Detector name: Coordinate Detector
Overview
The Coordinate Detector (CDET) serves a critical function in the
electron arm of the SBS experiments; in GEp it takes advantage of the
kinematic angular correlation between the elastic electron and the
recoil proton. CDET is also an integral part of the hadron arm of the
GMn and GEn experiments.
Function
One of the main features of the SBS detector system is the ability to
operate at very high luminosities. In the GEp experiment, finding the
correct proton track in the spectrometer in the presence of very high
background rates is possible only due to the angular correlation
between the elastic proton and electron. CDET is used to measure the
electron's vertical angle which, given the small size of the electron
beam, significantly reduces the proton track search region in the front
GEM chambers of the SBS. This allows reconstruction of the proton
trajectory with efficiencies greater than 95%. Once the proton's track is
determined, the kinematic correlations are used to isolate the elastic
peak from the inelastic background. CDET is placed directly in front of
the electromagnetic calorimeter (ECAL) in the GEp experiment. In the
GMn and GEn experiments, the CDET walls are placed in one plane
directly in front of the hadron calorimeter (HCAL-J), where it works as a
proton tagger in GMn and as a charged particle veto detector in GEn.
Performance Requirements
CDET should provide at least:
 CDET coordinate resolution of
 Time resolution of
 Efficiency, per plane, of
2 mm
2 ns
95%
The coordinate resolution for CDET listed above makes an in-plane
electron-proton angular correlation cut more than a factor of two
tighter than could be done without CDET’s information. CDET also
provides an important means of rejection of the high-energy photoproton events, which are a dominant physics background in the GEp
experiment.
Calibration
Initial calibration of CDET before the experimental runs will be done
using cosmic rays. During the experiments CDET will be calibrated
using elastic e-p events.
Physical Characteristics
CDET consists of two planes of extruded scintillator (similar to Saint
Gobain BC-408 type) plates with sensitive areas of 102 cm x 294 cm.
A single plane includes three 98-cm tall “modules”.
Each module consists of:
 392 scintillator plates of dimensions 0.5 cm x 4.0 cm x 51 cm with
individual readout. The scintillator will be produced at an FNAL
extrusion facility and milled to its final thickness by ELJEN.
CDET consists of:
 1,176 readout channels, per plane (3 modules).
 CDET’s total number of plates (6 modules) and readout channels is
2,352. Individual plates are oriented toward the target by using
20-50 micron shims between the plates.
 Light output from a single bar is collected via a 2-mm diameter
fast wavelength shifting (WLS) fiber (Saint Gobain BCF-92 type)
which passes through a 3 mm diameter central hole along the
length of each bar.
 WLS fibers are coupled to 16-channel multi-anode PMTs (maPMT,
Hamamatsu H8711 and H6568-R5900-M16 types with built-in
bases) for the processing of light-output.
 CDET will use a total of 168 maPMTs (14 pixels out of the 16 in a
maPMT will be used in order to account for the condition of
available 15 year old PMTs). The current divider of the individual
pixel output signal will be used for equalizing the signals at the
A/D card input.
Electronics
CDET electronics include the front-end, HV, and DAQ systems. In the
front-end system, the 16-channel maPMT will be connected to a
custom-built 16-channel amplifier/discriminator card, A/D, based on a
NINO chip. The A/D card has been developed by the University of
Glasgow. A total of 28 cards will be mounted on the frame of each
CDET module within 1 m of the maPMTs. A total of 168 A/D cards will
be required for the CDET.
For each readout channel the card produces two outputs: a logical
signal on the LVDS level for a 100 Ohm impedance twisted pair and an
analog for a 50 Ohm load. After propagation from the detector to the
shielded bunker via flat cables, the LVDS signals will be converted to the
ECL level using the level translators designed by JLab and digitized by
LeCroy 1877S TDCs. A time-over-threshold feature of the A/D card
allows crude measurement of the PMT signal amplitude. A total of 84
TDC modules will be required for the CDET (200 TDC modules have
been donated by Belle/KEK).
The HV system is based on a CAEN SY527 HV mainframe with 24channel CAEN 932AN HV modules. A total of 7 HV modules and 1 HV
crate will be required for the CDET. These units have been obtained on
loan from the CDF Collaboration at FNAL.
In CDET the DAQ will use logical signals. The DAQ system includes 9
Fastbus crates with 3 power supply units, 9 CPU units, and 9 SFI
modules, 84 TDC 1877S, and 3 ADC 1881m (used for initial calibration
only).