Spin Observables for Electromagnetic Nuclear Physics with Few-Body ystems
We discuss aspects of the physics program with polarized electrons and polarized few-body systems in Amsterdam. Polarized electrons were injected into the AmPS storage ring at a beam energy of 720 MeV, while a Siberian snake was employed to preserve longi
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Spin Observables for Electromagnetic Nuclear Physics with Few-Body ystems J.F.J. van den Brand Department of Physics, Vrije Universiteit, de Boelelaan 1081, 1081 HV Amsterdam, The Netherlands
Abstract. We discuss aspects of the physics program with polarized electrons and polarized few-body systems in Amsterdam. Polarized electrons were injected into the AmPS storage ring at a beam energy of 720 MeV, while a Siberian snake was employed to preserve longitudinal polarization at the interaction point. Vector-polarized deuterium was produced by an atomic beam source and injected into an open-ended cylindrical cell, internal to the electron storage ring. The spin correlation parameter A~d was measured for the (e, e' p) and (e,e'n) reactions at a four-momentum transfer squared of 0.21 (GeV/c? The data are sensitive to the spin structure of the deuteron and allow to extract a value for the charge form factor of the neutron.
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Introduction
Although the neutron has no net electric charge, it does have a charge distribution. Precise measurements [1] where thermal neutrons l.from a nuclear reactor are scattered from atomic electrons indicate that the neutron has a positive core surrounded by a region of negative charge. The actual distribution is described by the charge form factor GE, which enters the cross section for elastic electron scattering. It is related to the Fourier transform of the charge distribution and is generally expressed as a function of Q2, the square of the four-momentum transfer. Data on G E are important for our understanding of the nucleon and are essential for the interpretation of electromagnetic multipoles of nuclei, e.g. the deuteron. Since a practical target of free neutrons is not available, experimentalists mostly resorted to (quasi)elastic scattering of electrons from unpolarized deuterium [2, 3] to determine this form factor. The shape of G E as function of Q2 is relatively well known from high precision elastic electron-deuteron scattering [3]. However, in this case the cross section is dominated by scattering l.from the proton and, moreover, is sensitive to nuclear-structure uncertainties S. Oryu et al. (eds.), Few-Body Problems in Physics ’99 © Springer-Verlag Wien 2000
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and reaction-mechanism effects. Consequently, the absolute scale of GE still contains a systematic uncertainty of about 50%. Many of the aforementioned uncertainties can be significantly reduced through the measurement of electronuclear spin observables. The scattering cross section with both longitudinal polarized electrons and a polarized target for the 2H(e, e' N) reaction, can be written as [4]
where So is the unpolarized cross section, h the polarization of the electrons, and Pf (Pt) the vector (tensor) polarization of the target. Ae is the beam analyzing power. The vector and tensor target analyzing powers A~IT and spin-correlation parameters A~lT, depend on the orientation of the target spin. The polarization direction of the deuteron is defined by the angles ad and ifld in the
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