Neutron Detector

Quasielastic scattering events from the neutron will be identified by detecting the recoil neutron in coincidence with the scattered electron. The neutron detector will consist of an array of scintillators which will form a continuous wall. The front of the detector will be covered by a layer of E detectors for the identification of protons. A thin layer of lead (a few mm to 1 cm thick, depending on background conditions) will shield the detector from soft x-rays coming from the target.

The size of the neutron detector is determined by two factors:

1. The solid angle required to match the electron spectrometer solid angle and contain the Fermi-broadened neutron peak corresponding to the quasi-elastic electron peak.
2. The neutron flight path necessary to obtain the required energy resolution to separate the quasielastic recoiling neutron from background events, -excitation in particular.

To determine the required solid angle which will match the electron spectrometer solid angle we performed a Monte Carlo calculation to simulate the quasielastic d(e,e'n)p process. The neutron initial momentum was assumed to follow the nucleon momentum distribution in the deuteron. The spectrometer solid angle was assumed to be 10 msr with a momentum bite acceptance of . This results in a detector of size 2.6 m (vertical) by 1.3 m (horizontal). This detector is placed at a distance of 3.4, 3.4, 5.5, 8.0 m from the target, for the values of 0.5, 1., 1.5 and 2.0 (GeV/c) , respectively.

There are three major sources of non-quasielastic, background (e,e'n) events;

1. Coincidence events originating from the nitrogen in the polarized target.
2. Charge exchange reactions (discussed later).
3. Coincidence events originating from the process .

The first two can be determined by measuring the N(e,e'n) reaction under the same conditions, or by a careful measurement of the nitrogen quasielastic peak (which extends beyond the deuteron peak), and subtracted from the deuteron quasielastic events.

The expected spectra with this set up have been calculated using the Monte Carlo code MCEEP[20]. As input we have used the known deuteron momentum distribution, an experimental spectral function for N and He, the standard nucleon form factors, calculated neutron detector efficiencies and the known acceptance of the HMS. Examples of the resulting spectra (cut on the presence of a recoil neutron and on a value of rad (which corresponds to a cut in the component of the neutron momentum perpendicular to q) are shown in Figures 5 and 6.

 
Figure 5:   Electron energy loss spectrum for (e,e'n) for ND and N-target at Q =0.5 (GeV/c) .

 
Figure 6:   Electron energy loss spectrum for (e,e'n) for ND and N-target at Q =2.0 (GeV/c) .

The quasielastic cross section is at worse about equal to that of production. However, while the neutron detector covers the entire quasi elastic phase space, only a fraction of the phase space of the neutrons originating from -production processes will be covered by the neutron detector, thus eliminating the major part of the background. We have estimated that the electron-neutron coincidence requirement will cut the effective coincidence e-n background cross section to about 15% of the deuteron quasi-free background. This remaining contribution will be eliminated by measuring the neutron energy via TOF. For the flight paths assumed, and a conservative estimate of the time resolution obtainable ( = 0.3 ns, see below) we obtain an energy resolution of < 100 MeV. The small contribution to the background from the ensures that this energy resolution will suffice to separate the deuteron quasielastic events from the -production events.

The neutron detector will be assembled from 1.6 x 0.1 m scintillator bars 10 cm thick; a total thickness of 30 cm will yield about 40% neutron detection efficiency. The University of Virginia has presently operating a similar neutron detector 1.6 x 1.6 m , 20 cm thick, with the same scintillating units. With this detector and minimum ionizing particles, a timing resolution of 120 ps has been achieved. This detector will be extended to match the size and thickness listed above. At the present time, 48 of the 72 scintillator bars, the E detectors and the associated electronics are at hand; the rest will be procured in the course of 1993.

Part of the detector will be covered by a double layer of E detectors. This will allow us to check the efficiency in separating protons from neutrons. The analysis of the events from different planes of the E-detector will provide information to the same end.

The shielding in front of the detector needed to remove soft x-rays is expected to be in the thickness range of several mm to 1cm. A thickness of 1-3 mm of lead was sufficient to run at NIKHEF (duty cycle 1%, peak current of 30 A, target thickness of 50mg/cm ) a successful d(e,e'n) experiment that measured the neutron magnetic form factor by detecting recoil protons and neutrons simultaneously, using a similar setup as proposed here [19]. Given the much more favourable conditions (luminosity, duty cycle) of the proposed experiment, we expect no difficulties with high single rates or accidental rates.