Not all events identified as a coincidence between an electron and a neutron necessarily correspond to the (e,e'n) process. The reaction (e,e'p), followed by charge exchange (p,n) can produce events of similar signature.
Two types of charge exchange processes occur:
These same arguments are also valid for the
N(e,e'p) followed by a incoherent charge exchange reaction in the ND
,
target walls, superinsulation etc. The e-n coincidence events from the process
d(e,e'p) and subsequent (p,n) in the target etc. however have the same signature as
the genuine d(e,e'n) events and have to be removed via a calculated correction.
To estimate the contribution of this effect we extrapolate from measured A(p,n)
cross sections [11, 12, 13]. We assume the same energy
dependence of the zero degree A(p,n) cross section as for basic (p,n) scattering
because the dominant reaction process for emerging high energy neutrons is a
quasifree interaction in the region of interest (100 - 1000 MeV). Furthermore we
employ an A-dependence of A assuming that the reaction is mainly sensitive
to the surface. Such an A-dependence describes reasonably well the existing
data[11]. With this procedure, we estimate the contribution to the
asymmetry assuming a 2cm thick NH
-target to be <0.24% in out Q
-region.
The contribution is highest at the lowest Q
-point due to the high ratio of
cross sections d(e,e'p)/d(e,e'n). However the contribution is negligible for the
present proposal.
This is an advantage over measurements using the recoil polarization of the neutron to determine G. The heavy shielding in front of the polarimeter represents a very thick target that increases the probability for the process d(e,e'p) + (p,n) to a point where an accurate calculation is needed to correct for the effect. For example, at a proton energy of 800 MeV and a polarimeter shielding of 12 cm Pb, we estimate this process to contribute a false asymmetry of order 5%.