By analyzing the high-momentum features of the nucleon momentum distribution in light and complex nuclei, it is argued that the basic two-nucleon configurations generating the structure of the nucleon spectral function at high values of the nucleon momentum and removal energy can be properly described by a factorized ansatz for the nuclear wave function, which leads to a nucleon spectral function in the form of a convolution integral involving the momentum distributions describing the relative and center-of-mass motion of a correlated nucleon-nucleon pair embedded in the medium. The spectral functions of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ and infinite nuclear matter resulting from the convolution formula and from many-body calculations are compared, and a very good agreement in a wide range of values of nucleon momentum and removal energy is found. Applications of the model to the analysis of inclusive and exclusive processes are presented, illustrating those features of the cross section which are sensitive to that part of the spectral function which is governed by short-range and tensor nucleon-nucleon correlations. © 1996 The American Physical Society.

We report measurements of in-plane polarization transfer for nine natural-parity isoscalar transitions in the ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$(p→,p${\to }^{\prime }$${)}^{28}$Si reaction using a 198.5 MeV polarized proton beam. These measurements test in a new way the validity of an empirical effective NN interaction derived from cross section and analyzing power measurements for similar transitions. The test is successful, demonstrating the need to modify the free NN interaction in the nuclear medium. © 1996 The American Physical Society.

The reaction ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$(n,p${)}^{13}$B has been studied at ${\mathit{E}}_{\mathit{n}}$=65 MeV. Cross sections were measured for transitions to the ground state and excited states of ${}_{}{}^{13}{}_{}{}^{}\mathrm{B}$ from 0° to 40°. The 0° cross section to the ground state gives a value of 179±20 MeV ${\mathrm{fm}}^3$ for ${\mathit{V}}_{\mathrm{\sigma }\mathrm{\tau }}^{\mathit{c}}$, the spin-isospin central part of the volume integral of the effective N-N interaction. A theoretical calculation, 181.3 MeV ${\mathrm{fm}}^3$, is in good agreement with this result. Distorted-wave impulse-approximation calculations are compared with the experimental angular distributions. Good agreement is obtained for the dominant Gamow-Teller transition to the ground state of ${}_{}{}^{13}{}_{}{}^{}\mathrm{B}$. The evidence obtained here, together with results from the analogous reaction ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$(${\mathrm{\pi }}^{\mathrm{-}}$,γ${)}^{13}$B, suggests that transitions to states at 6.5 and 7.6 MeV are spin dipole in character and that the broad resonance at 10.2 MeV is chiefly the analog of the giant E1 resonance in ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$. © 1996 The American Physical Society.

The recoil detection method has been used to measure the differential cross sections at nucleon backward angles, 160°–180° (cm), for the ${}_{}{}^{12,13}{}_{}{}^{}\mathrm{C}$(p,p${)}^{12,13}$C and ${}_{}{}^{12,13}{}_{}{}^{}\mathrm{C}$(p,n${)}^{12,13}$N reactions at an incident proton energy of 200 MeV. These are the first reported data in this angular region. The elastic scattering cross sections show a shallow minimum at 180° and are about a factor of 15 larger than those predicted by optical model calculations using standard parameters. The (p,n) cross sections are lower than the (p,p), flat within the rather limited statistics, and about a factor of 5 greater than predicted. The backward-angle elastic scattering cross sections are selectively sensitive to the real central potential and can be brought into fairly good agreement with experiment without destroying the fit at more forward angles by greatly reducing both its depth and diffuseness. Adding a Majorana exchange term in the real central and spin orbit potential to a phenomenological potential that fits previously reported lower-angle ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ elastic scattering data does not significantly alter these fits while introducing a peak at 180°. © 1996 The American Physical Society.

The entrance-channel dependence of the distribution of reaction strength has been studied for three systems, namely ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$${+}^{182}$W, ${}_{}{}^{48}{}_{}{}^{}\mathrm{Ti}$${+}^{166}$Er, and ${}_{}{}^{60}{}_{}{}^{}\mathrm{Ni}$${+}^{154}$Sm, which all lead to the compound system ${}_{}{}^{214}{}_{}{}^{}\mathrm{Th}$ in complete fusion reactions. The cross sections for elastic/quasielastic scattering, deeply inelastic, and fissionlike processes were measured at beam energies of ${\mathit{E}}_{\mathrm{lab}}$ =166, 177, 222, 260 MeV for ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$${+}^{182}$W, ${\mathit{E}}_{\mathrm{lab}}$=220, 240, 270, 298 MeV for ${}_{}{}^{48}{}_{}{}^{}\mathrm{Ti}$${+}^{166}$Er, and ${\mathit{E}}_{\mathrm{lab}}$=339, 390, 421 MeV for ${}_{}{}^{60}{}_{}{}^{}\mathrm{Ni}$${+}^{154}$Sm, respectively. The maximum contribution of complete-fusion fission processes to the fissionlike cross section is estimated on the basis of expected angle-mass correlations for such reactions. The results show a strong entrance-channel dependence as predicted by the extra-push model. © 1996 The American Physical Society.

Reaction and total cross sections for ${\mathrm{\pi }}^{+}$ and ${\mathrm{\pi }}^{\mathrm{-}}$ on targets of ${}_{}{}^2{}_{}{}^{}\mathrm{H}$, ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$, C, Al, Si, S, and Ca have been measured for beam energies from 42 to 65 MeV. The cross sections are proportional to the target mass at 50 MeV, consistent with transparency to these projectiles. The cross sections are compared to theoretical calculations. © 1996 The American Physical Society.

To gain a better understanding of the production of exotic isotopes and provide information on the stability of nuclei along the path of the rapid-proton capture process, isotopic cross sections from the reaction ${}_{}{}^{78}{}_{}{}^{}\mathrm{Kr}$ + ${}_{}{}^{58}{}_{}{}^{}\mathrm{Ni}$ at 75 MeV/nucleon were measured at 0° using the A1200 fragment separator. Most notably the particle stability of ${}_{}{}^{69}{}_{}{}^{}\mathrm{Br}$ was thoroughly probed during this experiment and it appears to be particle unstable. The experimental production cross section data are compared to previous krypton isotope fragmentation data to explore the dependence of the N/Z ratio of the projectile on the observed isotopic distributions (‘‘memory effect’’) as well as with an intranuclear cascade code developed for higher energies (≳ 200 MeV/nucleon) and a semiempirical parametrization derived from high energy systematics. © 1996 The American Physical Society.

We have measured inclusive γ-ray production cross sections for ${}_{}{}^3{}_{}{}^{}\mathrm{-}$ and α-induced reactions on various targets in the mass range A=61–181 at projectile energies of 11–27 MeV. Except for α-induced yields on the lightest targets, all reactions show high-energy γ-ray yields 1–3 orders of magnitude larger than can be obtained from statistical model calculations. Angular distributions are strongly forward peaked in the center of mass frame for the highest γ-ray energies, providing model-independent evidence for a nonstatistical reaction mechanism. Calculations of direct radiative capture and semidirect excitation of the giant dipole resonance can qualitatively reproduce the angular distributions but cannot reproduce either the dependence of the γ-ray cross sections on ${\mathit{E}}_{\gamma }$ or the target dependence of the absolute cross sections. The ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$${(}^3$He,${\gamma }_0$${)}^{15}$O reaction was found to change from predominantly statistical to predominantly direct over this bombarding energy range. © 1996 The American Physical Society.

This experiment has measured cross sections, analyzing powers, and the spin observables P, ${\mathit{D}}_{\mathit{ss}}^{\prime }$, ${\mathit{D}}_{\mathit{sl}}^{\prime }$, ${\mathit{D}}_{\mathit{nn}}^{\prime }$, ${\mathit{D}}_{\mathit{ll}}^{\prime }$, ${\mathit{D}}_{\mathit{ls}}^{\prime }$ for the reaction ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$(p,${\mathit{p}}^{\prime }$) at 350 MeV populating three $4^{\mathrm{-}}$ states. These ‘‘stretched’’ states, at excitation energies of 17.79 MeV, 19.80 MeV (both mainly T=0), and 18.98 MeV (mainly T=1), have well known nuclear structure and are used as a nuclear filter to examine the tensor and spin-orbit components of the effective nucleon-nucleus interaction at 350 MeV. A complete set of spin observables was obtained at an angle of 22° in the laboratory frame, whereas the ${\mathit{D}}_{\mathit{ss}}^{\prime }$ and ${\mathit{D}}_{\mathit{sl}}^{\prime }$ observables were measured at laboratory angles of 17, 22, 27, and 32 degrees. The data are compared with distorted-wave impulse approximation calculations from several theoretical models. © 1996 The American Physical Society.

Inclusive and exclusive measurements of neutral pions in heavy-ion collisions around 100 MeV/nucleon, carried out in a near 4π geometry, have been analyzed to obtain information on the nuclear stopping of the projectile. Stopping of the projectile has been investigated by the analysis of the source velocity, of the distribution of the energetic products of the collisions, and of the associated rapidity distribution of the baryon matter. Collisions were classified according to their centrality by the charged particle multiplicity. Clear evidence for this phenomenon has been obtained by the study of different observables. Both stopping and reabsorption effects play an essential role in the interpretation of the results. © 1996 The American Physical Society.

The ${}_{}{}^{62}{}_{}{}^{}\mathrm{Ni}$(p,d${)}^{61}$Ni reaction has been studied with 65 MeV polarized protons. Angular distributions of the differential cross section and analyzing power have been measured for neutron hole states in ${}_{}{}^{61}{}_{}{}^{}\mathrm{Ni}$ up to an excitation energy of 7 MeV. The data analysis with a standard distorted-wave Born approximation theory provides transferred angular momenta l and j and spectroscopic factors for several strongly excited states. The 1${\mathit{f}}_{7/2}$ hole state spreads largely in the excitation energy region of 2–6 MeV, while the 1${\mathit{f}}_{5/2}$, 2${\mathit{p}}_{3/2}$, and 2${\mathit{p}}_{1/2}$ hole states into only 2–4 levels. The strength function of the 1${\mathit{f}}_{7/2}$ hole state is analyzed with an asymmetrical Lorentzian function. The damping mechanism of the single hole states is discussed. © 1996 The American Physical Society.

A study of the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$${(}^{24}$Mg${,}^{16}$O ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$${)}^4$He reaction has revealed ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ + ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ breakup from states in ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$ in the excitation region ranging from 32 to 38 MeV. The spins of the states near 33, 35, and 38 MeV of excitation, obtained from angular correlations, are measured to be (${10}^{+}$ or ${12}^{+}$), (${12}^{+}$ or ${14}^{+}$), and (${14}^{+}$ or ${16}^{+}$), respectively. These may be interpreted as arising from a highly deformed band in ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$. © 1996 The American Physical Society.

The dynamical evolution of central collisions induced by GeV light-ion projectiles is examined with two different Boltzmann-Uehling-Uhlenbeck (BUU) calculations. For projectile energies above 1 GeV incident on heavy target nuclei, a region of depleted density develops in the core of the nucleus at times of the order of 30 fm/c, producing hot residues with significantly depleted density at longer times. The simulations predict penetration of the target by the projectile momentum front at incident energies near 4–6 GeV, leading to a saturation of deposition energy. These results are examined in the context of marked changes in reaction observables reported for light-ion-induced collisions. © 1996 The American Physical Society.

Optical model and exact finite range distorted-wave Born approximation analyses were performed on neutron pair exchange between identical cores for ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$ and ${}_{}{}^{34}{}_{}{}^{}\mathrm{S}$ nuclei and on proton pair exchange between identical cores for ${}_{}{}^{30}{}_{}{}^{}\mathrm{Si}$ and ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$. The extracted spectroscopic factors were compared with theoretical ones deduced from Hartree-Fock calculations on these pairs of nuclei. The enhancement of the experimental cross sections with respect to the theoretical ones strongly suggests evidence for a nuclear Josephson effect. © 1996 The American Physical Society.

The preequilibrium (nucleon-in, nucleon-out) angular distributions of ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$, ${}_{}{}^{58}{}_{}{}^{}\mathrm{Ni}$, and ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$ have been analyzed in the energy region from 90 to 200 MeV in terms of the quantum moleculear dynamics theory. First, we show that the present approach can reproduce the measured (p,${\mathit{xp}}^{\prime }$) and (p,xn) angular distributions leading to continuous final states without adjusting any parameters. Second, we show results of a detailed study of the preequilibrium reaction processes, the stepwise contributions to the angular distribution, comparisons with the quantum-mechanical Feshbach-Kerman-Koonin theory, and the effects of momentum distribution and surface refraction/reflection to the quasifree scattering. Finally, the present method was used to assess the importance of multiple preequilibrium particle emission as a function of projectile energy up to 1 GeV. © 1996 The American Physical Society.

Based on the fireball model and the spectator-participant picture, a statistical simultaneous multifragmentation model is developed for heavy ion induced reactions, with the entrance channel characteristics taken into account. This model has the density as the only parameter and it predicts the excitation energy, the temperature, and the frgmentation cross section, etc., once the incident energy, the projectile, and the target nuclei are specified. As a first application, it has been applied to the central collision data taken at National Superconducting Cyclotron Laboratory on the ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$${+}^{45}$Sc reaction at incident energies ranging from 35 to 115 MeV/A. The study brings out the similarity in the characteristics of high energy proton induced and medium energy heavy ion induced reactions. It is observed that with rise of bombarding energy, the onset of simultaneous multifragmentation occurs at some energy between 50 and 60 MeV/A. © 1996 The American Physical Society.

Using Langer’s ImF method, we discuss the temperature dependence of nuclear fission width in the presence of dissipative environments. We introduce a low frequency cutoff to the spectral density of the environmental oscillators in order to mimic the pairing gap. It is shown that the decay width rapidly decreases at the critical temperature, where the phase transition from superfluid to normal fluids takes place. A possible relation to the recently observed threshold for the dissipative fission is discussed. © 1996 The American Physical Society.

In this paper we apply the semiclassical method based on the Feynman path integral formalism to sub-barrier fusion of heavy nuclei. Cross sections are calculated and compared to experimental data and to coupled-channel calculations for different mass systems: ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$${+}^{24}$Mg, ${}_{}{}^{58}{}_{}{}^{}\mathrm{Ni}$${+}^{64}$Ni, and ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$${+}^{208}$Pb. The semiclassical method and coupled-channel calculations give comparable results. It is found that the coupling produces a renormalization of the barrier that is responsible for the enhancement of sub-barrier fusion cross sections and a dissipative force along the classical tunneling path. © 1996 The American Physical Society.

The triton energy spectra of the charge-exchange ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$${(}^3$He,t) reaction at 2 GeV beam energy are analyzed in the quasielastic nucleon knockout region. Considering that this region is mainly populated by the charge exchange of a proton in ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ with a neutron in the target nucleus and the final proton going in the continuum, the cross sections are written in the distorted-wave impulse approximation. The t matrix for the elementary exchange process is constructed in the distorted-wave Born approximation, using one-pion-plus-ρ-exchange potential for the spin-isospin nucleon-nucleon potential. This t matrix reproduces the experimental data on the elementary pn→np process. The calculated cross sections for the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$${(}^3$He,t) reaction at 2° to 7° triton emission angle are compared with the corresponding experimental data, and are found in reasonable overall accord. © 1996 The American Physical Society.

Within the framework of the relaxation time approximation we have computed two components of the friction coefficient for various single-particle potentials and have found that the nondiagonal component of the friction coefficient depends generally on the diffuseness of the potential. The diagonal component of friction, which is related to the two-body dissipation mechanism, was found to be unreasonably large in these computations, especially for small temperatures. © 1996 The American Physical Society.