-------- Original Message --------
Subject: LOI for CPR replacement in Run 2b
Resent-Date: Mon, 18 Sep 2000 14:49:27 -0500 (CDT)
Resent-From: bed@FNALD.FNAL.GOV
Resent-To: BED@IMAPSERVER3.FNAL.GOV
Date: Mon, 18 Sep 2000 15:48:04 -0500
From: Joey Huston <huston@pa.msu.edu>
Organization: Michigan State University
To: goshaw@fnal.gov, bed@fnal.gov, ljn@anl.gov, kuhlmann@anl.gov

			A CPR Replacement for Run 2	

	It may be desireable to replace the current central preradiator
(CPR) for the start of Run 2b. The design of the current CPR will 
result in the detector integrating over 3 crossings (approx 400 ns).
In Run 1B, the CPR occupancy was 17% and in Run 2, it is expected
to suffer significant performance problems at 5E32, and to 
perhaps become useless. In
addition, even though the gain for the current CPR has been reduced,
there may be concerns with the cumulative effects of the charge
integrated by the CPR over the course of Run 2b. As discussed below,
it is possible to design a replacement CPR (CPR++) that will have close
to 100% coverage (compared to 72% for the current preradiator) and that
will be able to instrument the phi crack region between modules. 

        A design for a replacement detector should have a fast response
time, have a flexibility in the possible segmentation, be easily 
constructed and installed, and, most of all, be relatively cheap to
build. Such design criteria can be satisfied using a
scintillator-wavelength
shifting (WLS) fiber based CPR similar in design to the preshower
detector
in the endplug upgrade. To save on electronic costs, the existing
electronic channels for the CPR and CCR would be utlized, allowing for
a total of 64 channels per wedge. To save on scintillator costs, the
same
type of extruded scintillator would be used as that produced for MINOS.
The amount needed for CPR++ would be less than 0.1% of the MINOS
production.
The MINOS extrusions size is 4.1 cm wide by 1 cm thick, with a groove
along one surface to allow for a WLS fiber to be imbedded. A thickness
of 1 cm is probably appropriate for our purposes; if needed, a new die 
could be constructed for the extrusion of arbitrary width for about
$5K. The cost for the polystyrene is on the order of $5/kg; since we are
using less than 1000 kg, the cost from this source is small. 

There is a great deal of flexibility regarding the segmentation
possible, with the constraint that the total number of channels per
wedge not exceed 64 (thus allowing for the use of only the existing
electronics). For example, a segmentation of 32 channels in eta
and 2 in phi wil allow for the same ability to identify electrons and
photons as the current CPR, but will also allow for an improved jet
energy resolution (using the techniques of the dijet mass group). The
readout of the extruded scintillator will be by WLS fiber, possibly
spliced to clear fiber (using the same technology developed for the
endplug upgrade). The clear (or WLS) fibers would extend to the 
edges of the CPR. There the fibers will be terminated in optical
connectors
which will mate to optical cables, also constructed using the endplug
technology. The cables transport the light signals through the crack to
optical decoder boxes. In the decoder boxes, the light signals will
be multiplexed onto 16 channel MAPMT's. The plan would be to use the
same type of MAPMT's as were used by the new plug preshower (and by
MINOS) and a similar design of decoder box. The best location of these
decoder boxes is still an open question.

        It also is currently an open question as to how many WLS 
fibers to use per CPR scintillator segment. More than two may be
needed to increase the light yield. The pixels for the MAPMT
have a unifrom active area of 2.5X2.5 mm^2, allowing for a number
of fibers to be directed to the same pixel. 

        Such a detector can be constructed in a relatively short time
(less than 1 year) and will have a short installation time. The major
cost is that for the MAPMT's (about $50/channel). Other major expenses 
include the fibers (both WLS and clear) and the costs of construction.
Some optical cable stock remains from the endplug project, possibly
reducing the needs/costs in this area. A more complete cost estimate
is in progress but a reasonable range is from $300K-$500K.


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