Proton Knockout Research Articles

Exclusive Measurement of Proton Quasifree Scattering and Density Dependence of the Nucleon-Nucleon Interaction

Differential cross sections and analyzing powers ( ${A}_{y}$) have been measured for the proton knockout reaction from ${s}_{1/2}$ orbits of three kinds of target nuclei at ${E}_{p}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}392\mathrm{MeV}$. The ${A}_{y}$ values are reduced from those of free proton-proton scattering and the reduction rate is found to depend on target. For the present setting, where the recoil momentum is almost zero, an averaged density seen through this reaction can be estimated and it reaches almost half of the saturation density in some cases. The reduction of ${A}_{y}$ shows a monotonically decreasing function of the averaged density, which strongly suggests the existence of a nuclear medium effect on the nucleon-nucleon interaction.

Interaction dependence in a relativistic DWIA description of proton knockout

Abstract Analyzing powers and cross sections have been obtained for the (p,2p) reaction on 16O and 40Ca for a variety of incident proton energies and outgoing angle pairs. The calculations are based on a finite-range, relativistic DWIA formalism that includes dynamical recoil effects and have been carried out with three different input t-matrices of relativistic Love-Franey form. Distortions are incorporated by means of relativistic, energy-dependent optical potentials. It is found that the dependence of the calculated observables on the choice of input t-matrix is rather weak.

Photon-induced proton knockout from 208Pb

Cross sections have been measured for the reaction 208Pb(γ,p) leading to the low-lying ( 1 2 + , 3 2 + ) and ( 11 2 − , 5 2 + ) doublets and 7 2 + state in 207Tl using a high energy-resolution tagged photon beam of 41 to 57 MeV. The data are compared with results obtained with the reaction 208Pb(e,e′p) in the same recoil-momentum range. The recoil-momentum distributions derived from the (γ,p) data do not coincide with those obtained from the (e,e′p) data. Distorted-wave impulse-approximation (DWIA) calculations of which the input is constrained by the (e,e′p) data underestimate the (γ,p) data by typically one order of magnitude. An estimate of meson-exchange currents (MEC) effects using the Siegert theorem brings the DWIA-calculations close to the data. Random-phase approximation calculations also give a fair account of the data, but in this case MEC-effects are predicted to be less important.

Medium effects on spin observables of proton knockout reactions.

Medium modifications of the properties of bound nucleons and mesons are investigated by means of intermediate energy quasifree proton knockout reactions with polarized incident protons, by comparison of quantities which are insensitive against wave distortions. The sensitivity of the spin observables of the quasifree proton-proton cross section to modifications of the nucleon and meson properties is studied using the Bonn one-boson exchange model of the nucleon-nucleon interaction. A method proposed to extract the pp analyzing power in medium from the (p, 2p) asymmetries indicates a reduction of this quantity compared to its free space value. This reduction is linked to modifications of masses and coupling constants of the nucleons and mesons in the nucleus. The implications of these modifications for another spin observable to be measured in the future are discussed.

Physics results from the AmPS electron scattering facility

The programme in intermediate energy hadronic physics at NIKHEF (Amsterdam) with the Amsterdam Pulse Stretcher and Storage Ring (AmPS) is now in full operation. With the ring operating in stretcher mode a beam with a current of ≥ 10 μA and with a (50 – 70) % duty factor has been used to probe high momentum components in 208Pb with the quasi-free proton knockout reaction. Ongoing coincidence experiments of type ( e, e′ p) and ( e, e′2 p) with broadband proton detectors of 500 msr solid angle are presented. We discuss an experiment with the beam in storage mode interacting with a tensor-polarized deuterium target, a storage cell fed by an atomic beam source.

An experiment on the electroproduction of multi-hadron final states

Electroproduction of final states including protons and pions was measured on 12C in close to 4π geometry using a BGO crystal ball at the 3-spectrometer facility at MAMI. The energy transfer regime studied at three incident electron energies ranged from the quasielastic proton knockout to the quasifree Δ excitation region. The 3-momentum of the virtual photon ranged from about 400 to 800 MeV/c. The performance of the experimental setup is discribed and missing energy distributions of the reactions 12C(e,e′p) and 12C(e,e′pp) are presented.

Proton knockout from 16O at 504 MeV and limitations of the DWIA

Analyzing powers have been calculated for the (p, 2p) reaction on 16O at 504 MeV incident energy using a finite-ranged, relativistic DWIA formalism that includes both dynamical and kinematical recoil effects. A relativistic, Love-Franey parametrization is employed for the NN t-matrix, and distortions are incorporated by means of relativistic, energy-dependent global optical potentials. The results are compared with the data for knockout from the 1 S 1 2 , 1 P 1 2 , and 1 P 3 2 bound states. The calculated analyzing powers for P-state knockout agree fairly well with the data. For knockout from the 1 S 1 2 state, however, the DWIA results and the data are in significant disagreement. Further investigation has revealed a relationship between this disagreement and the spin-orbit parts of the phase-equivalent Schrödinger distortion potentials. Specifically, when the spin-orbit parts of the distortion potentials are omitted, the DWIA analyzing powers move closer to the data, particularly in their energy dependence. In so far as the presence and strength of the data, particularly in their energy dependence. In so far as the presence and strength of the spin-orbit potential is well established by other considerations, this is not a solution to the disagreement, but it may provide a clue for future work.

Effects of Coulomb distortion and final state interaction on the fourth and fifth structure functions.

We examine the question of whether the fourth and fifth structure functions (called ${\mathrm{W}}_{\mathrm{LT}}$ and ${\mathrm{W}}_{\mathrm{LT}\ensuremath{'}}$) defined by the plane wave Born approximation (PWBA) for the reaction (e,e\ensuremath{'}p) are still meaningful quantities in the presence of electron Coulomb distortion. For a heavy target (${\mathrm{Pb}}^{208}$), the apparent structure functions obtained in the presence of the electron Coulomb distortion have shapes (in terms of the number of maxima or minima) similar to those defined in PWBA; however, the magnitudes are changed greatly. For the fourth structure function, the distortion can change the magnitude, even for ${\mathrm{O}}^{16}$, by more than 15%. For proton knockout from (l\ifmmode\pm\else\textpm\fi{}12) spin-orbit doublet states, the changes caused by electron distortion have opposite effects, increasing the magnitude of the one while decreasing the other. The Coulomb effects on the fifth structure function depend on the out of plane angle used in the extraction. For a small angle (e.g., 10\ifmmode^\circ\else\textdegree\fi{}), the electron distortion can be more than 15% in ${\mathrm{O}}^{16}$. However, for certain kinematic choices, the Coulomb distortion effects on the fifth structure function can largely be removed. For proton knockout from spin-orbit doublet states the ratio of the fifth structure functions is of interest. We find that this ratio is nearly independent of the central part of the proton final state interaction and has the power of investigating the spin-orbit interaction of optical potentials.

Quasielastic proton knockout from 16O.

The spectral function of the $^{16}\mathrm{O}$(e,e'p${)}^{15}$N reaction has been measured in quasielastic parallel kinematics. Momentum distributions are extracted for transitions to several discrete states, with emphasis on the low-lying positive parity states of $^{15}\mathrm{N}$. Spectroscopic factors and bound state wave functions are deduced from a DWIA analysis employing five different optical potentials. Coupled channels effects are investigated for the first four states of $^{15}\mathrm{N}$ and are found to be minimal. The spectroscopic results of the 1/${2}^{\mathrm{\ensuremath{-}}}$ ground state and the first 3/${2}^{\mathrm{\ensuremath{-}}}$ excited state indicate a 28%\ifmmode\pm\else\textpm\fi{}5.4% reduction of the valence 1p shell strength with respect to the IPSM sum rule limit for $^{16}\mathrm{O}$. The spectroscopic strength for knockout of protons from the 2s1d shell, integrated up to a missing energy of 40 MeV, is found to be 0.269\ifmmode\pm\else\textpm\fi{}0.033. The distribution of proton spectroscopic strength is determined from a multipole decomposition of the $^{16}\mathrm{O}$ spectral function.

Proton knockout from 16O and relativistic recoil effects in the Dirac impulse approximation

Recoil effects in exclusive nucleon knockout reactions, such as the p2p reaction, are examined within a relativisitic distorted-wave impulse approximation based on the Dirac equation. Starting from a tree-level Feynman diagram in which the Bethe-Salpeter propagator is replaced by the three-dimensional “smooth” propagator, a prescription is developed which preserves the Dirac equation and which reduces to the correct results in the non-relativistic and infinite target-mass limits. Distortions are incorporated in the scheme in a manner consistent with the plane-wave result. Within the scheme, the wave functions of the single-particle bound state and the various distorted waves are evaluated in different two-body and three-body centre-of-momentum frames, which are then boosted, in principle, to a common reference frame. It is found that this feature of the recoil prescription, i.e., the evaluation of the various wave functions in the appropriate centre-of-momentum frames, in conjunction with energy-momentum conservation, is the single most important feature of the recoil prescription. When recoil effects are neglected, the various centre-of-momentum frames coincide with one another, and the imposition of energy-momentum conservation then causes the momentum dependence of the bound-state wave function to be incorrectly folded into the p2p matrix element. Other recoil effects, mainly relativistic in nature, are found to be numerically less important. The scheme is applied to an analysis of the p2p reaction on 16O at 200 MeV incident energy. Overall, the results give a reasonably good account of the existing TRIUMF data, although at certain angle pairs, the calculated cross sections are somewhat too low. The cross sections exhibit a dependence on the bound-state momentum distribution similar to that observed in earlier calculations for a 40Ca target. Unlike the 40Ca results, however, the oxygen results also show a significant dependence on the elastic distorting potentials used, indicating that the elastic data for oxygen does not sufficiently constrain the oxygen optical potential to fix its role in the p2p reaction.

Neutrino-nucleus quasifree neutral current reactions and the nucleon strange quark content.

Differential cross sections for neutrino- (and antineutrino-) induced nucleon knockout are calculated for neutral-current reactions from nuclei. A relativistic Fermi gas model with binding energy corrections is used. We examine the accuracy with which strange quark axial and vector current parameters can be extracted from both existing and (possible) future experiments. Both high (above 1 GeV) and low (near 150 MeV) neutrino energies and the knockout of neutrons and protons are considered.

E-p off-shell cross section in quasielastic (e,e'p) reactions.

By carrying out calculations on $^{208}\mathrm{Pb}$(e,e'p) reactions at different electron beam energies for fixed knocked-out proton kinematics, we examined whether, even in the presence of Coulomb distortions of the electron waves, the (e,e'p) cross section can still be separated into a reduced cross section ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{red}}$(${\mathit{p}}_{\mathit{m})}$ and the e-p off-shell cross section. We found that if one uses an effective momentum approximation in calculating the e-p off-shell cross section, one gets a reduced cross section which is independent of the electron kinematics within a few percent.

Coincident proton emission induced by 200 MeV protons onAu197

Coincident cross sections were measured for the reaction [sup 197]Au([ital p],[ital p][prime][ital p][prime][prime]) at an incident energy of 200 MeV. The protons emitted from the continuum were detected at primary laboratory angles of [minus]40[degree], [minus]50[degree], and [minus]60[degree], for secondary angles ranging between 20[degree] and 150[degree]. The theoretical model which was used to interpret the coincident spectra describes the reaction mechanism in terms of a quasifree nucleon-nucleon interaction between the projectile and a bound target nucleon, folded with a probability for emission of the quasifree knocked-on nucleon after inelastic rescattering off the residual target nucleus. The validity of the distorted-wave impulse approximation which describes the initial quasifree interaction was confirmed by measuring the quasifree knockout of valence-orbital protons from [sup 197]Au. The measured coincident continuum cross sections were then compared with model calculations for primary proton energy cuts around 90, 70, and 40 MeV. The reasonable agreement found between the calculated and measured cross sections indicates that the signature of quasifree scattering persists in the continuum emission from a nucleus as heavy as [sup 197]Au, and is not obscured by multiple scattering efects.

The (e, eṕ) reaction on 142Nd and 146Nd: A search for deformation effects

In a search for nuclear deformation effects in the (e, e'p) reaction, proton knockout from the spherical nucleus 142Nd and the transitional isotope 146Nd has been investigated. Momentum distributions for the transitions to the discrete states below 1.3 MeV excitation energy in 141Pr and below 0.6 MeV in 145Pr have been measured and compared to results of calculations with the Nilsson + BCS model, assuming 146Nd to be deformed and including Coriolis coupling. For the transitions to the 7 2 + states ( 1g 7 2 knockout) the spectroscopic strength changes in going from 142Nd to 146Nd but the shape of the momentum distribution remains fairly unchanged. The same is true for the 5 2 + states. For the transition to the 3 2 + state at 0.348 MeV in 145Pr, the measured momentum distribution is influenced by a mixture of ∼ 25% s-wave in agreement with the prediction of a close-lying 1 2 + state in the deformed shell model. The theory is well able to account for the increased level density in 145Pr and for the other experimental features observed, but the calculated spectroscopic factors are systematically larger than the experimental ones.

Proton-induced nucleon knockout from polarized 3He at 220 MeV.

A high pressure polarized $^{3}\mathrm{He}$ gas target has been used to measure spin observables and absolute cross sections for (p\ensuremath{\rightarrow},2p) and (p\ensuremath{\rightarrow},pn) nucleon knockout in quasielastic kinematics at 220 MeV. The neutron knockout spin observables exhibit large deviations from plane wave impulse approximation (PWIA) calculations and show the importance of rescattering corrections. At this energy, the proton knockout spin observables differ slightly from PWIA calculations, especially at low energy transfers (\ensuremath{\omega}), but this discrepancy may be resolved when final state interactions between nucleons of the two-body recoil scattering state are included. The large rescattering effects observed in the (p\ensuremath{\rightarrow},pn) results may have implications for experiments which consider polarized $^{3}\mathrm{He}$ as a polarized neutron.

Quasi-elastic electron scattering off nuclei

A survey is given of results of electron-induced quasi-elastic proton knockout experiments. We present results of experimental investigations of the (e,e′p) reaction-mechanism, among which are a separation of three of the four structure functions, and the determination of the momentum-transfer dependence of the cross section. Furthermore, the experimental momentum distributions are compared to Distorted Wave Impulse Approximation calculations that employ bound-state wave functions evaluated in a mean-field potential. The mass- and energy-dependence of the deduced spectroscopic strengths are discussed on the basis of modern calculations of the nuclear response that include short-range correlations.

Nucleon knockout from polarized 3He by polarized protons at 290 MeV.

The $^{3}\ensuremath{\rightarrow}$(p\ensuremath{\rightarrow},2p) and $^{3}\ensuremath{\rightarrow}$(p\ensuremath{\rightarrow},pn) knockout reactions were observed in quasielastic kinematics at 290 MeV. Spin observables ${\mathit{A}}_{\mathit{n}0}$, ${\mathit{A}}_{0\mathit{n}}$, and ${\mathit{A}}_{\mathit{n}\mathit{n}}$ were obtained for both reactions. Calculations using the plane wave impulse approximation (PWIA) and the free nucleon-nucleon scattering amplitudes were performed and compared with the data. The proton knockout data are in qualitative agreement with the calculations which predict ${\mathit{A}}_{\mathit{n}\mathit{n}}$ to be sensitive to the presence of mixed-symmetry S' components in the $^{3}\mathrm{He}$ wave function. Proton spins are expected to be exactly paired at momenta q\ensuremath{\approxeq}80 MeV/c, in agreement with the ${\mathit{A}}_{\mathit{n}\mathit{n}}$ data. Spin observables ${\mathit{A}}_{0\mathit{n}}$ and ${\mathit{A}}_{\mathit{n}\mathit{n}}$ for the $^{3}\ensuremath{\rightarrow}$(p\ensuremath{\rightarrow},pn) reaction deviate significantly from the PWIA predictions. The qualitative differences between proton and neutron knockout results may be caused by final-state interaction effects.

Electron-induced proton knock-out from 30Si, 31P and 32S

Electron-induced proton knock-out from the isotones 30Si, 31P and 32S is studied. The extracted r.m.s. radii of the bound-state wave functions, momentum distributions and spectral functions are presented and the systematics are discussed. The momentum distributions and r.m.s. radii for knock-out from the 2s12 and 1d52 orbitals are found to be similar for all three isotones. The 2s12 orbit is filled smoothly in going from 30Si to 32S, but it is neither completely empty nor completely filled in 30Si and 32S, respectively. The experimental results are compared to shell-model calculations and to the results of a quasi-particle phonon model calculation. Although the distribution of spectroscopic strength as a function of energy can be reproduced reasonably well by the latter model, its value, integrated up to 24 MeV excitation energy, is not compatible with the model value.

The 90Zr(n, p) 90Y reaction at En=98 MeV

Double-differential cross sections of the 90Zr(n, p) reaction have been measured at 98 MeV in the angular range 0°–30° for excitation energies up to 40 MeV. The spectra have been interpreted in terms of direct and multistep reactions. The contributions from the latter have been calculated using a statistical theory of multistep direct reactions. The direct parts of the spectra have been decomposed into their different multipole components by using peak-fitting and multipole-decomposition techniques, based on sample angular distributions calculated within the distorted-wave Born approximation. An extensive microscopic calculation based on the random-phase approximation has also been performed and the results are compared with the experimental data. A value S β + = 1.7±0.2 is extracted for the Gamow-Teller strength, suggesting an occupancy ν 2 = 0.09±0.01 for the 1 g 9 2 proton orbital. This value has been compared with recent results from (d, 3He) proton pickup and (e, e′p) knockout reactions.

Production of 3H at large momentum in α− 12C collisions at 2 A GeV

Calculations of the longitudinal and transverse momentum distributions for 3H production at large momentum in α− 12C colli sions near 2 A GeV are compared to experiment. Triton exchange and final-state interactions are shown to represent large corrections to the impulse approximation for proton knockout. A method for calculating interference effects for inelastic fragmentation is discussed. Good agreement with experiment is found using a phenomenological overlap function for 3H-p successful in describing 3H production in pion-induced reactions. Comparisons to momentum distributions obtained through (p, 2p) and (e, ep) reactions on 4He are made.