The exact solution of the many-body problem in the framework of the nuclear shell model with a realistic residual Hamiltonian makes it possible to study the fragmentation of simple configurations as a function of excitation energy and interaction strength. The analysis is performed for 839 states with quantum numbers ${\mathit{J}}^{\mathrm{\pi }}$T=$0^{+}$0 in a system of 12 valence particles within the sd shell. Our statistics allow us to establish the generic shape of the strength function in the region of strong mixing. For the realistic interaction, the strength function is close to Gaussian in the central part and has exponential wings. The spreading width is larger than predicted by the standard golden rule. At the artificially suppressed interaction strength, we recover the Breit-Wigner shape and the golden rule for the spreading width. The transition between these regimes agrees with theoretical considerations based on the idea of chaotic dynamics. © 1996 The American Physical Society.

Energy-level statistics are considered for nuclei whose Hamiltonian is divided into intrinsic and collective-vibrational terms. The levels are described as a random superposition of independent sequences, each corresponding to a given number of phonons. The intrinsic motion is assumed chaotic. The level spacing distribution is found to be intermediate between the Wigner and Poisson distributions and similar in form to the spacing distribution of a system with classical phase space divided into separate regular and chaotic domains. We have obtained approximate expressions for the nearest neighbor spacing and cumulative spacing distribution valid when the level density is described by a constant-temperature formula and not involving additional free parameters. These expressions have been able to achieve good agreement with the experimental spacing distributions. © 1996 The American Physical Society.

Collective radial flow of light fragments from ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$${+}^{45}$Sc reactions at beam energies between 35 and 115 MeV/nucleon has been investigated using the Michigan State University 4π Array. The mean transverse kinetic energy 〈${\mathit{E}}_{\mathit{t}}$〉 of the different fragment types increases with event centrality and increases as a function of the incident beam energy. Comparison of our measured values of 〈${\mathit{E}}_{\mathit{t}}$〉 shows agreement with predictions of Boltzmann-Uehling-Uhlenbeck model and WIX multifragmentation model calculations. The radial flow extracted from 〈${\mathit{E}}_{\mathit{t}}$〉 accounts for approximately half of the emitted particle’s energy for the heavier fragments (Z≥4) at the highest beam energy studied. © 1996 The American Physical Society.

The total photoabsorption cross section for ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$, C, Al, Cu, Sn, Pb has been measured in the energy range 300–1200 MeV at Frascati with the jet-target tagged photon beam. A 4π NaI crystal detector and a lead-glass shower counter were used, respectively, to measure hadronic events and to reject the electromagnetic background. Data above 600 MeV clearly indicate a broadening of higher nucleon resonance peaks in nuclei and a reduction of the absolute value of the cross section per nucleon with respect to the free-nucleon case. This large broadening suggests a strong influence of the nuclear medium in the resonance propagation and interaction, while the systematic reduction of the measured cross sections might be due to a depletion of the resonance excitation strength and to the onset of the shadowing effect around 1 GeV. Moreover, our systematic study indicates that also the Δ-resonance excitation parameters are not the same for all nuclei, being its mass and width increasing with the nuclear density. © 1996 The American Physical Society.

Total reaction cross sections, ${\mathrm{\sigma }}_{\mathit{R}}$, of 20–60A MeV ${}_{}{}^{4,6,8}{}_{}{}^{}\mathrm{He}$, ${}_{}{}^{6–9,11}{}_{}{}^{}\mathrm{Li}$, and ${}_{}{}^{10}{}_{}{}^{}\mathrm{Be}$ were measured by injecting magnetically separated, focused, monoenergetic, identified secondary beams of those projectiles into a Si detector telescope and measuring their energy-deposition spectra. These ${\mathrm{\sigma }}_{\mathit{R}}$’s, accurate to about 3%, were compared with predictions of optical, strong absorption, and microscopic models. The latter gave the best overall fit to the data, providing long-tailed matter densities were assumed. The best available optical potentials generally overpredicted the data by about 10%. Strong absorption calculations, in which the isospin-dependent term is quite important, were often unsuccessful, especially for projectiles with large neutron excess. Two-neutron removal cross sections were measured for ${}_{}{}^6{}_{}{}^{}\mathrm{He}$ and ${}_{}{}^{11}{}_{}{}^{}\mathrm{Li}$; the ${}_{}{}^{11}{}_{}{}^{}\mathrm{Li}$ data were slightly overpredicted by a microscopic model which includes correlation effects for the ${}_{}{}^{11}{}_{}{}^{}\mathrm{Li}$ valence neutrons. Both 2n and 4n removal from ${}_{}{}^8{}_{}{}^{}\mathrm{He}$ were observed, in about a 2:1 ratio. Subtraction analysis of the data indicates that ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ is a good core within ${}_{}{}^6{}_{}{}^{}\mathrm{He}$ and ${}_{}{}^8{}_{}{}^{}\mathrm{He}$, as is ${}_{}{}^9{}_{}{}^{}\mathrm{Li}$ within ${}_{}{}^{11}{}_{}{}^{}\mathrm{Li}$. © 1996 The American Physical Society.

The production of intermediate mass fragments (IMF’s) from the four reactions 55A MeV ${}_{}{}^{124,136}{}_{}{}^{}\mathrm{Xe}$ + ${}_{}{}^{112,124}{}_{}{}^{}\mathrm{Sn}$ is studied with an experimental apparatus which is highly efficient for the detection of both charged particles and neutrons. The IMF’s are more localized in the midvelocity region than are the light charged particles, and the detected multiplicity of IMF’s depends linearly on the charge lost from the projectile and increases with the neutron excess of the system. Remnants of the projectile, with very little velocity reduction, are found for most of the reaction cross section. Isotopic and isobaric fragment yields in the projectile-velocity region indicate that charge-to-mass ratio neutralization is generally not achieved but is approached when little remains of the projectile. For all systems, the fragments found in the midvelocity region are substantially more neutron rich than those found in the velocity region dominated by the emission from the projectile. This observation can be accounted for if the midvelocity source (or sources) is either more neutron rich or smaller, with the same neutron-to-proton ratio, than the source with the velocity of the projectile. Taken together, the observations of this work suggest that the intermediate mass fragments are, to a large extent, formed by multiple neck rupture of the overlap material, a process which might enhance the neutron-to-proton ratio of the primary source material and/or limit the size of the sources. This scenario is reminiscent of low-energy ternary fission and one predicted by Boltzmann-Uehling-Uhlenbeck calculations. However, these calculations predict too much velocity damping of the projectile remnant. The calculations improve, in this regard, when the in-medium nucleon-nucleon cross sections and the cost of creating low density material are reduced. © 1996 The American Physical Society.

Inclusive spectra of protons, α particles, and heavy ejectiles as well as coincidence spectra of protons and α particles with heavy ejectiles were measured for ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$${+}^{11}$B system at several angles and at two laborabory energies of ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$: 70 and 80 MeV. A Hauser-Feshbach model analysis was performed for inclusive and coincidence spectra to extract the fusion cross section values 800 ± 50 mb and 750 ± 50 mb at ${\mathit{E}}_{\mathrm{lab}}$ 70 and 80 MeV, respectively. These values are in agreement with the limitation of the fusion imposed by entrance channel conditions and by properties of the compound nucleus, in contradiction to results of earlier investigations which suggested an anomalous energy dependence of the fusion. © 1996 The American Physical Society.

Angular distributions for the differential cross section and three deuteron analyzing powers ${\mathit{iT}}_{11}$, ${\mathit{T}}_{20}$, and ${\mathit{T}}_{22}$ of the reaction d→p${\to }^3$He${\mathrm{\pi }}^0$ have been measured over the whole angular domain at 20 energies close to threshold (0.03<${\mathit{T}}_{\mathrm{\pi }}^{\mathrm{c}.\mathrm{m}.\mathrm{}}$<10.2 MeV). The differential cross section and tensor analyzing power ${\mathit{T}}_{20}$ both show strong variation in energy and angle due to interference between S- and P-wave pion production, whereas ${\mathit{iT}}_{11}$ and ${\mathit{T}}_{22}$ remain consistent with zero over the whole experimental range. All the data at different energies and angles fall on universal curves when plotted as functions of the single variable ${\mathit{p}}_{\mathrm{\pi }}$cosΘ, evaluated in the c.m. The broad features of the results are in line with theoretical expectations. © 1996 The American Physical Society.

Differential cross sections for neutrino scattering on ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ are calculated within the (continuum) random phase approximation model. The charged current (${\nu }_{\mathit{e}}$,${\mathit{e}}^{\mathrm{-}}$) and (${\nu }_{\mathrm{\mu }}$,${\mathrm{\mu }}^{\mathrm{-}}$) capture reactions on ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ are measured by the LSND Collaboration at LAMPF. We investigate and discuss the merits of such studies, especially the information that can be extracted from data for differential neutrino scattering cross sections. © 1996 The American Physical Society.

Mass and charge identified ejectiles, spanning from ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ to ${}_{}{}^{22}{}_{}{}^{}\mathrm{Ne}$, have been detected in the 123 MeV ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$${+}^{56}$Fe reaction. The coexistence of deep inelastic collision (DIC) and incomplete fusion (IF) mechanisms has been observed. The shape of the energy spectra and their behavior with angle allowed us to identify two components: The less dissipative one was dominating near the grazing angle. For both components experimental optimum Q values were derived. Two approaches based on the sum rule (SR) model of Wilczyński were used to calculate DIC and IF contributions to the complex fragment cross sections. Both prescriptions fit reasonably well experimental ejectile cross sections and Q optimum values. Results of the present investigation support the idea that the DIC can be treated on the same footing as IF in the SR model once the first process is confined in an inner angular momentum window, starting from the maximum fusion angular momentum, with respect to quasielastic processes. © 1996 The American Physical Society.

Double differential cross sections have been measured for inclusive (p,${\mathit{p}}^{\prime }$) reactions on ${}_{}{}^{115}{}_{}{}^{}\mathrm{In}$, ${}_{}{}^{167}{}_{}{}^{}\mathrm{Er}$, ${}_{}{}^{173}{}_{}{}^{}\mathrm{Yb}$, and ${}_{}{}^{181}{}_{}{}^{}\mathrm{Ta}$ at incident proton energies of 120, 150, 175, and 200 MeV and for ${}_{}{}^{141}{}_{}{}^{}\mathrm{Pr}$ at 120 and 200 MeV. These targets were chosen to investigate a possible mass dependence of the effective interaction strength when comparisons are made with multistep direct calculations based on the Feshbach, Kerman, and Koonin theory. The inclusion of two-nucleon emission is also investigated. The multiparticle emission contributes significantly at low emission energies and forward angles, but it is shown that the energy dependence of the strength of the effective interaction must also be taken into account to explain the discrepancy in this region. © 1996 The American Physical Society.

The ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$(γ→,np)X reaction has been studied using the Ladon polarized and tagged γ-ray beam, in the energy region between 50 and 75 MeV. The data have been compared with the quasideuteron mechanism. At the highest photon energy the applied model leads to a satisfactory description of both the unpolarized cross section and beam polarization asymmetry. © 1996 The American Physical Society.

Proton pickup data have been obtained through the ${}_{}{}^{55}{}_{}{}^{}\mathrm{Mn}$(d${,}^3$He${)}^{54}$Cr (${\mathit{E}}_{\mathit{d}}$ = 46 MeV) reaction in the 6°–30° angular range using a magnetic spectrometer with a resolution of ∼ 40 keV full width at half maximum. Spectroscopic factors associated with transitions to 24 ${}_{}{}^{54}{}_{}{}^{}\mathrm{Cr}$ final states (${\mathit{E}}^{\mathrm{*}}$≤ 6.104 MeV) were determined from local, zero-range distorted-wave Born approximation analyses of the measured angular distributions allowing for l = 0, 1, 2, and 3 transfer. A spin-dependent sum-rule analysis of the 0${\mathit{f}}_{7/2}$ proton transfer data has been performed using complementary stripping data from a study of the ${}_{}{}^{55}{}_{}{}^{}\mathrm{Mn}$(α,t${)}^{56}$Fe reaction. The 0${\mathit{f}}_{7/2}$ proton transfer data have also been compared to results from a 0f1p shell-model calculation based on a new effective interaction for A = 41–66 nuclei. © 1996 The American Physical Society.

The reaction cross section for ${}_{}{}^8{}_{}{}^{}\mathrm{B}$ and ${}_{}{}^7{}_{}{}^{}\mathrm{Be}$ and the breakup cross section for ${}_{}{}^8{}_{}{}^{}\mathrm{B}$ on silicon have been measured for incident energies between 10 and 40 MeV/nucleon with a stack of silicon detectors. Secondary ${}_{}{}^8{}_{}{}^{}\mathrm{B}$ and ${}_{}{}^7{}_{}{}^{}\mathrm{Be}$ beams were obtained by use of the LISE spectrometer at GANIL. The stack of silicon detectors had the multiple purpose of slowing down the incident secondary beams, identifying the reaction products, and measuring their energy. The separate contributions of diffraction dissociation and absorption to the breakup have been determined. The parallel momentum distribution of ${}_{}{}^7{}_{}{}^{}\mathrm{Be}$ resulting from the breakup of ${}_{}{}^8{}_{}{}^{}\mathrm{B}$ has also been determined. The data are compared to theoretical calculations. © 1996 The American Physical Society.

Disintegration of a heavy nucleus into three charged fragments with the one fragment at rest is possible if a certain relation between the masses and the charges of the moving fragments is fulfilled. A two-arm spectrometer of the angle-velocity-energy correlations has been used at the 1 GeV proton beam to detect such a process. Disintegrations with all three moving fragments have also been observed. The possibility of the identification of the collinear three-body splitting of heavy nuclei at low energies is discussed. © 1996 The American Physical Society.

Based on a three-body formulation of a nuclear reaction, we show that a new type of long-range interaction can exist in the imaginary part of the effective potential for nuclear reactions if the final-state nucleus has an excited neutron-core halo state. The existence of such a long-range interaction can lead to an enhancement of the nuclear cross section. Comparison of the new long-range effective interaction with other previously known ones is given. © 1996 The American Physical Society.

The kaon photoproduction is investigated at the threshold energy. We discuss (i) the difference between the pseudoscalar (PS) and the pseudovector (PV) coupling descriptions on the KNY interaction, (ii) the contributions of t-channel poles of vector and axial vector mesons, and (iii) the contributions of nucleon and hyperon resonances to the ${\mathit{E}}_0^{+}$ amplitude. We suggest that total cross sections near threshold can be used to estimate the leading coupling constant, ${\mathit{g}}_{\mathit{KN}}$Λ and to make clear which description is the better of PS and PV coupling schemes for this reaction from future experiments. © 1996 The American Physical Society.

An inverse scattering method of Lipperheide and Fiedeldey [Z. Phys. A 286, 45 (1978); 301, 81 (1981)] has been used to construct an energy-dependent potential from the elastic-scattering phase shifts of the recently developed K model of Kaneko, LeMere, and Tang [Phys. Rev. C 44, 1588 (1991); 46, 298 (1992)] for the n-α and n${\mathrm{-}}^{40}$Ca systems. The local momentum of the inversion potential is subsequently used to recover the Wigner transforms of the K model. The results obtained indicate that it is possible to find, via inversion, an l-independent Wigner transform, which, when calculated at all energies, can provide us with the full nonlocality. © 1996 The American Physical Society.

Nuclear capture of antiprotons from atomic states is studied. Partial widths for single-nucleon capture events leading to cold residual nuclei are calculated. Recent CERN experiments that compare the neutron and proton captures are analyzed. Nuclear density distributions at the extreme nuclear surface are calculated and tested against the experimental results. © 1996 The American Physical Society.

The root mean square matter radii of halo nuclei provide a basic measure in constructing, constraining, and assessing theoretical models of halo structures. We consider corrections to static density (optical limit) Glauber model calculations of reaction cross sections of such nuclei at high energy giving careful consideration to their intrinsic few-body structure and the adiabatic nature of the halo nucleus-target interaction. We take as important examples the loosely bound two- and three-body systems ${}_{}{}^{11}{}_{}{}^{}\mathrm{Be}$, ${}_{}{}^6{}_{}{}^{}\mathrm{He}$, ${}_{}{}^{11}{}_{}{}^{}\mathrm{Li}$, and ${}_{}{}^{14}{}_{}{}^{}\mathrm{Be}$. The contribution of the valence particles to the calculated reaction cross sections are shown to be significantly reduced, requiring increased halo radii to reproduce experimental data. The implications of these changes for structure models of extended two- and three-body systems are discussed. © 1996 The American Physical Society.