Showing papers in "Physical Review C in 1989"

Soft-core baryon-baryon one-boson-exchange models. II. Hyperon-nucleon potential.

Abstract: The results of the Nijmegen soft-core potential model are presented for the low-energy YN interactions. The YN version of the model is obtained by a straightforward extension of the NN model through the application of SU(3). The potentials are due to the dominant parts of the \ensuremath{\pi}, \ensuremath{\eta}, \ensuremath{\eta}', \ensuremath{\rho}, \ensuremath{\omega}, \ensuremath{\varphi}, \ensuremath{\delta}, \ensuremath{\varepsilon}, and ${S}^{\mathrm{*}}$ Regge trajectories. This gives the traditional one-boson-exchange potentials. In addition to these, the J=0 contributions from the tensor f,f',${A}_{2}$ and Pomeron trajectories are included in the potentials. The latter give potentials of the Gaussian type. Also the form factors from Regge poles are Gaussian, which guarantees that the potentials have a soft behavior near the origin. The multichannel Schr\"odinger equation is solved in configuration space for the (partially) nonlocal potentials. We work on the particle basis and include the Coulomb interaction exactly.The meson-baryon coupling constants are calculated via SU(3), using the coupling constants of the NN analysis as input. Charge symmetry breaking in the \ensuremath{\Lambda}p and \ensuremath{\Lambda}n channels is included. An excellent description is achieved of the available low-energy data per degree of freedom (${\ensuremath{\chi}}^{2}$\ensuremath{\approxeq}0.58 for 35 YN data). In particular, we were able to fit the inelastic capture ratio at rest perfectly. We have ${r}_{R}$=0.471, where experimentally the average value is ${r}_{R}$=0.468\ifmmode\pm\else\textpm\fi{}0.010. The obtained values for the adjustable mixing angles and F/(F+D) ratios agree very well with the literature. We find ${\ensuremath{\alpha}}_{\mathrm{PV}=0.355}$ and ${\ensuremath{\alpha}}_{V}^{m}$=0.275. For the scalar-meson mixing angle we obtain ${\ensuremath{\theta}}_{s}$=40.895\ifmmode^\circ\else\textdegree\fi{}, which lies between the ideal mixing angles for the scalar ${q}^{2}$q\ifmmode\bar\else\textasciimacron\fi{} $^{2}$ and qq\ifmmode\bar\else\textasciimacron\fi{} states. In the \ensuremath{\Lambda}p system we find a cusp at the ${\ensuremath{\Sigma}}^{+}$n threshold, but there is on the second Riemann sheet no pole in the vicinity causing this cusp. The predictions of the total cross sections up to the pion production threshold are given and compared to the experimental data.

Theory of cluster radioactive decay and of cluster formation in nuclei

Abstract: A new model is proposed for the mechanism of cluster formation and then penetration of the confining nuclear interaction barrier in radioactive nuclei. The cluster formation is treated as a quantum-mechanical fragmentation process and the WKB penetrability is found analytically. Applications of the model are made to $^{14}\mathrm{C}$ decay of $^{222\mathrm{\ensuremath{-}}224}\mathrm{Ra}$ and $^{24}\mathrm{Ne}$ decay of $^{232}\mathrm{U}$. The branching ratio for $^{14}\mathrm{C}$ decay of $^{232}\mathrm{U}$ is also calculated and is found to be incredibly small as compared to that for its $^{24}\mathrm{Ne}$ decay.

Problems of describing spin-(3/2 baryon resonances in the effective Lagrangian theory

Abstract: We investigate some important theoretical issues associated with the treatment of the spin-(3/2 baryons in the effective Lagrangian theory. These are the form of the spin-(3/2 particle propagator, off-shell parameters involving the spin-(3/2 field; strategies of implementing gauge invariance, and unitarity. We comment on previous works by Peccei, Nath et al., Williams, and Adelseck et al., the last three works being in the context of recent revival of interest of baryon resonance structure in quantum chromodynamics. Our experience on the \ensuremath{\Delta}(1232) resonance is invoked as a concrete example of dealing with these problems. Examples of some related problems in theories of massive vector and spin-(3/2 particles, in pion decay and supersymmetry, respectively, are also discussed. These discussions should trigger new theoretical and experimental investigations in the study of baryon resonances excited from free hadrons and in complex nuclei.

Multifragmentation, fragment flow, and the nuclear equation of state

Abstract: The quantum molecular dynamic method is used to study multifragmentation and fragment flow and their dependence on in-medium cross sections, momentum dependent interactions, and the nuclear equation of state, for collisions of $^{197}\mathrm{Au}$${+}^{197}$Au and $^{93}\mathrm{Nb}$${+}^{93}$Nb in the bombarding energy regime from 100 to 800A MeV. Time and impact parameter dependence of the fragment formation and their implications for the conjectured liquid-vapor phase transition are investigated. We find that the inclusive fragment mass distribution is independent of the equation of state and exhibits a power-law behavior Y(A)\ensuremath{\sim}${A}^{\mathrm{\ensuremath{-}}\ensuremath{\tau}}$ with an exponent \ensuremath{\tau}\ensuremath{\approxeq}-2.3. True multifragmentation events are found in central collisions for energies ${E}_{\mathrm{lab}\mathrm{\ensuremath{\sim}}30--200}$ MeV/nucleon. The associated light fragment (d,t,\ensuremath{\alpha}) to proton ratios increase with the multiplicity of charged particles and decrease with energy, in agreement with recent experiments. The calculated absolute charged particle multiplicities, the multiplicities of intermediate mass (Ag4) fragments, and their respective rapidity distributions do compare well with recent 4\ensuremath{\pi} data, but are quite insensitive to the equation of state.On the other hand, these quantities depend sensitively on the nucleon-nucleon scattering cross section, and can be used to determine \ensuremath{\sigma} experimentally. The transverse momentum flow of the complex fragments increases with the stiffness of the equation of state. Reduced (in-medium) n-n scattering cross sections reduce the fragment flow. Momentum dependent interactions increase the fragment flow. It is shown that the measured fragment flow at 200A MeV can be reproduced in the model. We find that also the increase of the ${p}_{x}$/A values with the fragment mass is in agreement with experiments. The calculated fragment flow is too small as compared to the plastic ball data, if a soft equation of state with in-medium corrections (momentum dependent interactions plus reduced cross sections) is employed. An alternative, most intriguing resolution of the puzzle about the stiffness of the equation of state could be an increase of the scattering cross sections due to precritical scattering in the vicinity of a phase transition.

Consistent three-nucleon forces in the nuclear many-body problem

Abstract: Describing an assembly of an infinite number of nucleons in interaction via a two-body potential as a nonrelativistic many-body problem in the first place, we envisage corrections to this picture due to suppressed degrees of freedom at the level of the two-body potential At variance with relativistic many-body theory, the solution of the nonrelativistic problem with a two-body potential only is sufficiently under control at present so that evaluating corrections in this framework is of particular interest These corrections come primarily from additional three-body forces either due to finite-density effects (Pauli blocking of fermions) or are of genuine origin: relativistic dynamical processes and effects from the intrinsic structure of the nucleon Recalling the successful treatment of electromagnetic interactions in nuclei in terms of meson-exchange currents, we establish novel consistency requirements between the initial two-body force and the well-identified residual three-body force In this way no new parameters enter in the three-body force, save for the controversial mass of the fictitious scalar ``\ensuremath{\sigma}'' meson We show further that the nucleon-antinucleon pair term required in the analysis of meson-exchange currents has a genuine three-body counterpart resulting from time-ordered diagrams containing a single Z branch Its contribution to the energy per particle is repulsive and varies with a high power of the density Thereby we obtain the important saturating effect present in relativistic mean-field approaches We envisage next the role of the first radial nucleon resonance ${N}^{\mathrm{*}}$((1/2,1)/2) (Roper resonance) in inducing a specific three-body force The meson-nucleon-Roper coupling constants and form factors are evaluated in a relativistic quark model Gathering all self-consistent corrections to the binding energy per particle of infinitely many nucleons, we find that the final equation of state is solely governed by the density dependence of medium corrections to the free \ensuremath{\sigma}-meson mass We discuss a first attempt to extract this density dependence from an empirical equation of state

Effective masses in relativistic approaches to the nucleon-nucleus mean field

Abstract: In relativistic descriptions of the mean field in nuclei or in nuclear matter, the expression ``effective mass'' has been used to denote different quantities. The relationship between these various quantities is clarified. It is exhibited which one among them is most closely related to the effective mass that is derived from nonrelativistic analyses of scattering and bound-state data. This nonrelativistic-type effective mass has a characteristic energy dependence near the Fermi energy whenever one goes beyond the relativistic Hartree or Hartree-Fock approximations. By making use of dispersion relations that connect the real and imaginary parts of the microscopic mean field, it is shown that the occurrence of this ``Fermi surface anomaly'' is quite general. It has the same origin as in the nonrelativistic case, namely the frequency dependence of the mean field. Despite this qualitative similarity between the relativistic and nonrelativistic cases, a striking difference exists between the size of the Fermi surface anomaly in the two cases. The physical origins of the effective mass are also shown to be very different in the relativistic and nonrelativistic descriptions.

Spontaneous fission properties of 258Fm, 259Md, 260Md, 258No, and 260

Abstract: We have measured the mass and kinetic-energy distributions from the spontaneous fission of {sup 258}Fm, {sup 258}No, {sup 259}Md, {sup 260}Md, and {sup 260}(104). All are observed to fission with a symmetrical division of mass. The total-kinetic-energy distributions strongly deviated from the Gaussian shape characteristically found in the fission of all other actinides. When the total-kinetic-energy distributions are resolved into two Gaussians, the constituent peaks lie near 200 and 233 MeV. We conclude that both low- and high-energy fission modes occur in four of the five nuclides studied. We call this property bimodal fission.'' Even though both modes are possible in the same nuclide, one generally predominates. We offer an explanation for each mode based on shell structures of the fissioning nucleus and of its fragments. The appearance of both modes of fission in this region of the nuclide chart seems to be a coincidence in that the opportunity to divide into near doubly magic Sn fragments occurs in the same region where the second fission barrier is expected to drop in energy below the ground state of the fissioning nucleus. Appropriate paths on the potential-energy surface of deformation have been found by theorists, but no physical grounds have beenmore » advanced that would allow the near equal populations we observe traveling each path. We suggest that this failure to find a reason for somewhat equal branching may be a fundamental flaw of current fission models. Assuming the proposed origins of these modes are correct, we conclude the low-energy, but also mass-symmetrical mode is likely to extend to far heavier nuclei. The high-energy mode will be restricted to a smaller region, a realm of nuclei defined by the proximity of the fragments to the strong neutron and proton shells in {sup 132}Sn.« less

Systematics of momentum distributions from reactions with relativistic ions.

Abstract: The momentum distributions of projectile and target residues from spallation reactions induced by relativistic projectiles already in the literature are transformed into consistent quantities and compared. The momentum imparted to the residual nucleus is presented in terms of a longitudinal velocity, 〈${\ensuremath{\beta}}_{?}$〉, and a root-mean-squared momentum, ${P}_{\mathrm{rms}}$. These parameters from all spallation products from many disparate systems display the same systematic dependence on observed mass loss. The rms momentum is shown to depend on the square root of the observed mass loss due to momentum conservation in either of three competing processes with no dependence on the initial reaction. The longitudinal momentum is shown to depend on the observed mass loss and therefore on the excitation energy with only a kinematical factor from the entrance channel. Thus, the longitudinal momenta depend on the velocity but not the mass of the reaction partner.

Coupled-rearrangement-channel Gaussian-basis variational method for trinucleon bound states

Abstract: To the $^{3}\mathrm{H}$ and $^{3}\mathrm{He}$ ground states, we apply the coupled-rearrangement-channel variational method with Gaussian-basis functions which has successfully been used in precise calculations of muonic molecular ions, Coulomb-interacting three-body systems. The trinucleon wave function is decomposed into angular-momentum-projected three-body channels as done in the Faddeev equations method, but the interaction is fully incorporated with no partial-wave decomposition. The radial part of the channel amplitudes is expanded with a sufficient number of Gaussian-tail basis functions of the Jacobi coordinates. The Gaussian ranges are taken to be geometrical progressions which run from very short ranges through large enough ones. This ab initio variational approach is found to describe accurately both the short-range correlations and the asymptotic behavior. The Argonne ${V}_{14}$ potential is used as an example of realistic two-nucleon interactions; for $^{3}\mathrm{He}$, the Coulomb potential is included nonperturbatively. The calculation reproduces precisely the results of the Faddeev calculations for $^{3}\mathrm{H}$ and $^{3}\mathrm{He}$ for binding energy, probabilities of the S, S', P, and D states, and the S- and D-wave asymptotic normalization constants. Convergence of the present results is seen at a much smaller number of the three-body channels than in the Faddeev calculations. This is because the interaction is truncated in the angular momentum space in the Faddeev calculations but the full interaction is taken in the present method.

Spin-one Kemmer-Duffin-Petiau equations and intermediate-energy deuteron-nucleus scattering.

Abstract: We present a description of the spin-one Kemmer-Duffin-Petiau equations. An effective second-order equation is obtained for various types of interactions. An argument is given for the use of one particular form of interaction for deuteron-nucleus scattering. This provides the basis for a generalization of the standard Watanabe model in which Dirac scalar and vector nucleon-nucleus potentials are used as input. Parameter-free calculations are performed using both phenomenological and microscopic nucleon-nucleus potentials, and results are compared with deuteron-nucleus elastic scattering data at 400 and 700 MeV. Qualitative agreement is generally obtained. A good description of the forward-angle vector analyzing power data is achieved.

High-energy reaction cross sections of light nuclei

Abstract: The high-energy reaction cross sections of Li and Be isotopes are calculated using a simplified Glauber model and densities constrained by the empirical binding energies. We find excellent agreement with experiment, reproducing the large increase for the most neutron-rich nuclei.

Temperatures and excitation energies of hot nuclei in the reactions of 32S+Ag and 16O+Ag at 30 MeV/nucleon.

Abstract: For the reactions of 30 MeV/nucleon /sup 16/O and /sup 32/S with Ag, singles and coincidence measurements have been made for heavy residues, fragments (3less than or equal tozless than or equal to14), and light charged particles (zless than or equal to2). Mass-velocity correlations for the residues and fragment-residue coincidences indicate that increasing residue velocities do correspond to increasing excitation energy. Excitation energies as high as 90% of those which would result from complete fusion are reached. The spectra of light particles detected in coincidence with residue groups having different average velocities are analyzed with a moving source fit. When recoil effects are properly taken into account, excellent fits to the data are obtained. From the energy spectra and multiplicities of particles emitted from a fusion-like source the initial temperatures of the primary composite nuclei are determined. The results suggest that a plateau temperature near 6.5 MeV is reached above excitation energies of 3 MeV/nucleon. The temperatures are compared to those resulting from various model calculations.

Systematics of fusion-fission time scales

Abstract: New pre-scission neutron multiplicity (..nu../sub pre/) data for /sup 7/Li-, /sup 16/O-, and /sup 20/Ne-induced fission are presented, spanning a fissility range from 0.60 to 0.85. Fission time scales have been deduced for two extreme assumptions regarding the mean excitation energy during fission. It is found for fusion-fission reactions that the fission time scale is independent of fissility, within a factor 1.5. A comparison of time scales deduced from ..nu../sub pre/ measurements for fast-fission with quasifission time- scales deduced from the rotation angle of the composite system allows a minimum fusion-fission time scale of 30/times/10/sup /minus/21/ s to be determined. For the most fissile system, fast-fission (fission without barrier) is shown to be up to three times faster than fusion-fission. Using a model to interpret the fusion-fission time scales, it is concluded that motion in the fission direction is strongly overdamped. This means that the dynamics of fission are dominated by a slow diffusion towards scission, and not by the potential energy surface. It is shown that such a picture can explain the observed lack of dependence of the fission time scale on fissility.

Binary decay of 56Ni formed in the 32S

Abstract: Fully energy-damped yields from the {sup 32}S+{sup 24}Mg reaction have been measured at center-of-mass energies of {ital E}{sub c.m.}=51.6 and 60.5 MeV with the use of an experimental arrangement where both of the resulting heavy fragments could be detected in coincidence. Energy, velocity, and angular distributions of the reaction fragments have been determined. The cross sections prior to secondary light-particle emission have been deduced for the breakup of the compound system into different mass channels. These data are discussed in terms of two possible reaction mechanisms: fusion followed by fission and deep-inelastic orbiting.

Gamow-Teller strength deduced from charge exchange reactions on 54Fe at 300 MeV.

Abstract: Angular distributions of the {sup 54}Fe({ital p},{ital n}){sup 54}Co and {sup 54}Fe({ital n},{ital p}){sup 54}Mn cross sections have been measured to test the Gamow-Teller sum rule ({ital S}{sub {minus}}{minus}S{sub +}=3(N{minus}Z)) in a case where the Gamow-Teller strength is large for both channels. The results for {ital S}{sub {minus}} and {ital S}{sub +} are compared to several models which have moderate success in describing the data. Large scale shell-model and quasiparticle random-phase-approximation calculations correctly predict the distribution of Gamow-Teller strength but overestimate the total strength. A model that approximates the nuclear surface to be a semi-infinite slab describes the cross sections well in the quasielastic scattering region if 2p-2h correlations are included.

Relativistic Vlasov-Uehling-Uhlenbeck equation for nucleus-nucleus collisions.

Abstract: A relativistic Vlasov-Uehling-Uhlenbeck equation for the nuclear phase-space distribution function is derived from the Walecka model through the use of semiclassical, local, and Born approximations.

Charge distributions of 208Pb, 206Pb, and 205Tl and the mean-field approximation.

Abstract: Charge distributions of $^{208}\mathrm{Pb}$, $^{206}\mathrm{Pb}$, and $^{205}\mathrm{Tl}$ have been calculated within the Hartree-Fock and Hartree-Fock+BCS approximations with the Skyrme interaction. Using the force ${\mathrm{SkM}}^{\mathrm{*}}$ designed without any reference to this particular problem, we find good agreement with elastic electron scattering data. The role of pairing correlations beyond the mean field is studied by applying the Lipkin-Nogami method of variation after approximate projection on the good number of particles. We argue in this paper that, in our opinion, there is no significant discrepancy between the experimental data and the Hartree-Fock calculations using reasonable effective interactions. In particular, we do not see any compelling need for a large depletion of the occupation number of the proton 3${s}_{1/2}$ orbital.

Two-loop corrections for nuclear matter in the Walecka model

Abstract: Two-loop corrections for nuclear matter, including vacuum polarization, are calculated in the Walecka model to study the loop expansion as an approximation scheme for quantum hadrodynamics. Criteria for useful approximation schemes are discussed, and the concepts of strong and weak convergence are introduced. The two-loop corrections are evaluated first with one-loop parameters and mean fields and then by minimizing the total energy density with respect to the scalar field and refitting parameters to empirical nuclear matter saturation properties. The size and nature of the corrections indicate that the loop expansion is not convergent at two-loop order in either the strong or weak sense. Prospects for alternative approximation schemes are discussed.

Single nucleon removal in relativistic nuclear collisions.

Abstract: We implement a simple approach to the inclusive cross section for single nucleon removal by relativistic nucleons and nuclei. We first develop the projectile and target dependence of the mean number, {ital N}({ital b}), of nucleon-nucleon collisions as a function of impact parameter in the peripheral region. Using the Glauber approximation, we obtain a simple parametrization for a critical impact parameter {ital b}{sub {ital c}} such that the reaction cross sections for both {ital N}-{ital B} and {ital A}-{ital B} collisions are well represented by {pi}{ital b}{sub {ital c}}{sup 2}. Further study of the {ital b} dependence of {ital N}({ital b}) around {ital b}={ital b}{sub {ital c}} allows us to develop a parametrization of single nucleon abrasion cross sections. Next, we employ the Weizsacker-Williams approximation with {ital b}{sub {ital c}} as the cutoff impact parameter to calculate the Coulomb contribution to the single nucleon removal process. The results are compared with recent data which suggest that the Weizsacker-Williams approximation is inadequate for heavy projectiles. Using our estimates for the nuclear contribution, we find that the data yields good agreement with the Weizsacker-Williams results for virtually all projectile-target combinations. We therefore conclude that the measured deviations from the Weizsacker-Williams results domore » not represent new physics, but rather reflect uncertainties in the estimation of the nuclear contribution to the single nucleon removal process. As an elementary example of the possible new physics that may be observed in this process, we calculate the contributions from a coherent nuclear process and the possible interference effects. For heavy projectiles, we find that the interference effects are comparable to the present experimental uncertainties.« less

One- and two-dimensional analysis of the factorial moments in 200 GeV/nucleon p, 16O, and 32S interactions with Ag and Br nuclei.

Abstract: Scaled factorial moments, corrected for the shape of the single-particle pseudorapidity distribution, are analyzed in pseudorapidity and in two-dimensional (pseudorapidity and azimuth angle) space. An intermittent, power-law growth of the moments with decreasing bin size is found, with two-dimensional analysis revealing a much stronger effect than for one-dimensional for nucleus-nucleus data. The intermittent patterns are more evident for proton-nucleus than for nucleus-nucleus collisions, with the heaviest nucleus, {sup 32}S, showing the weakest effect.

Relativistic Hartree calculations of odd-A nuclei

Abstract: Ground-state properties of odd-{ital A} nuclei near closed shells are calculated using self-consistent, relativistic mean-field models of baryon-meson dynamics Contributions from {sigma}, {omega}, and {rho} mesons and the photon are included Comparisons are made between linear and nonlinear mean-field models, and with calculations including vacuum polarization in a local density approximation Results are given for (intrinsic) binding energies, rms radii, magnetic and quadrupole moments, currents, and elastic magnetic scattering form factors, with comparisons to other calculations and to experiment Bulk systematics are well reproduced by the nonlinear model and, as expected, isoscalar magnetic moments in light nuclei are close to Schmidt predictions for all models At higher momentum transfer, currents are enhanced compared to nonrelativistic single-particle predictions, in disagreement with experiment Extensions to include pions and derivative corrections to the vacuum polarization current are discussed

Collective modes in hot and dense matter

Abstract: We propose a model for a relativistic many-body system at finite temperature in the framework of thermo field dynamics, which is a real-time formalism of finite-temperature field theory. Our model contains the scalar (\ensuremath{\sigma}) and the vector (\ensuremath{\omega}) mesons as well as the Dirac nucleon. The full propagator and self-energy for each particle are presented in terms of spectral representations. The Feynman rules for a perturbation expansion are shown. They are applied to the study of collective modes in hot and dense matter within the random-phase approximation. The dispersion relations of the longitudinal and transverse collective modes in the meson branch are calculated. We also estimate the effective meson mass which is defined as the energy needed to create one meson at rest in extreme matter. The effects of vacuum fluctuations are also examined. They contribute a fair amount to the collective modes through the effective nucleon mass.

Nilsson parameter set in the A≊120–140 region

Abstract: New Nilsson (\ensuremath{\kappa},\ensuremath{\mu}) parameter sets for the proton N=4, 5, and 6 harmonic oscillator shells are deduced for the A\ensuremath{\approxeq}120--140 mass region by fitting a potential-energy-surface calculation, using the Nilsson-Strutinsky method, to 28 experimental bandhead energies. The results from both the standard and the fitted Nilsson (\ensuremath{\kappa},\ensuremath{\mu}) parameter sets are compared with experimental bandhead energies. The fitted (\ensuremath{\kappa},\ensuremath{\mu}) parameter sets achieve a significant improvement in the bandhead energies over this region. In addition, calculated single-particle levels using both the Woods-Saxon and the Nilsson potentials are compared.

Analysis of pre- and post-scission neutrons emitted in the reaction 169Tm(36Ar,f) at Elab

Abstract: Pre- and post-scission neutron multiplicities for the reaction {sup 169}Tm({sup 36}Ar,{ital f}) at {ital E}{sub lab}=205 MeV were measured in coincidence with fission fragments of different masses and total kinetic energies. The mass and total kinetic energy dependence of the total neutron multiplicity as well as the width of the out-of-plane fission fragment correlation angle are well described by evaporation calculations. An average time before scission of several 10{sup {minus}20} s is deduced from the average pre-scission neutron multiplicity. The mass dependence of the post-scission neutron multiplicity is consistent with an energy division at scission proportional to the mass of the fragments. For the first time clear evidence for an increase in pre-scission neutrons with increasing total kinetic energy values has been observed. Possible interpretations of this unexpected behavior are discussed.

Electromagnetic interactions of extended nucleons.

Abstract: An electromagnetic current operator is deduced from the most general form of the extended pion-nucleon vertex function using the minimal substitution prescription. It is proved that the sum of the obtained current operator and the isolated-pole contribution satisfies the Ward-Takahashi identity derived for the pion photoproduction. The minimal-coupling interaction is applied to the calculation of the one-pion exchange current regularized by the pion-nucleon form factors. It is found that the one-pion exchange current operator including hadronic and electromagnetic form factors satisfies the Ward-Takahashi equation for the nucleon-nucleon interaction.

Low-lying structures in the Gamow-Teller strength functions for the double-beta-decaying nuclei 76Ge, 82Se, 128Te, and 130Te.

Abstract: Excitation-energy distributions of transition strength to ${1}^{+}$ states excited via the (p,n) reaction at 134.4 MeV on targets of $^{76}\mathrm{Ge}$, $^{82}\mathrm{Se}$, $^{128}\mathrm{Te}$, and $^{130}\mathrm{Te}$ were measured for excitation energies up to 25 MeV. Structures observed in the neutron spectra with forward-peaked (\ensuremath{\Delta}L=0) angular distributions were identified as ${1}^{+}$ states, except for the isobaric analog transitions. The total ${1}^{+}$ strength in these reactions was extracted by normalizing the intensity in the ${1}^{+}$ peaks to the Fermi transition strength observed in the isobaric analog state. The Gamow-Teller strength observed in ${1}^{+}$ peaks above a fitted polynomial background is typically 55% of the sum rule obtained by assuming that the strength of ${\ensuremath{\beta}}^{+}$ transitions is negligible. The portion of this strength found at excitation energies less than that of the Gamow-Teller giant resonance varied from 15% for $^{128}\mathrm{Te}$ to 38% for $^{76}\mathrm{Ge}$. Experimental results are compared with predictions of a shell model that includes a pairing force and a long-range Gamow-Teller force in both parent and daughter nuclei. A comparison of the strength functions of the tellurium isotopes is made; this comparison is relevant in determining whether double-beta decay without neutrino emission (0\ensuremath{ u} decay) is observed in these isotopes.

European Muon Collaboration effect in nuclear Drell-Yan processes.

Abstract: We compute the effects of nuclear binding in the Drell-Yan process pA..--> mu../sup +/..mu../sup -/+X using a relativistic nuclear model. Comparisons are made for A = He, Fe, and Au.

Excitation function of the quasifree contribution in the 2H(7Li, alpha alpha )n reaction at E0=28-48 MeV.

Abstract: The $^{2}\mathrm{H}$${(}^{7}$Li, \ensuremath{\alpha}\ensuremath{\alpha})n reaction was studied in a kinematically complete experiment at energies between 28 and 48 MeV. Coincidence spectra show the contribution from the quasifree p${+}^{7}$Li\ensuremath{\rightarrow}\ensuremath{\alpha}+\ensuremath{\alpha} reaction. The excitation function of the quasifree reaction cross section at small neutron momenta was extracted and compared with the behavior of the free reaction cross section.