models of the halo nucleus ${}_{}{}^{11}{}_{}{}^{}\mathrm{Li}$.

Cross sections for the ${}_{}{}^8{}_{}{}^{}\mathrm{Li}$(d,n${)}^9$Be (ground state) and ${}_{}{}^8{}_{}{}^{}\mathrm{Li}$(d,t${)}^7$Li reactions, both important to primordial nucleosynthesis in the inhomogeneous models, have been measured using a radioactive beam technique. The cross section for the former reaction is found to be small, so it is important only for synthesis of ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$. The cross section for the latter reaction, however, is found to be large enough to destroy significant quantities of ${}_{}{}^8{}_{}{}^{}\mathrm{Li}$, and thus could affect predictions of primordial nucleosynthesis yields.

Multifragment production was measured for the 60 MeV/nucleon ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$${+}^{27}$Al, ${}_{}{}^{51}{}_{}{}^{}\mathrm{V}$, and ${}_{}{}^{\mathrm{nat}}{}_{}{}^{}\mathrm{Cu}$ reactions. The branching ratios for binary, ternary, quaternary, and quinary decays were determined as a function of the excitation energy E and are independent of the target. The logarithms of these branching ratios when plotted vs ${\mathit{E}}^{\mathrm{-}1/2}$ show a linear dependence that strongly suggests a statistical competition between the various multifragmentation channels. This behavior seems to relegate the role of dynamics to the formation of the sources, which then proceed to decay in an apparently statistical manner.

Phase shift parameters for p-d elastic scattering are extracted from new measurements of spin observables at ${\mathit{E}}_{\mathit{p}}$=3 MeV. The experimental phase shifts are compared with predictions obtained from Faddeev calculations. The agreement between theory and experiment is generally good. However, we find that there are significant discrepancies for the ${}_{}{}^2{}_{}{}^{}$${\mathit{S}}_{1/2}$ phase shift and for the S-D mixing parameters.

Inelastic scattering of 50 MeV/nucleon ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ on ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ was measured in coincidence with protons in order to investigate the excitation and decay of highly excited states in ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$. The proton decay pattern corresponding to an excitation region around 34 MeV shows the characteristic behavior expected of the direct decay of a two-phonon state. Thus, by using this novel method, the presence of a double-phonon state built with isoscalar giant resonances in ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ is demonstrated.

The dependence on invariant mass M and transverse momentum ${\mathit{q}}_{\mathrm{\perp }}$ of lepton pairs emitted by quarks during the early stage of central collisions between gold nuclei at √s =200A GeV is investigated within the parton cascade model. For fixed transverse mass ${\mathit{M}}_{\mathrm{\perp }}$ between 2 and 3 GeV, the spectrum is thermal shaped at low ${\mathit{q}}_{\mathrm{\perp }}$ and exhibits a power law behavior at larger ${\mathit{q}}_{\mathrm{\perp }}$. A suspected ${\mathit{M}}_{\mathrm{\perp }}$ scaling is not observed in the calculated distribution, but the significant shape of the spectrum and the large yield from preequilibrium emission may nevertheless serve as a characteristic signal for a parton plasma formation.

Relativistic mean field theory in the rotating frame is used to describe superdeformed nuclei. Nuclear currents and the resulting spatial components of the vector meson fields are fully taken into account. Identical bands in neighboring rare earth nuclei are investigated and excellent agreement with recent experimental data is observed.