Charge exchange reactions

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:

• Charge exchange within the nucleus that produced the (e,e'p) reaction.
• Charge exchange in the rest of the target and the shielding between target and detectors.

The former process can not be distinguished experimentally from (e,e'n). For the special case of the deuteron the contribution can be calculated accurately. A calculation by Arenhövel confirms that the effect is <2% for the asymmetry in our Q -range. The (e,e'p) events from N followed by a coherent (p,n) final state interaction have the same signature as the N(e,e'n) events and are automatically subtracted out when removing the wide nitrogen quasielastic peak from the the narrow deuteron quasielastic peak.

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%.