Introduction

The form factors of the proton and neutron give information on fundamental properties of the nucleon, and provide a critical testing ground for models based on QCD. A detailed knowledge of these quantities is also essential to our understanding of the electromagnetic response functions of nuclei.

Our present knowledge of the neutron electric form factor is inadequate. The slope of at is accurately known from neutron-electron scattering. At higher systematic errors are very large. There, has been extracted from elastic e-d scattering, or inclusive quasielastic e-d scattering. In both cases removal of the proton contribution requires information about the deuteron structure and large uncertainties are introduced. Uncertainties in the theoretical description of the deuteron (mostly from FSI and MEC contributions) have especially negative consequences. As a result, , until very recently, was known with a systematic error of about 100 % . A new experiment at Saclay [1] on e-d elastic scattering has improved the situation at ; the resulting systematic errors are 30%. Serious doubts remain as a great deal of theoretical input on non-relativistic deuteron structure, relativistic effects and MEC are needed to infer from elastic e-d data. Figure 1 shows the best fits to the inferred obtained by different models for the N-N interaction necessary to compute the deuteron structure. Such uncertainties and ambiguities are unsatisfactory for a quantity as fundamental as . With the experiment proposed here we will be able to determine without large theoretical corrections.

The large systematic errors in the past experiments result from two difficulties.

• For elastic e-d scattering the deuteron structure is very important: Accounting for it introduces errors which are magnified with the subsequent subtraction of the dominant proton contribution.
• For quasielastic e-d scattering, the longitudinal/transverse Rosenbluth separation introduces large systematic errors for the small term (charge). Furthermore the necessary subtraction of the dominant proton contribution increases the large systematic errors.

 
Figure 1:   Two parameter fits to data for deduced from the d(e,e) data using deuteron wave functions calculated with the Paris (solid), RSC (dotted), Argonne V14 (dashed) and Nijmegen (dash-dotted) potentials. From Reference [1].

To improve this situation we need to study a reaction which is insensitive to the deuteron structure, which avoids a subtraction of the proton contribution and which avoids longitudinal/transverse Rosenbluth separation.

In this proposal we describe in detail an alternative way of extracting the Sachs Coulomb form factor , by measuring the spin-dependent part of the elastic e-n cross section. To this effect, we plan to detect quasielastically scattered electrons from a longitudinally polarized beam incident on polarized deuterium nuclei in deuterated ammonia ( ). The determination of the asymmetry in the cross section for two opposite orientations of either polarization, yields the product . In the remainder of the proposal we will review the exact relation between and the experimental asymmetry, explore the kinematic region where the method may be applied, and discuss the technical details of the polarized target, the electron and the neutron detector systems, polarimeter and the auxiliary devices involved. An analysis of the estimated uncertainties as well as a relation of the count rates and beam time request complete the proposal.