Speaker
Description
Summary
HADES is a High Acceptance DiElecton
Spectrometer currently installed at GSI Darmstadt
(Germany), which has as main goal the detection of
electron pairs produced in relativistic pion-nucleus
and nucleus-nucleus collisions, with high
invariant-mass resolution and high acceptance, to
obtain information about the modification of the
properties of vector mesons in nuclear matter, both
normal and hot and compressed. HADES consists of
several subdetectors providing triggering, and particle
identification and discrimination capabilities. Among
these subdetectors there is a TOF system, built of
plastic scintillator rods read by photo-multiplier
tubes at large angles and of Resistive Plate Chamber
(RPC) detectors at low angles, where the particle rate
is low enough. This new low angle ESTRELA detector,
which has recently been approved, covers a polar angle
between 18 and 85 deg. with 2pi azimuthal acceptance, and
consists of a RPC wall containing 1000 double-sided
readout detectors (2000 channels) distributed in 6
sectors, covering an active area of 8 squared-meters.
The Front-End electronics consists of 2 kind of boards:
4-channel daughter boards (DB) and 32-channel
motherboards in which 8 DB are allocated. Accurate
timing measurement are performed by adjustable
threshold discriminators.
The charge of RPC signals are measured from the
time-over-threshold of the integrated signals (with
sliding reference threshold in the next upgrade) via a
comparator with latch-enable, and are encoded as LVDS
signals. Output signals contain information about
both detection time and ionization charge. Time
information is given by the rising edge of
the output signal, which gives the detection time.
Charge information is given by the width of the output
pulse. The 6-layer DB boards provide a digital LVDS
output signal containing accurate time and charge
information in a compact design, using a reduced number
of commercially available and inexpensive components.
The MB is a 8-layer board providing voltage
regulation. Stable thresholds for time-of-flight and
time-over-threshold are set by DAC circuits
daisy-chained on the motherboard and remotely
programmable by Serial Peripheral Interface. A 3-stage
circuit of summing operational amplifiers generates a
multiplicity signal to be used for low level trigger purposes.
The motherboards are 8-layer PCBs, use the novel
technique of plugged vias and are completely impedance
matched to reduce signal reflections and distortions.
Connectors have been carefully selected, and are high
frequency, differential and impedance matched.
Measurements for both electronic pulses and gamma ray
sources show, respectively, about 15 ps (for
pulses above 100 fC) and 40+-5 ps TOF resolutions values.
Studies
of data with several channels firing simultaneously
show levels of cross-talk below 1% for a threshold of
25 fC, and a worsening of the time resolution of 10 ps
at most. Recent data for cosmic rays and secondaries
from 1 GeV C-C collisions show efficiencies larger than 90%
and a time resolution about 75 ps (including the
detector response).
