The CLEO III upgrade is primarily driven by the desire to study
rare decays. Operationally, this means
providing space for particle identification by reducing the tracking length
relative to CLEO II
while maintaining the tracking performance by simultaneously reducing the
amount of material.
The material in the various devices of CLEO II which a charged particle
under normal incidence traverses, is listed in Table
. The
CLEO III design was arrived at by studying the effects of changes to the inner
detector geometry and material on the track parameters: d,
,
,
, and
. All
designs were compared with a similar model for CLEO II.
In these studies, the various tracking layers were defined by their
radii and intrinsic spatial resolution, multiple scattering was added
at discrete radii, and the track parameter resolutions for a given momentum
and angle were derived by a matrix inversion technique.
The designs studied
were constructed out of modular pieces consistent with the new IR.
The cell design for the tracking chambers was based on that used in
the CLEO II drift chamber. The modular
pieces for the parametric model were chosen among the specifications shown
in Table
.
The track parameter studies showed that only two tracking devices are necessary, an inner silicon detector followed by a large, uninterrupted, low mass drift chamber. Both the silicon and the inner section of the drift chamber will follow the stepped structure of the machine around a 300 mrad line to achieve an active detector region beginning at 330 mrad. The minimum achievable drift chamber radius is set to be 17.5 cm by the radius of the accelerator rare-earth, permanent quadrupole. The maximum drift chamber radius is set at 82 cm to allow 20 cm of space for the particle identification system.
The critical parameter for achieving the best momentum resolution in every
design was found to be the uninterrupted length of the drift chamber;
i.e., with the outer
radius determined, the inner edge should extend to the smallest radius
without encountering any walls or additional material. In the CLEO III
design all walls and/or material between the silicon and the drift chamber have
been reduced to the equivalent of one 0.5 mm
beryllium layer. Furthermore, a drift chamber gas with twice the
radiation length of argon-ethane ( m for Ar-ethane) is needed
to maintain the tracking
resolution equal to that of CLEO II. We note that under these circumstances
the addition of any tracking chamber between the silicon and drift chamber
degrades the performance of the detector.
The silicon detector begins immediately after the beam pipe which is chosen
by background considerations to have a radius of 2.2 cm.
The detector has four layers at the approximate radii of 2.5 cm,
3.75 cm, 7.5 cm, and 10.0 cm. The very high resolution of a silicon detector
compared to a wire chamber leads to a separated function detector. Namely, the
silicon measures the impact parameter in and polar angle whereas the drift
chamber measures curvature. Both devices contribute to measuring the impact
parameter in
and the azimuthal angle.
As stated earlier, the mark of a successful design is that it at least
reproduce the current CLEO II tracking system performance.
In Table we show the five track parameters generated
by the parametric studies for
the CLEO II detector, the CLEO II/Si detector in which the
PTL is replaced with a 3 layer silicon detector, and the proposed CLEO III
detector. For every parameter and at all momenta the CLEO III design
is comparable to or better than the previous detectors.
The CLEO III design described above has the added benefit that
by moving the outer radial edge of the drift chamber to smaller radius, the
solid angle of ``good barrel'' crystals increases from 70%to 77%of .
This has the effect of deemphasizing the importance of the endcaps.
We note that increasing the magnetic field above 1.5 Tesla would also
improve the
tracking as would an improvement in the spatial resolution of the
tracking chambers. There is no mechanism, however, for increasing the CLEO
magnetic field and there is no obvious technology that would allow a
significant improvement in drift chamber spatial resolution technology beyond
the present DR 2, so we have not pursued these possibilities. We also note that
the CLEO III design is especially sensitive to the placement of the material:
the addition of 0.5% at
cm has the effect of deteriorating the
tracking by an amount equivalent to placing 2.5%
at
cm.
Care in placement of material is also important for the energy
resolution and efficiency of the crystal calorimeter.