Study of the 4He(e,e′d)pn Reaction

 The cross section for the ⁴He(e,e′d)pn reaction has been measured in parallel and in (q, ω)-constant kinematics for values of the three-momentum transfer of 406, 495 and 595 MeV/c, and for a range in missing momentum. Just above threshold this reaction can be characterized as a spin/isospin flip transition of the involved pn pair. By using two electron energies (576 and 370 MeV) the longitudinal and transverse structure functions could be separated. The cross sections turn out to be purely transverse, as expected for a spin/isospin flip transition. The data are well described by new covariant and current-conserving calculations that include the major final-state interaction effects. Received August 20, 1998; modified October 12, 1998; accepted for publication January 30, 1999     

Measurement of the exclusive D(p,e+e−)3He and D(p, γ)3He reactions at 98 and 176 MeV

A high resolution magnetic pair spectrometer for medium energy photons has been used for studies of the exclusive (p, e⁺e⁻) and (p, γ) reactions on deuterium at 98 and 176 MeV. The (p, e⁺e⁻) cross sections, obtained at 40° and 80°, are compared with the corresponding data for the (p, γ) reaction measured with the same apparatus. At small virtual photon masses the (p, e⁺e⁻) cross sections show significant enhancement over what is expected from model calculations, a behaviour which has earlier been observed also for the ¹¹B(p, e⁺e⁻)¹²C reaction.     

Relativistic Description of 3He (e, e�� p)2H

The relativistic distorted-wave impulse approximation is used to describe the ³He(e, e?? p)²H process. We describe the ³He nucleus within the adiabatic hyperspherical expansion method with realistic nucleon-nucleon interactions. The overlap between the ³He and the deuteron wave functions can be accurately computed from a three-body calculation. The nucleons are described by solutions of the Dirac equation with scalar and vector (S?CV) potentials. The wave function of the outgoing proton is obtained by solving the Dirac equation with a S?CV optical potential fitted to elastic proton scattering data on the residual nucleus. Within this theoretical framework, we compute the cross section of the reaction and other observables like the transverse-longitudinal asymmetry, and compare them with the available experimental data measured at JLab.     

Exclusive (e,e′pp) knockout reactions

Exclusive cross sections of the ¹⁶O(e,e′pp) ¹⁴C knockout reaction are calculated for transitions to the low-lying discrete final states of the residual nucleus. Short-range correlations and two-body currents, due to the excitation of a Δ resonance in the intermediate state, are included in the calculations. Final-state interactions are taken into account by means of phenomenological spin-dependent optical potentials. Recoil-momentum distributions for transitions to states with different angular momentum exhibit different shapes, which are basically determined by the cm orbital angular momentum of the initial proton pair.     

Inclusive (e,e′ N), (e,e′ NN), (e,e′ π), …v reactions in nuclei

We study the inclusive (e, e′ N), (e, e′ NN), (e, e′ π), (e, e′ πN) reactions in nuclei using a Monte Carlo simulation method to treat the multichannel problem of the final state. The input consists of reaction probabilities for the different steps evaluated using microscopical many body methods. We obtain a good agreement with experiment in some channels where there is data and make predictions for other channels which are presently under investigation in several electron laboratories. The comparison of the theoretical results with experiment for several kinematical conditions and diverse channels can serve to learn about different physical processes occurring in the reaction. The potential of this theoretical tool to make prospections for possible experiments, aiming at pinning down certain reaction probabilities, is also emphasized.     

He原子(e,3e)反应中末态波函数的动量相关

在前期工作的基础上,进一步考虑了He原子(e,3e)反应中末态四体的动量相关.用改进后的理论计算了高能电子入射双电离He原子的五重微分截面(FDCS).与前期理论相比,所得结果大大地改善了理论曲线与实验结果的吻合程度.     

Multi-scattering effects in (e,e′p) knockout

Multistep direct scattering of the ejectile proton is considered in (e,e′p) reactions in the quasi-free region as an improvement over the usual treatment of final-state interactions by means of an optical potential. The theory is applied to ⁴⁰Ca(e,e′p) as a case example. Important contributions of two- and three-step processes are found at high missing energy and momentum in agreement with the experimental trend.     

散射粒子的相互作用对He(e,3e)五重微分截面的贡献

计算了敲出电子等能分享几何安排下,入射能量分别为5 599 eV和1 099 eV时He(e,3e)反应五重微分截面,并比较在He(e,3e)过程中散射粒子的相互作用和它们之间的干涉效应对五重微分截面的贡献.结果表明,理论计算数据在误差范围内与实验数据基本吻合,并且散射粒子的相互作用以及干涉效应对五重微分截面的影响都是不容忽视的.     

Analyzing Power of 3He(n, n)3He Between 1.60 and 5.54 MeV

We report the measurements of the analyzing power A _y (θ) of n-³He elastic scattering with unprecedented accuracy and angular coverage at five incident neutron energies between 1.60 and 5.54 MeV. To this end, we employed the polarization-transfer reactions ³H(p, n)³He and ²H(d, n)³He at 0° for neutron generation and a recently developed high-pressure ³He gas scintillator as an active target, enabling neutron-time-of-flight and ³He recoil-energy determinations. We obtained simultaneously the neutron polarization with a ⁴He-based polarimeter, capitalizing on the well-known n-⁴He A _y (θ). Our n-³He A _y (θ) data are compared to rigorous four-nucleon calculations using high-precision nucleon–nucleon potential models. The agreement between data and calculations is fair at the lower energies and becomes less satisfactory with increasing neutron energy. However, in comparison to the pure isotriplet p-³He system in the same energy region, the agreement between measured and calculated A _y is much better for the mixed (isotriplet and isosinglet) n-³He system, indicating large and somewhat counterintuitive isospin effects. We note that the incorporation of a Δ-mediated three-nucleon force in calculations of n-³He scattering was found to affect A _y (θ) negligibly, suggesting the need for the reconsideration of fundamental interactions in resolving the four-nucleon analyzing power puzzle first established about a decade ago in p-³He scattering.     

Higher-q Quasi-elastic 3He Transverse (e,e��) Response in a Two-Fragment Model

Recently (Efros et?al. in Phys Rev C81:034001, 2010) we have computed the ³He transverse response in the quasielastic region for momentum transfers up to 700?MeV/c using the AV18 potential with the UrbanaIX three body potential. These calculations were carried out in the active nucleon Breit (ANB) frame. Owing to the use of the ANB frame the disagreement of the conventional non-relativistic calculations with experiment has been removed for q values up to 500?MeV/c. However, for higher q values some disagreement still persists. In order to further study the issue we present here results for R _T (q, ??) computed in the Breit and Lab frames with the use of the two-fragment model.     

Towards spin degrees of freedom in (e,e′p)

The expeprimental (e, ep) program at Mainz is presented. The reaction (e, ep) without spin degrees of freedom is compared with the case when polarization observables are considered. It is demonstrated on some examples how the new information can be used to extend the knowledge of the reaction mechanism and nucleon/nuclear structure. Construction and operation of a focal-plane proton polarimeter at Mainz is described.Lectures held at the Indian-Summer School on Electromagnetic and Weak Interactions of Particles with Nuclei, Sázava, Czechoslovakia, 6–11 September 1992.     

Transverse (e,e′) Response Functions for 4He

This poster presentation is intended to show preliminary results from calculations of the inclusive (e,e′) transverse response functions of ⁴He. Up to now we have only included the E1 and M1 multipoles and therefore no comparison with experiment is shown. That will be done once all relevant multipoles have been calculated. Details of the calculation are summarized in the text below.     

Correlations and the cross section of exclusive (e,e′p) reactions for 16O

The reduced cross section for exclusive (e, e′p) reactions has been studied in DWIA for the example of the nucleus ¹⁶O using a spectral function containing effects of correlations. The spectral function is evaluated directly for the finite nucleus starting from a realistic nucleon-nucleon interaction within the framework of the Green's function approach. The emphasis is focused on the correlations induced by excitation modes at low energies described within a model-space of shell-model configurations including states up to the sdg shell. Cross sections for the p-wave quasi-hole transitions at low missing energies are presented and compared with the most recent experimental data. In the case of the so-called perpendicular kinematics the reduced cross section derived in DWIA shows an enhancement at high missing momenta as compared to the PWIA result. Furthermore the cross sections for the s- and d-wave quasi-hole transitions are presented and compared to available data at low missing momenta. Also in these cases, which cannot be described in a model without correlations, a good agreement with the experiment is obtained.     

Study of the 3He(4He,γ)7Be and 3H(4He,γ)7Li Reactions in an Extended Two-Cluster Model

 The ³He(⁴He, γ)⁷Be and ³H(⁴He, γ)⁷Li reactions are studied in an extended two-cluster model which contains α + h/t and ⁶Li + p/n clusterizations. We show that the inclusion of the ⁶Li + p/n channels can significantly change the zero-energy reaction cross sections, S(0), and other properties of the ⁷Be and ⁷Li nuclei, like the quadrupole moments Q. However, the results agree with the known correlation trend between S(0) and Q. Moreover, we demonstrate that the value of the zero-energy derivatives of the astrophysical S-factors are more uncertain than currently believed. Received June 17, 1999; revised February 22, 2000; accepted for publication April 17, 2000     

Final-state interactions in (e,e′p) reactions with polarized nuclei

The cross section for coincidence, quasielastic proton knock-out by electrons from a polarized ³⁹K nucleus is computed in DWIA using an optical potential in describing the wave function of the ejected nucleon. The dependence of the FSI on the initial polarization angles of the nucleus is analyzed and explained in a new, semi-classical picture of the reaction in which the nuclear transparency decreases as a function of the amount of nuclear matter that the proton has to cross, thus providing a method for obtaining detailed information on its mean free path in finite nuclei. We propose a procedure to find the best initial kinematical conditions for minimizing the FSI which will be useful as a guide for future experiments with polarized nuclei.     

低能电子离化He原子(e,2e)反应中的动力学屏蔽

在三体索末菲参量修正的基础上,进一步对(e,2e)反应的末态He⁺中核外电子的有效屏蔽给出修正-这种修正反映了末态四粒子间的动力学屏蔽-并用修正后的索末菲参量计算了入射能量为50eV时,共面不对称几何条件下电子离化He原子的三重微分截面-所得结果与convergent close-coupling理论结果及最新绝对测量的实验数据进行比较,发现当敲出电子能量为10eV时,理论曲线更接近实验数据- 关键词：     

Effects of the form factor on exclusive (e, e′p) reactions

For the exclusive (e, e′p) reaction, we discuss the possibility of observing effects due to swollen nucleons in the nuclear medium, such as ⁴⁰Ca and ²⁰⁸Pb, by introducing form factors of the nucleon in the nuclear medium. These form factors include effectively the change of nucleon properties in the nuclear medium. This calculation is performed by using a Dirac-Hartree single particle model for a bound state and a relativistic optical model for a continuum state with inclusion of the electron Coulomb distortion. The effect of the form factor, which increases with higher momentum transfer (q ≥ 400MeV/c) is too small to be discerned from the errors on the available experimental data. But it affects the determination of spectroscopic factors to some extent.     

Comparison of Monte Carlo simulation and direct multistep scattering theory in (e,e′p) nuclear reactions

Two methods to deal with final state interactions in (e,e′p) reactions in nuclei are compared. One of them uses a Monte Carlo semiclassical approach while the other uses a statistical quantum mechanical approach. The comparison serves to give support to both approaches, showing at the same time their limitations.     

(n,3He)的反应截面的系统学

本文研究了中、重质量核的14MeV中子的(n,~3He)反应截面的系统学特性,在对实验数据分析,评价的基础上,给出了系统学的经验公式,计算了一批核素的反应截面,同实验值符合较好,并讨论了可能的反应机制。 The systematic characters of (n, ~3He) reaction cross sections have been studied formedium and heavy mass nucleus at 14MeV. A set of the empirital parameters have been eatracted onthe bases of analyzing and evaluating available data. The (n,~3He) reaction cross sections of somenuclei have been calculated, and a good agreement with the experimental data have been obtained.Most possible reaction mechanisms are also discussed.