The Forward Time Projection Chamber (FTPC) in STAR
--------------------------------------------------
1. Introduction
---------------
The FTPCs provide particle acceptance in the pseudorapidity range of 2.5<[eta]<4.0
and allow the measurement of pion and proton production rates as well as for charged
and neutral strange particles. Due to the high multiplicity, approx. 1000 charged
particles in a central collision, also event-by-event observables like and
charged particle fluctuations can be studied. The extented acceptance improves the
general event characterization and allows the study of asymmetric systems like p-A
collisions.The design and construction was carried out by the MPI Munich with
contributions from LBNL Berkeley, BNL Brookhaven, UC Davis, UCLA Los Angeles, and
MEPhI Moscow.
2. Detector Design
------------------
2.1. Conceptual Design
The design was determined mainly by two considerations: Firstly by the high particle
density with tracks under small angles in respect to the beam direction and
secondly by the restricted available space inside the TPC, where the FTPCs are located.
In Fig. (JB 97, Abb.30) the final design is shown. It is a cylindrical structure,
75 cm diameter and 120 cm long, with a radial drift field and readout chambers
located on the outer surface. The radial drift field was choosen to improve
the two-track-separation in the region close to the beam pipe where the particle
density is highest. The radial field is formed by the inner HV-electrode,
a thin metalized plastic tube, and the outer cylinder wall at ground potential.
The field region at both ends is closed by a ring structure of concentric rings,
made out of thin aluminum pipes. The front end electronics, which amplifies,
shapes, and digitizes the signals, is mounted on the back of the readout chambers.
Each particle trajectory is sampled up to 10 times by 9600 pads (1.6x20mm2).
The above design has some unusual and new features for a TPC: (i) The electrons drift
in a radial electrical field perpendicular to the solenoidal magnetic field.
(ii) Curved readout chambers are used to keep the radial field as ideal as possible.
(iii) A two-track separation of 1-2 mm is needed which is an order of
magnitude better than in all previous built TPCs. To meet these requirements a R+D
program was initiated, including the selection of the most suitable gas mixture, the
development of the fabrication technology for the curved readout chambers, and the
optimization of the wire and pad geometry for the readout chambers.
2.2. Selection of Gas Mixture
Due to the short drift length of only 23cm a cool gas mixture with CO2 or DME
can be used. It has a low diffusion coefficient for electrons and a small Lorentz
angle [Nim paper]. After extensive measurements the decision was made to use a
Ar/CO2 (50%/50%) mixture which is nonflammable, shows no or little aging effect in
comparison to hydrocarbons and is chemical less agressive than a mixture with DME.
Fig.:(proposal,fig.24) shows the measurements of drift time, cluster sizes and
the deflection angle due to the Lorentzforce in the magnetic field for the Ar/CO2
gas mixture.
2.3. Readout Chambers
In a conventional TPC the anode (amplification) wires are orthotogonal to the axial
direction of the pads. This is impossible in the case of a curved readout chamber. But
the wires can not be parallel to the pads and therefore to the cylinder axis,
because focusing effects lead to distortions of the position measurement in this
case. This is demonstrated in fig.: (JB97,fig.32). However, if two or more wires
cross the pad under a small angle this effect already vanishes.
For the FTPC design an angle of 17.4 degrees was chosen having three
crossing wires for the selected pad-wire geometry. The anode wires are first glued
on the flat pad plate with conductive epoxy. Afterwards the plate is bended between
three rollers to the final curvature without breaking the wires. A complete
readout chamber with 2 padrows is shown in fig.: (JB 99,fig.78). With only
1.5mm distance between the anode wires and the padplane the spread of the signal
(the so-called Pad Response Function) is small. This together with the low electron
diffusion and the radial drift principle gives the required two-track-separation
of 1mm as can be seen in fig.: (JB 98, fig.34).
2.4. Readout Electronics
The two FTPCs have 19.600 channels of electronics, capable of measuring time samples.
The drift time of 50 microsec for the 23 cm drift length is subdivided into 256 time
bins. For the slow drift gas a risetime of 350 nsec is used. The sampling rate is
5 MHz.
The front end electronics closely follow the electronics of the central TPC [bet96,
from proposal].Each pad is read out by a low-noise STAR preamplifier/shaper (SAS),
which sends signals to a switched capacitor array/ADC chip (SCA/ADC). Four of
these chip sets, handling 64 channels, are mounted on a small FEE card, which is
positioned directly on the detector, parallel to the readout chambers. Fifteen
FEE cards are read out by a readout board, which sends the signals over a
1.2 Gbit/sec fiber-optic link to the data aquisition system. The readout board
also controls the FEE cards, based on signals from the clock and trigger distribution
system and the slow control links. For temperature control the FEE and readout
boards are water cooled using a leakless, low pressure circulation system [...].
2.5. Laser Calibration System
The laser calibration system serves for three primary purposes: i) It provides
calibrated, straight tracks to infer corrections for spacial distortions due to
mechanical or drift field imperfections; ii) it will help to calibrate the drift
velocity in the nonuniform radial drift field; and iii) it allows detector
testing independent from the colllider operation.
The primary laser beams are provided by the Nd:YAG lasers, run in the fourth
harmonic (266 nm), which are also used by the STAR-TPC laser system. These
beams with 30mm diameter are transported to the FTPCs, one on each side. Two
remote controlled mirrors in each transfer path and two CCD cameras allow the
precise beam steering toward the detector. A distribution box subdivides the
incoming beam into three beams with 8mm diameter each. Two 45 degree mirrors
bring the smaller beams to fused silica windows in the gas containment wall.
Inside the FTPC they hit five 1mm pickoff mirrors mounted on a 5mm ceramic tube
at different radii. The five beams are arranged in a plane with three beams
parallel to the detector axis and two are tilted to allow measurements at mani
radii. The three planes are displaced by approx. 120 degrees in azimuth.
3.0. Simulation and Reconstruction of Experimental Data
The first step in the reconstruction of tracks is to calculate the track points
(cluster finding) from the charge distribution measured by the readout electronics.
In a second step (track finding), these track points are grouped to tracks. Using
the magnetic field map, the up to ten position measurements per track are then
used to fit the momentum.
3.1. Cluster Finding
The reconstruction of track points is done by the FTPC Cluster Finding program. It is
optimized to deal with high track densities while minimizing the use of computing
time. The program reads in the electronic signal data from the data aquisition
system, looks for areas of nonzero charge (cluster), deconvolutes clusters and
fits the point coordinates. The transformation from pad position and drift time
in cartesian coordinates includes the correction of distortion introduced by the
magnetic field. For a typical central Au-Au collision with 1000 particles in
the both FTPCs the program needs about 5 seconds on a 400 MHzIntel PentiumII
processor.
3.2. Track Reconstruction
The second step in the analysis of FTPC data is the reconstruction of the particles
tracks and their momenta. The designed FTPC track reconstruction code is based on
an algorithm developed for fast online reconstruction. It is a conventional track-
following algorithm optimized for minimum use of computing power. In this code
all position calculations are done in a transformed coordinate system in which points
appear on a straight line if they form a helix in cartesian coordinates. This
processing step is known as conformal mapping. It saves calculation time in the
track fitting, because all fits can be done by linear regression. With the same code
a primary vertex position and the particles momentum can be determined.
Fig.:(JB 99, fig80,unten) shows a reconstructed HIJING event for a central
Au-Au collision at 200 GeV per nucleon pair. From 14.745 space points 1026
particle tracks were reconstructed in 6 seconds.
4.0. Physics Simulation Studies
Simulation studies demonstrate the capability of distinguishing different
theoretical models of nucleus-nucleus collisions such as HIJING, NEXUS and VNI
with measurements in the FTPCs. Fig. xx shows the rapidity distribution of
net positive charges which follow the p-antip distribution and characterise
the baryon stopping in the reaction. Fig. yy shows histograms of the effective
temperature as determined event-by-event. Such measurements will be usedto study and
search for flucyuations of event properties and to selectspecial event classes.
5.0. Summary
Based on the prototype measurements and simulations it is expected to obtain a
position resolution of 100 micro-m, a two-track-separation of 1mm, a momentum
resolution between 12 and 15%, and an overall reconstruction efficiency between
70 and 80%.