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.
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.