The Enge spectrometer which was used in E89-009 will be installed as a spectrometer to analyzes scattered electron momentum. However, as already mentioned, the spectrometer will be tilted vertically by 2.25 degrees, which was not the case in the previous E89-009 experiment.
The detectors required at the Enge are also summarized in Table 3. For the Enge spectrometer, tracking of the electrons are required to achieve good momentum resolution since it is tilted and the focal plane is no more the one originally designed. The expected rate is only a few MHz and is 2 order of magnitude smaller than the case of E89-009, in which the beam intensity as limited by this Brems electron rate.
The optics of the combined system of Splitter plus the tilted Enge spectrometer
shows the same general features about focal plane geometry and momentum
dispersion as the original untilted geometry used in E89-009.
However, with the introduced tilted angle with respect to the
horizontal plane, the momentum correlates to all the focal plane parameters, x, x', y,
and y', where the x' and y' are the in-plane and out-of-plane angles, respectively. Thus,
a full tracking including both position and angular measurements is needed. The
momentum resolution is studied as a function of position and angular errors as shown
in Fig.18.
The dominant contributions are from x and x', which means that the
measurements in y and y' are less crucial.
The results showed that using a 4th-order
optical matrix and the momentum resolution can be better than
3
10
(FWHM),
if the position error is about 0.15 mm (r.m.s.) and the angular error is
about 1 mr (RMS).
Such precision can be easily reached by the conventional wire chamber technique and
the multiple scattering contributions from the light vacuum window material used in
the HNSS experiment and wire chambers is small, if the first tracking plane is located
along the focal plane. With a central momentum about 300 MeV/c, the resolution
contribution is about 90 keV (FWHM) or less, thus small compare to other
contributions.
Similarly segmented scintillation hodoscope array as used in the HNSS experiment
will be used. The thickness will be increased to improve the time resolution of 250 ps
achieved in the HNSS experiment for better signal /accidental separation. An on-line
coincidence between the e' and K
will be used for this proposed experiment to reduce
the data size.
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