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A forward magnetic spectrometer system for high-energy heavy-ion experiments
Nuclear Instruments and Methods in Physics Research A 438 (1999) 282}301
A forward magnetic spectrometer system for high-energy heavy-ion experiments Forward Spectrometer Team of BNL-AGS E802/866 Collaboration K. Shigaki!,*,1, Y. Akiba",1, C. Chasman#, Z. Chen#, H. Hamagaki",2, A. Kumagai$, K. Kurita$,3, Y. Miake$, H. Sako",4, O. Sasaki%, S. Ueno-Hayashi$, H.E. Wegner#,5, F. Zhu# !Department of Physics, University of Tokyo, Hongo, Tokyo 113-0033, Japan "Institute for Nuclear Study, University of Tokyo, Tanashi, Tokyo 188-0002, Japan #Physics Department, Brookhaven National Laboratory, Upton, NY 11973-5000, USA $Institute of Physics, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan %KEK, Tsukuba, Ibaraki 305-0801, Japan Received 19 April 1999; accepted 10 June 1999
Abstract A small aperture magnetic spectrometer has been built to study hadron production in 197Au#197Au collisions at the AGS energy of 11.6A GeV/c. It operates in the forward angular range from 6 to 303 with respect to the incident beam axis and covers the mid-rapidity region for heavy particles such as protons. The detector components of the spectrometer system include two time projection chambers, four drift chamber modules and a time-of-#ight scintillation counter wall. A few new technologies are implemented in the design of the system to achieve the performance goals. The spectrometer has proved to function properly under the high particle-density environment encountered in experiments with the heavy-ion colliding system. The achieved momentum resolution is 1.3% in r.m.s. for pions at 1 GeV/c and 1.6% for protons at the same momentum. With the time-of-#ight resolution of 76 ps in r.m.s., the particle identi"cation momentum limit extends to 4 GeV/c for pions, 3 GeV/c for kaons, 5 GeV/c for protons, and 4.5 GeV/c for anti-protons. The tracking e$ciency stays above 86% for tracks up to 5 GeV/c with as many as 10 tracks in the spectrometer aperture. ( 1999 Elsevier Science B.V. All rights reserved. Keywords: Magnetic spectrometer; Relativistic heavy-ion collision; Tracking chambers; Particle identi"cation
* Corresponding author. Tel.:#1-516-344-7801; fax: #1516-344-7841. E-mail address: [email protected] (K. Shigaki) 1 Now at KEK, Tsukuba, Ibaraki 305-0801, Japan. 2 Now at Center for Nuclear Study, University of Tokyo, Tanashi, Tokyo 188-0002, Japan. 3 Now at Radiation Laboratory, RIKEN, Wako, Saitama 351-0198, Japan. 4 Now at GSI, Darmstadt D-64291, Germany. 5 Deceased.
1. Introduction In studies of relativistic nucleus}nucleus collisions, the measurement of semi-inclusive spectra of identi"ed particles (pions, kaons, protons, anti-protons and so forth) is essential to the understanding of reaction dynamics and particle production
0168-9002/99/$ - see front matter ( 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 6 8 0 - 4
K. Shigaki et al. / Nuclear Instruments and Methods in Physics Research A 438 (1999) 282}301
mechanisms. For example, the rapidity distribution of protons provides a measure of baryon stopping power, and it can be used to estimate the baryon density of the matter formed in the colliding system. Anti-proton yields can provide further insight into the baryon density because of the large annihilation cross section. Strangeness production, expected to be a signal for exotic states of matter, can be studied through measurement of charged kaons. The primary purpose of the E866 experiment at the AGS is to study particle production in 197Au#197Au collisions at 11.6 A GeV/c [1}5]. The experiment measures semi-inclusive spectra of pions, kaons, protons, anti-protons and even heavier hadrons over a wide kinematic region with global event characterization detectors establishing the collision geometry. The AGS-E802/859 experiments have previously measured those spectra in reactions induced by proton and lighter ion beams up to 28Si [6}9]. In 197Au#197Au collisions, however, a much higher baryon density is predicted in the reaction system [10,11] with consequently increased interest in the particle spectra. The major experimental challenge with the heavier colliding system is the higher spatial density of produced particles in the forward angular region. The spatial density of charged particles produced in a typical central 197Au#197Au collision at 11.6 A GeV/c is more than 4 times higher compared to a 28Si#197Au case at 14.5 A GeV/c. With this higher particle density, the E802 spectrometer [12] performs well only in the angular region larger than 203 in the laboratory frame which means that spectra of particles heavier than pions at mid-rapidity (y&y "1.6) are not covered by the spec#.4 trometer. Measurement of particle production is of particular importance at mid-rapidity, where the baryon rapidity density should peak in central 197Au#197Au collisions. A small aperture magnetic spectrometer has hence been built to complement the E802 spectrometer by covering the forward angular range down to 63. The angular region covered by the spectrometer corresponds to the mid-rapidity region of protons and kaons. The forward spectrometer was "rst commissioned in 1993, and operated through 1995 in 197Au beam runs at the AGS.
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This paper describes the design, characteristics, and performance of the forward spectrometer system. An overview of the design of the spectrometer is given in Section 2. Section 3 describes the characteristics of each component of the spectrometer. The performance of the spectrometer is discussed in Section 4, and the conclusions are given in Section 5.
2. Design concept 2.1. Acceptance of the spectrometer system Fig. 1. shows the kinematic coverage of the forward spectrometer and the E802 spectrometer for pions, kaons and protons at the AGS energies. The angular coverage and ranges of measurable momentum are converted onto the phase space of rapidity and transverse momentum. The E802 spectrometer, which properly functions with the particle density up to 0.3}0.5/msr, does not cover the mid-rapidity region for particles heavier than pions. The forward spectrometer deals with about 10 times higher particle density to cover the most interesting kinematic region. The useable momentum range of the forward spectrometer is limited on the high side by its particle identi"cation capability. The lowest measurable momentum is determined by the "eld strength of the two bending magnets and is 0.4 A GeV/c for the nominal settings. With a twodimensional particle identi"cation method based on momentum and time-of-#ight, the particle identi"cation limit reaches 4 GeV/c for pions, 3 GeV/c for kaons, 5 GeV/c for protons, and 4.5 GeV/c for anti-protons. See Section 4.3 for details. 2.2. Design principles and basic layout The con"guration of the forward spectrometer is schematically shown in Fig. 2, in which the spectrometer is placed at its most forward angle, i.e. 63 with respect to the incident beam axis. The spectrometer is placed on the opposite side to the E802 spectrometer in the AGS-B1 experimental area. The spectrometer is designed to sweep a region from 63 to about 303 in the laboratory frame with
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Fig. 1. The coverages in the rapidity and transverse-momentum space of the two arms of the spectrometer; for pions (top), kaons (middle), and protons (bottom). Solid lines: the forward spectrometer (FS); broken lines: the E802 spectrometer (HH). The forward spectrometer covers an angular range from 63 to 283 and a momentum range from 0.4 to 4.0 GeV/c for pions, to 3.0 GeV/c for kaons and to 5.0 GeV/c for protons; the E802 spectrometer covers from 203 to 563 and from 0.5 to 5.5 GeV/c for pions, to 3.6 GeV/c for kaons and to 5.8 GeV/c for protons.
respect to the incident beam, covering about 33 in polar angle at one time.6 The design emphasis is on its capability to deal with high particle-density events. Its solid angle is rather small (4.7 msr) so
6 The aperture is about 43.
that the multiplicity of particles entering the aperture is moderate, 6}10 in average per central 197Au#197Au collision at the most forward angle. The solid angle is still su$cient for measurement of single-particle spectra, even for particles with small production cross sections such as anti-protons, and of closely correlated particle pairs. The distance between the target and the center of the "rst chamber station is 2.5 m to reduce the spatial particle density at the detectors. Shielding made of heavy metal is placed limiting the acceptance to reduce the background rate in the tracking chambers and hence to make the spectrometer system function properly even at the most forward angles. E!ects of the shielding are discussed in Section 3.1. A twomagnet arrangement is adopted to reduce background from the shielding. A dipole magnet (FM1) is placed in front of the "rst tracking chambers to curl up secondary particles with low momenta, which are generated in various material in the experimental apparatus including the target, the beam pipe and the shielding. It also serves to reduce the spatial particle density at the chambers by de#ecting the detected particles. The momenta of the detected particles are determined by the second magnet (FM2), with more bending power, placed between the two tracking stations. Speci"cations of the bending magnets are in Section 3.2. The forward spectrometer is equipped with two tracking chamber stations in front of and behind the analyzing magnet, FM2. The "rst station consists of a time projection chamber (TPC1) and two drift chamber modules (FT1, FT2) sandwiching TPC1; the second likewise contains a time projection chamber (TPC2) and two drift chamber modules (FT3, FT4). The time projection chambers are the primary devices to "nd and de"ne tracks, making the best use of their high capability of threedimensional pattern recognition with good twoparticle separation. The drift chambers are to improve the spatial resolution of the tracking system by "xing the endpoints of the track segments found in the time projection chambers. The three-dimensional tracking capability of the time projection chambers not only reduces the number of incorrectly identi"ed tracks but also diminishes the requirements on the computational power needed for track reconstruction. Characteristics of the
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Fig. 2. A schematic view of the experimental setup of the AGS-E866. The lower half shows the forward spetrometer built for the E866 experiment, at the most forward angle setting at 63. FM1, FM2: dipole magnets; TPC1, TPC2: time projection chambers; FT1}4: drift chamber modules; FTOF: a time-of-#ight wall. The upper half shows the E802 spectrometer.
tracking chambers are described in Sections 3.4 and 3.5. A high-segmentation time-of-#ight scintillation counter wall (FTOF) is used for particle identi"cation. An on-tube discriminator technique is developed to improve its timing resolution. The characteristics and performance of FTOF, as well as the technique, are described in Section 3.6. A photograph of the forward spectrometer is shown in Fig. 3.
3. Characterization of the individual subsystems In this section, we describe characteristics of each subsystem of the forward spectrometer, i.e. detectors and auxiliary components. We de"ne a coordinate system on the spectrometer as follows: the origin is at the target position; the z-axis is a line perpendicular to the surfaces of the magnets and the tracking chambers as shown in
Fig. 2; the y-axis is vertically upward; the x-axis is taken to form a right-handed rectangular system. The spectrometer angle is de"ned as the angle between the incident beam line and the z-axis of the spectrometer coordinates. 3.1. Collimator A collimator made of heavy metal (90% tungsten, 6% nickel and 4% copper) is placed in front of FM1, i.e. at the most upstream of the spectrometer system. Its front surface with a square window of 52 mm high and 52 mm wide is placed 76 cm away from the target. It de"nes the opening angles of the spectrometer aperture to be 4.03 in the horizontal direction and $2.03 in the vertical, leading to the solid angle of 4.7 msr. The thickness of the collimator is 56 cm, corresponding to 5.6 nuclear interaction length (1.5]102 radiation length). It prevents charged particles emitted outside the detector acceptance bump into surrounding
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3.2. Dipole magnets The forward spectrometer has two dipole magnets: FM1 is placed upstream of the "rst tracking chamber station to sweep out background particles with low momenta; FM2 is an analyzing magnet with an e!ective length of 94 cm and bending power of about 0.9 T m at its full strength. Both of the magnets are asymmetric window-frame type with iron yokes and "eld clumps at their entrances and exits. Their characteristic parameters are summarized in Table 1. Field maps of FM1 and FM2 have been measured with Hall probes at 0.6 and 0.4 T settings, respectively [15]. The magnets are placed so that the z-axis of the spectrometer coordinate system is perpendicular to the entrance surfaces and 10 mm inside of the edges of the gaps. The distances from the target to the centers of FM1 and FM2 are 175 and 351 cm, respectively. 3.3. Trigger Fig. 3. A photograph of the forward spectrometer viewed from the upstream of the beamline. The spectrometer is seen on the right side of the beam pipe.
materials, e.g. the return yoke of the "rst magnet and frames of the tracking chambers, and create background into the tracking system. The e!ect of the collimator was studied extensively using the GEANT simulation packages7 [13] with the GHEISHA hadron shower generator [14]. For central 197Au#197Au collisions, the average number of good tracks from the interaction point is 6}7 per event at the entrance of the "rst tracking station. Number of background particles, most of which are hadrons from secondary production, is as large as 12}13 without any shielding, but can be reduced by a factor of 3}4 with a lead collimator of the particular geometry and by a factor of 8}10 with a tungsten one. The heavy metal is selected as the shielding material due to the space limitation.
7 Versions 3.13 and 3.14 of the simulation code were used for the Monte Carlo study.
3.3.1. External trigger The readout electronics system of the forward spectrometer is provided with three levels of external triggers by the data acquisition system of the E866 experiment. A fast-timing pre-trigger8 (LVL0) signals an interaction of a beam particle in the target region. A trigger accepted by the data acquisition system is called a "rst-level trigger (LVL1). The experiment also has a second-level trigger9 provided to the readout electronics as a fast clear signal (LVL2). The LVL0 and the LVL1 signals are available on the spectrometer platform, where most readout electronics for the tracking chambers are placed, approximately 200 and 800 ns after the interaction, respectively. The LVL2 comes 60 ls later if it applies.
8 The pre-trigger is implemented as de"ned only with the beam counters and the beam-fragments counter, and is created near the experimental apparatus for the fast timing. 9 The E802 spectrometer is capable of particle-identi"cation triggers and provides a second-level trigger to both the spectrometer arms when they run in parallel. The forward spectrometer is not equipped with its own second-level trigger.
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Table 1 Characteristic parameters of the dipole magnets Magnet Gap size (w]h]d) E!ective "eld length Maximum "eld
(mm3) (cm) (T)
FM1
FM2
229]159]508 62 &1.0
597]283]762 94 &1.0
3.3.2. Spectrometer trigger To e!ectively collect minimum bias data with the small-aperture spectrometer, a hardware trigger is formed to signal that at least one particle passes through it. The forward spectrometer trigger (FS) is formed on the spectrometer platform using the "rst two x-planes of FT2 drift chamber module and the "rst two x-planes of FT3, as FS,MFT2(x)XFT2(x@)NWMFT3(x)XFT3(x@)N
(1)
where FTn(w) stands for that as least one wire is "red on the w-plane of the n-th drift chamber module. See Section 3.5 for descriptions of the drift chambers. Requiring at least one particle behind FM2 e!ectively triggers on a particle "ring a chamber in front of the analyzing magnet (FMz) and one particle passing through the whole forward spectrometer. The OR of two planes in each chamber module is to minimize ine$ciency of the tracking chambers. The enhancement factor of the spectrometer trigger over the simple interaction trigger is approximately 2 at the most forward angle of 63 but as large as 4}9 at backward angles of 14}243, where the average multiplicity in the solid angle of the forward spectrometer is much less than 1 per interaction. In the E866 experiment, the FS trigger signal is sent to the data acquisition system and handled as one of the inputs to the LVL1 trigger. 3.4. Time projection chambers At the center of each tracking chamber station, a time projection chamber is set as a main tracking device. The two time projection chambers are designed for operation under a high particle-density condition [16,17] adopting the segmented anode wire readout scheme. Conceptual schematics of the
Fig. 4. A schematic view of segmented anode wires of a time projection chamber. A particle trajectory and drift path of electrons are also shown.
scheme, which has been developed for relativistic heavy-ion collision experiments [18}20], is shown in Fig. 4. The most important advantage of the scheme over the commonly-used cathode pad scheme is better two-particle separation. While the separation with cathode pads is primarily limited by the distribution of the induced electric charge on the pads, that with the segmented anode wire readout scheme is determined dominantly by the di!usion of the drift electrons which can be kept reasonably small by choosing an appropriate gas mixture. Other advantages of the scheme include the smaller amount of the data and availability of pipeline time-to-digital converter (TDC) modules with a lower cost compared to modules needed to read out cathode pads.
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Both chambers are positioned normal to the zaxis of the spectrometer coordinates; the distances from the target to their centers are 250 and 458 cm, respectively. 3.4.1. Mechanical structure Each of the time projection chambers consists of three major parts as shown in Fig. 5, i.e. an aluminum cage holding the gas, a "eld cage to supply the drift "eld for electrons and an end-cap multi-wire proportional chamber (MWPC) with a gating grid [21]. Characteristic parameters of the time projection chambers are summarized in Table 2. To keep the electric "eld in the sensitive volumes uniform, the "eld cage of each chamber has a top electrode with a high negative voltage and side strips and wires to keep a uniform gradient of the potential. The side walls of the cage are made of G10. Copper "eld strips on one side of each wall are staggered to strips on the other side for better "eld uniformity. Since the amount of material is another concern with the front and back sides of the cage, a 6 lm thick copper foil (1.0]10~4 radiation
Fig. 5. A schematic structure of the upstream time projection chamber, TPC1.
Table 2 Characteristic parameters of the time projection chambers Chamber Sensitive volume (w]h]d) Lever arm Number of rows Row-to-row pitch Chamber gas Drift "eld Maximum drift length
TPCI
TPC2
(mm)
29]20]28 230 6 46
77]35]33 270 6 54
(V/cm) (cm)
Ar/iso-C H (75 : 25) 4 10 800 26
Ar/CH (80 : 20) 4 160}220 42
(cm3) (mm)
Number of anode wires Total number of readout
(/row)
96 576
192 1152
Anode-to-anode pitch Anode-to-cathode gap E!ective anode wire length Anode wire diameter Operation voltage
(mm) (mm) (mm) (lm) (kV)
3.0 3.0 7 15 1.85}1.95
4.0 4.0 7 15 2.10}2.20
Gating grid wire pitch Grid-to-cathode distance Gating grid operation voltage
(mm) (mm) (V)
1.0 8.0 $80
1.0 8.0 $50
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Fig. 6. A cross sectional view of a part of the end cap wire chamber of the upstream time projection chamber, TPC1.
length) pasted on a 25 lm kapton "lm (4.2]10~4 radiation length) is used for each of the entrance and exit windows of the "eld cage to shield the outer electric "eld, with a help of another row of wires (0.9]10~4 radiation length in average) inside each window. 3.4.2. Gas and drift xeld Each chamber has several gas inlets distributed over its bottom surface, through its end-cap MWPC as shown in Fig. 6, and two outlets on its side wall for e!ective gas circulation. The upstream chamber, TPC1, is operated with a gas mixture of argon (75%) and isobutane (25%) to obtain a small electron di!usion size10 and, consequently, a small pixel size. It is an essential requirement for the chambers in the "rst tracking station where the particle density is higher. As TPC2 has a longer drift length and its pixel size requirement is less stringent, it is operated with a mixture of argon (80%) and methane (20%) which requires a less strong electric "eld for stable operation. The drift "eld for TPC1 is set at
10 The di!usion size of electrons for 1 cm drift in the gas mixture is 210 lm in r.m.s. [22]
800 V/cm, slightly below the plateau region of the drift velocity of electrons. That for TPC2 is 160}220 V/cm which is on the plateau. 3.4.3. End-cap multi-wire proportional chamber The end-cap chamber is composed of four layers of wires or pads, i.e. a gating grid, a cathode grid, segmented anode wires and a cathode plane from the top to the bottom, on "ve layers of G10 boards with single-sided printed circuits as shown in Fig. 6. A connector board for the pre-ampli"ers, which also has connectors for high voltages and the gas inlet, is at the bottom of the layers. The anode wires are 15 lm diameter goldplated tungsten, and the cathode grid wires are 100 lm diameter gold-plated Cu}Be. See Table 2 for more characteristic parameters of the MWPCs. 3.4.4. Gating grid system Each of the time projection chambers is equipped with a gating grid system to suppress distortion of its drift "eld caused by positive ions drifting back from the MWPC ampli"cation part to the sensitive gas volume [23,24]. Fig. 7 shows the schematics of the gating grid in its open and closed states. The gating grid system is designed as to have the
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Fig. 7. The open (the top two plots) and closed (the bottom two plots) states of the gating grid of TPC2, simulated with the GARFIELD [25]. The left two plots show trajectories of electrons from the sensitive region (upside); the right two plots show trajectories of positive ions drifting back from the anode wires.
two sets of grid wires with operation voltages of opposite polarities placed symmetric with respect to every sense wire of the chamber; the induced pulses on the sense wires are canceled out with the con"guration to minimize the dead time of the readout electronic modules. The gating grid plane has 100 lm diameter Cu}Be wires in 1 mm pitch, and is 8 mm above the cathode grid.
Voltages on the gating grid wires of each time projection chamber are controlled by a gating grid driver with an external trigger (see Section 3.3). Based on the design of drivers for the PEP-4 experiment at Stanford Linear Accelerator Center and the TOPAZ experiment [26] at KEK, Japan, it has been upgraded to operate with a higher o!set voltage up to 3 kV. A schematic circuit diagram of the gating grid driver is shown in Fig. 8.
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Fig. 8. A schematic circuit diagram of the gating grid driver for the time projection chambers. < is the o!set voltage and < is the 3%& * operation voltage. Three optical couplings are used for electric isolation.
3.4.5. Readout electronics The readout electronics for the time projection chambers uses a current sensitive pre-ampli"er of Redeka type [27] and an ampli"er/ discriminator. They are designed by the KEK electronics group originally for experiments at the Superconducting Kaon Spectrometer [28] at KEK. The pre-ampli"er boards contain 16 each hybrid ampli"er chips, and are directly mounted on the bottom planes of the chambers. The quasi-differential output of the pre-ampli"er is sent to the ampli"er/discriminator card. The cards are arranged in 6-unit VME crates on the spectrometer platform to minimize the transmission length of the analog signal and reduce the electric noise. A common threshold voltage is provided to the cards in each crate. FASTBUS pipeline TDCs (LeCroy model 1879) are used to record the timing of the leading and trailing edges of the discriminated signals. Each pipeline TDC module contains 96 bu!er channels 1024 bins deep. The widths of the recorded pulses re#ect the energy deposit of the particles and provide supplementary information for identifying composite particles with the electric charge larger than one.
The data acquisition trigger, LVL1, signal is used with a delay to stop the pipeline TDCs through FASTBUS calibration and trigger (CAT) modules (LeCroy model 1810). The readout system of each of the two time projection chambers has a CAT module which is also used to program the full range of the TDCs to cover maximum drift time of the chamber, i.e. about 6 ls for TPC1 and 8 ls for TPC2. The stop signal is accordingly delayed by 7 and 9 ls for TPC1 and TPC2, respectively. The TDC time binning (smaller than 10 ns) is "ne enough considering the cluster/pixel size of the chambers in the drift (y-) direction. A fast clear signal from the second level trigger, LVL2, is provided to the TDCs through the CAT modules to stop data conversion for unwanted events and to reduce the dead time. The gating grid drivers on the other hand require fast triggering, since it takes 0.5}1.0 ls to fully open the gating grids. They are hence triggered by the pre-trigger, LVL0, which comes within 200 ns after the interaction. The grids are closed again in 700 ns unless the corresponding LVL1 signal is provided during the period, in which case the grids are kept open for the time duration covering the maximum drift time of the each time projection chamber.
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K. Shigaki et al. / Nuclear Instruments and Methods in Physics Research A 438 (1999) 282}301 Table 3 Performance of the upstream time projection chamber, TPC1, measured in test beams. Variation of the values represent measurements with di!erent drift lengths. x- and y-directions are transverse and parallel to the drift direction, respectively. Cluster size per row
(x) (y)
(mm) (mm)
4.0}4.6 2.4}2.6
Position resolution per row
(x) (y) (x) (y)
(lm) (lm) (mrad) (mrad)
560}660 285}330 2.7}3.3 1.4}1.6
(%)
96.7}99.4
Angular resolution with 6 rows
E$ciency per row
The cluster size11 listed in Table 3 represents the e!ective pixel size of the chamber in the x}y plane. It is a measure of the two-particle separation capability, one of the most important performance parameters for chambers under high particle-density conditions. While the cluster size depends on several factors including the applied clustering algorithm and correlated to the chamber gain and threshold, i.e. the detection e$ciency, the main factor governing the two-particle separation is the di!usion size of electrons on the drift path. Note that the di!usion size of TPC1 is 800 lm in r.m.s. averaged over its sensitive volume. That of TPC2 is larger due to the di!erent gas mixture and the longer drift length, leading to larger clusters. Fig. 9. A schematic logic diagram of the readout system for the time projection chambers (TPC), the drift chambers (FT) and the time-of-#ight wall (FTOF). Boxes lableled G.G. stand for gate generators. See text for descriptions.
Fig. 9 shows a schematic logic diagram of readout electronics for the time projection chambers, the drift chambers and the time-of-#ight wall. 3.4.6. Performance The performance of TPC1 was tested with beams at the BNL AGS and the KEK PS in 1992 [29] and is summarized in Table 3.
3.5. Drift chambers Four projective drift chamber modules, FT1}4, are utilized for tracking, complimentary to the time projection chambers, to improve spatial resolution of the tracking system. All the drift chamber modules are positioned normal to the z-axis of the spectrometer coordinates. The distances from the target to their centers are 225, 276, 427 and 489 cm, respectively.
11 Hits created by a particle on a row of segmented sense wires form a cluster in the two-dimensional space of wire position and drift time. The size of the cluster is measured in the converted geometric space, i.e. in the x}y plane.
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3.5.1. Mechanical structure Characteristic parameters of the drift chambers are summarized in Table 4. In order to handle high multiplicity events, the chambers are highly segmented, with the maximum drift length ranging from 2 mm for FT1 to 5 mm for FT4. Each chamber module has alternating layers of foil planes and wire planes. Aluminized mylar foils of 25 lm thick are stretched and glued to the frame boards made of G10 with printed circuits. Proper electric contacts between both sides of the foils and the detector frames are ensured by conducting glue. The wires of the chambers are wound controlling the pitch of the wires with grooved bars. The wires are gold-plated tungsten, and their diameters are listed in Table 4. Wires are glued onto aligned frame boards and subsequently soldered. The tension on the wires is controlled by adjustable weights during the wiring procedure. After chamber fabrication the tension and uniformity of the wires are checked by observing the resonant frequency of the forced vibration induced by applying electric pulses of about 200 V to the sense wires on the frame. 3.5.2. Gas and drift xeld The drift chambers are operated with a gas mixture of argon (50%) and isobutane (50%). The
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applied drift voltages for the chambers are listed in Table 4 along with the cell sizes. 3.5.3. Readout electronics The basic structure of the readout electronics for the drift chambers consists of a current sensitive pre-ampli"er followed by a combination of ampli"er and discriminator. FASTBUS pipeline TDCs (LeCroy model 1879) are used to read out the drift time. The dynamic range of the pipeline TDCs is set to be the minimum for the highest time resolution, but is still large (1 ls) compared to the maximum drift time of all the chambers which is within 150 ns. That allows reducing the required number of TDC channels (and the cost) by half, by multiplexing two wire channels into one TDC channel after delaying one of the signals by an appropriate amount of time. Fig. 10 illustrates the design schematically. The high segmentation of the chambers implies a small probability of having more than one particle in a single cell, and relieves the need for the electronics to distinguish two pulses from the same wire. Grounded-base pre-ampli"ers with an impulse response of 2 ns are plugged on the chambers in groups of 16 channels each, except at some edges where only 8 channels are left to group. The
Table 4 Characteristic parameters of the drift chambers. The x-planes have vertical wires, and wire orientations in y-, u- and v-planes are 903, !303 and 303, respectively, measured counterclockwise from the vertical, looking into the beam Chamber module Sensitive area (w]h) Number of wire planes Plane order Wire-to-foil gap Number of wires per plane
FT1
FT2
FT3
FT4
(mm2)
238]174
353]233
644]320
803]360
(mm)
10 xx@uu@yy@vv@xx@ 1.6
10 xx@uu@yy@vv@xx@ 1.6
8 xx@uu@vv@xx@ 2.4
8 xx@uu@vv@xx@ 2.4
x y u v
56 40 64 64 560
56 36 64 64 552
78 * 80 80 632
78 * 80 80 632
(mm) (lm) (kV)
2.0 10.0 2.40
3.0 12. 2.55
4.0 15. 2.75
5.0 15. 2.80
Total number of wires Maximum drift length Anode wire diameter Operation voltage
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Fig. 10. Schematics of the drift chamber readout electronics which combines two adjacent wire channels into one TDC channel with delaying one of the signals.
pre-ampli"er12 is developed at BNL for applications requiring a fast response and short pulse shaping. The unit is tested to ensure required timing, hence position, resolution of the chambers. Besides the electronic properties, the compact design, 19 mm]13 mm with hybrid technology, allows dense packing of the chips in a small board which can be plugged directly onto the chambers. A resistor of 62 ) before the output is used to eliminate cross talks between neighboring channels. Ribbon twisted pair cables of 6 m in length are used to propagate analog signals to the ampli"erdiscriminator boards. The signal from the pre-ampli"er is further ampli"ed by a fast ECL line receiver MC10115 with a typical rise time of 2 ns. The ampli"ed signal is fed into the discriminator circuit. The discriminator consists of three stages of di!erential ampli"cation with a feedback, which stretches the output logic signal to ensure proper functioning of the following ECL delay chip. Since the delay chip requires the minimum width of 100 ns for the input pulse, longer pulses of about 200 ns wide are produced for
12 The pre-ampli"er is code-named BNL IO-354.
the channels intended for the delay lines, while the pulse width for the other channels is set at 120 ns. The delay time is about 400 ns with a jitter smaller than 1 ns. Two channels from neighboring wires, one without delay and one with, are joined together with a wired-OR. The logic signals are regenerated and propagated in twisted pair cables to the FASTBUS TDCs. The ampli"er and discriminator circuit is fabricated on a 4-layer printed circuit board and housed in a 9-unit VME crate. Each board comprises 64 input channels and 32 output channels. A single threshold voltage is supplied to each board, and a strobe signal is provided to cut o! pulses with unwanted timing. One of the concerns in the physical layout of the readout electronics boards is possible cross talk between neighboring channels sharing the same printed circuit board and twisted pair cable. At the nominal threshold, no cross talk is observed in pulser tests up to the point where the output of the pre-ampli"er saturates. An upper limit of electronic cross talk is also found to be about 1.5% from the probability of two neighboring wires "red simultaneously in the real data. The LeCroy 1879 pipeline TDCs receive the LVL1 signal as the common stop signal directly from their front panels, without using a CAT module. Their full range is set at the minimum, i.e. 1 ls, to obtain the best time binning of 1 ns. Since the LVL1 arrives at the modules on the spectrometer platform approximately 800 ns after the particles pass through the chambers, no additional delay is needed for the stop timing. The TDCs receive the LVL2 signal as a fast clear. 3.5.4. Performance Each pair of planes of the drift chambers is staggered with "eld wires of one plane aligned on top of the sense wires of the other plane. The sum of the two TDC values from the two staggered cells should hence be a constant value, corresponding to the size of the cells assuming a constant drift velocity. The distribution of this timing sum is used to evaluate the timing resolution of the chamber. Resolution of 3 ns in r.m.s., corresponding to spatial resolution of about 150 lm, is achieved in a beam test at the BNL AGS with the operating
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voltages listed in Table 4 and with e$ciencies higher than 95% per plane. A consistent resolution is also obtained with pulser signals. 3.6. Time-of-yight wall High capability of particle identi"cation is required for systematic studies of semi-inclusive spectra of mesons and baryons produced in relativistic heavy-ion collisions. A time-of-#ight scintillation counter wall (FTOF), located at the most downstream end of the spectrometer system, is adopted as the particle-identi"cation device. It is tilted on the x}z plane clockwise by 63 with respect to the x-axis, as seen in Fig. 2, in order to maximize its e!ective segmentation. The distance from the target to the beam-side edge of FTOF is 600 cm. 3.6.1. Mechanical structure The time-of-#ight wall has 100 segments. Each segment, called a slat, is a rod of plastic scintillator with square cross section made of Bicron BC404. Fig. 11 shows a sketch of FTOF, and Table 5 summarizes its geometric characteristic parameters. Each slat of FTOF is read out with two photomultiplier tubes (Hamamatsu model R3478S), through light guides on its ends. The tubes are operated at !1.80 kV. Since the photo-multiplier tube is wider than the scintillator rod, the light guide is bent by 303 and arranged alternatively out of the FTOF plane as shown in Fig. 11 to accommodate all the tubes. A newly developed on-tube
Table 5 Geometric characteristic parameters of the time-of-#ight wall Sensitive area (w]h) Number of segments Segment size (w]h]d)
(cm2) (mm3)
126]42 100 12.4]419]12.5
discriminator board with two outputs, analog and discriminated, is mounted on each tube [30]. 3.6.2. On-tube discriminator The main factor which determines the timing resolution of a photo-multiplier tube is the number of photo-electrons. Assuming that N photo-electrons are emitted at the same time on a photocathode of a photo-multiplier tube whose transit time spread is p , the best timing obtainable with TTS the tube is p /JN. TTS Time-of-#ight resolution in real experiments is worse than the pure photo-multiplier tube timing due to many factors such as electric noise, spread of the photon arrival time onto the photo-cathode and so forth. There is hence room to improve the actual time-of-#ight resolution for a given tube by reducing those factors. One of the attempts made in this system is to improve the time-of-#ight robustness against electric noise. It is achieved by making the signal rise time as short as possible at discrimination. When a leading edge discriminator is used, the detected arrival time jitters due to the electric noise superimposed on the signal shape as indicated in Fig. 12. It is a random noise and cannot be corrected for. The time jitter p introduced by this noise is 5*.% p p " /0*4% 5*.% Dd
Fig. 11. A sketch of the time-of-#ight wall, FTOF.
295
(2)
where p is the noise level and d
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Fig. 12. How a higher voltage gradient d
therefore introduced what we call on-tube discriminator to the time-of-#ight system to eliminate the necessity of signal transmission through cables. It is on a 5 cm]20 cm circuit card which is directly mounted onto a photo-multiplier tube as shown in Fig. 11. The high-voltage bleeder and the discriminator circuit are implemented on the card sharing the same ground. This feature is another advantage in reducing the electric noise. The signal from the tube is "rst passively split into two. One side of the split signal is discriminated by the on-board circuit to send out a NIM timing signal; the other side is directly sent as an analog output. 3.6.3. Readout electronics After the discriminated timing signal is sent from the experimental area to the counting house though a 68 m long foam-polyethylene RG58 A/U coaxial cable (Belden type 8219), it is discriminated again with a NIM discriminator (Phillips model 710). The discriminator output is then sent to a CAMAC TDC (Phillips model 7186). Each TDC module contains 16 channels of time digitizers with the minimum binning of 25 ps and full range of 100 ns. The TDCs are operated in common start mode with the LVL0 signal as the start. Unless the LVL1
signal is received within 200 ns, a fast clear signal is generated and all the TDC modules get cleared together. The recovery time is 850 ns. When the LVL1 signal is received in time, the hit register bits for the modules whose LAM is set13 are read out "rst. Only the corresponding channels which got hit are read out. The LVL2 signal is also provided to the TDCs as a fast clear. On the other hand, the analog output of the on-tube discriminator is put into a FASTBUS analog-to-digital converter (ADC) module (LeCroy model 1882N) in the counting house, with an additional delay of 250 ns. The module has 96 channels each of charge sensitive ADC, with minimum binning of 50 fc and the full range of 200 pc. The ADC information is used to correct for the timing shift due to the pulse height variation (the `slewing correctiona) to improve the time-of-#ight measurement, as well as for a charge cut to discriminate hadrons from photons and d-rays. The ADC gate pulse is locally generated from the LVL1 signal and fed into the ADC modules through a CAT module (LeCroy model 1810). The CAT also distributes the LVL2 signal as a fast clear to the ADCs. 3.6.4. Performance A bench test and a beam test were performed to evaluate the on-tube discriminator. At the bench test, a pulsed laser (Hamamatsu PLP-02) was used as the light source with a time jitter less than 10 ps/3C. Fig. 13 shows the output voltage gradient d
13 LAM is set if the hit register of the TDC module is non-zero.
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297
are evaluated from the residual distribution with the track projection onto the detector. Fully reconstructed p~ tracks within the momentum range from 1.0 to 3.0 GeV/c are selected for the evaluation to minimize the e!ect of multiple scattering and of contamination by other particle species. Table 6 summarizes the performance of the detectors in the 197Au beam runs in 1994. All the values listed are averaged over the sensitive areas, while the detection e$ciency of the time projection chambers is found to have a slight position dependence. Note that all the listed values of resolution are in r.m.s.. 4.2. Tracking performance Fig. 13. Observed voltage gradient d
4. Spectrometer performance 4.1. Detector performance The e$ciency and the spatial and timing resolutions of each detector in the forward spectrometer
The e$ciency of the tracking system is a function of particle multiplicity in the spectrometer. Under high multiplicity conditions, the hardware ine$ciency of each detector increases due to "nite granularity and resolution. Tracking algorithms can also be confused by misassociation of hits and lose an increasing fraction of tracks. With the smallaperture spectrometer, the e!ect is more apparent for particles with higher momenta since they pass through the detectors almost parallel to other tracks, while particles with lower momenta are swept to regions where local particle density is lower.
Table 6 A summary of the performance of the detectors in the forward spectrometer in the 197Au beam runs. Variation of TPC performance represents di!erent spectrometer settings. All the listed values are in r.m.s. Time projection chamber E$ciency per row Resolution per row Cluster size
(x) (y) (x) (y)
(%) (mm) (mm) (mm) (mm)
Drift chamber E$ciency per plane Resolution per plane
(%) (mm)
Time-of-#ight wall Timing resolution Position resolution
TPC1
TPC2
87}97 0.47}0.65 0.28}0.37 4.6}5.0 1.9}2.3
83}94 0.81}1.20 0.56}0.85 7.1}7.6 4.3}5.3
FT1
FT2
FT3
FT4
97 0.25
96 0.30
97 0.34
93 0.37
FTOF
(y)
(ps) (mm)
76 8.5
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Fig. 14. Estimated tracking e$ciency as a function of total multiplicity and momentum of the track.
The e$ciency of the tracking system is evaluated by merging a Monte Carlo track into real events. Fig. 14 shows the result as a function of particle multiplicity in the spectrometer for merged tracks at di!erent momenta. The tracking e$ciency stays above 86% for tracks up to 5 GeV/c with a total multiplicity as high as 10. In an extreme condition with 20 particles in the spectrometer, the e$ciency goes down to 92% at 1 GeV/c and 76% at 3 GeV/c. 4.3. Particle identixcation capability Fig. 15 shows a scatter plot of measured squared mass versus signed momentum in a 197Au beam run with the spectrometer at 143 and a magnet setting favoring negative particles. The cuts used for particle identi"cation are also shown in the "gure. Pions, kaons and protons are identi"ed on a particle-by-particle basis from the measured momentum and time of #ight. Misidenti"cation of other particle species sets the upper limit of the identi"able momentum range, especially for particles with relatively low yields. An asymmetric cut on the squared mass is applied to suppress the contamination and extend the high momentum limit. Additional cuts are applied to identify anti-
Fig. 15. A scatter plot of measured squared mass versus signed momentum in a 143 run with a magnet setting favoring negative particles. See text for a description on the particle identi"cation cuts indicated by the solid lines.
protons whose yield is more than three orders of magnitude lower than protons. The optional cuts include energy deposit and isolation on the timeof-#ight wall [31]. The particle identi"cation limit reaches 4 GeV/c for pions, 3 GeV/c for kaons and 5 GeV/c for protons with less than 1% contamination from other particle species, and 4.5 GeV/c for anti-protons with a systematic error from contamination estimated to be less than 5%. The e$ciency of particle identi"cation is limited by the time-of-#ight wall segmentation. Timing information of second or later hits on each FTOF slat is lost since the readout electronics lacks multiple hit capability. Even the timing and position information of the "rst hit can be distorted if more than one particle hits a slat with a time interval shorter than the light propagation time in the scintillator. The particle identi"cation e$ciency is evaluated from real data as a function of spectrometer setting, trigger type, FTOF slat number and time of #ight. Fig. 16 shows the e$ciency plots for minimum-bias and central 197Au#197Au events with the spectrometer at 63 and 243.
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Fig. 16. Particle identi"cation e$ciency due to the "nite segmentation of FTOF. The e$ciency is plotted as a function of the FTOF slat number and time of #ight. A smaller slat number corresponds to the beam pipe side. The top two plots are for minimum-bias 197Au#197Au events; the bottom two are for central 197Au#197Au events. The left two plots are with the spectrometer at 63; the right two plots are at 243.
4.4. Momentum resolution The momentum resolution of the forward spectrometer is evaluated from the experimental mass resolution for pions, kaons and protons. Its dependence on the momentum and particle species can be deconvoluted into three contributing factors: the intrinsic angular resolution of the tracking system, the timing resolution of the time-of#ight system, and the e!ect of multiple scattering [32]. Fig. 17 shows the result of the momentum resolution evaluation by the method for pions and protons in a 143 run as a function of momentum. Resolution of 1.3% in r.m.s. is achieved for pions at 1.0 GeV/c, and 1.6% for protons at the same momentum. The time-of-#ight resolution evaluated by this method is 76$4 ps in r.m.s. and is consistent with the evaluation using high momentum p~ tracks also 76 ps.
Fig. 17. The experimental resolution of momentum for pions (solid line) and protons (dotted line) in a 143 run as a function of momentum.
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Fig. 18. A typical particle spectrum measured with the forward spectrometer [5]. Invariant cross section divided by the trigger cross section for identi"ed protons in central 197Au#197Au collisions at 11.6 A GeV/c, in di!erent rapidity intervals as a function of transverse kinetic energy m !m . The solid points 5 0 are data measured with the forward spectrometer (FS); the open ones are with the E802 spectrometer (HH). The bin width of the rapidity interval is 0.1, and *y denotes the absolute di!erence of the measured rapidity of each spectrum from the centre-of-mass rapidity y "1.6. The spectra are scaled down successively by #.4 a factor of 10. The error bars are statistical only, either shown or smaller than the data point.
4.5. Particle spectra Fig. 18 shows a typical particle spectrum measured with the forward spectrometer, invariant cross section divided by the trigger cross section for protons as a function of transverse kinetic energy. The data were collected in the 1994 runs of the E866 experiment from central 197Au#197Au collisions at 11.6 A GeV/c, with event selection in use of the zero-degree calorimeter. Single particle spectra measured with the spectrometer are presented in Ref. [4,5]. 5. Conclusions A small aperture magnetic spectrometer has been built to study hadron production in 197Au#197Au
collisions at the AGS energies. It operates in the forward angular range down to 63 with respect to the incident beam axis and covers the mid-rapidity region for heavy particles such as protons. The detector components of the forward spectrometer include two time projection chambers, four drift chamber modules and a time-of-#ight scintillation counter wall. A few new technologies are implemented in the design of the system to achieve the performance goals. The spectrometer has proved to function properly under the high particle-density environment encountered in experiments with the heavy colliding system. The achieved momentum resolution is 1.3% in r.m.s. for pions at 1 GeV/c and 1.6% for protons at the same momentum. With time-of-#ight resolution of 76 ps in r.m.s., the particle identi"cation limit reaches 4 GeV/c for pions, 3 GeV/c for kaons, 5 GeV/c for protons, and 4.5 GeV/c for antiprotons. The tracking e$ciency stays above 86% for tracks up to 5 GeV/c with as many as 10 tracks in the spectrometer aperture.
Acknowledgements We acknowledge Mr. K. Asselta, Mr. E. Baker, Mr. H. Diaz, Mr. J. Dioguardi, Mr. S. Kato, Mr. Y. Matsuyama and Mr. R.A. Scheetz for their technical supports. This work is supported in part by the US Department of Energy under contract DE-AC02-98CH10886 with BNL and in part by Japanese Ministry of Education, Science, Sports and Culture.
References [1] C. Chasman, H. Hamagaki et al., Proposal submitted to the AGS Program Committee, 866 (1990). [2] E802/866 Collaboration, F. Videbaek et al., Nucl. Phys. A 590 (1995) 249c. [3] E802/866 Collaboration, K. Shigaki et al., Nucl. Phys. A 590 (1995) 519c. [4] E802/866 Collaboration, L. Ahle et al., Phys. Rev. Lett. 81 (1998) 2650. [5] E802/866 Collaboration, L. Ahle et al., Phys. Rev. C57 (1998) R466. [6] E802 Collaboration, T. Abbott et al., Phys. Lett. B271 (1991) 447.
K. Shigaki et al. / Nuclear Instruments and Methods in Physics Research A 438 (1999) 282}301 [7] E802 Collaboration, T. Abbott et al., Phys. Rev. D45 (1992) 3906. [8] E802 Collaboration, T. Abbott et al., Phys. Lett. B291 (1992) 341. [9] E802 Collaboration, T. Abbott et al., Phys. Rev. C50 (1994) 1024. [10] H. Sorge et al., Phys. Lett. B 243 (1990) 7. [11] S.H. Kahana et al., Proceedings of HIPAGS'93, BNL preprint 48888 (1993). [12] E802 Collaboration, T. Abbott et al., Nucl. Instr. and Meth. A290 (1990) 41. [13] CERN Application Software Group, CERN Program Library W5013 (1992). [14] H.C. Fesefeldt, Technical Report, PITHA 85-02 (1985). [15] E802/866 Collaboration, to be published. [16] K. Shigaki, Master's Thesis, University of Tokyo (1992). [17] H. Sako, Master's Thesis, University of Tokyo (1993). [18] C.R. Gruhn et al., Proposal submitted to the CERN SPSC 84-13 (1984). [19] C.R. Gruhn, Nucl. Phys. A 461 (1987) 391c.
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[20] E810 Collaboration, B.E. Bonner et al., Nucl. Phys. A 498 (1989) 523c. [21] P. NeH methy et al., Nucl. Instr. and Meth. 212 (1983) 273. [22] V. Palladino, B. Sadoulet, Nucl. Instr. and Meth. 128 (1975) 323. [23] D. Friedrich et al., Nucl. Instr. and Meth. 158 (1979) 81. [24] F. Sauli, Z. Phys. C 38 (1988) 339. [25] R. Veenhof, CERN Program Library W5050 (1991). [26] TOPAZ-TPC Group, T. Kamae et al., Nucl. Instr. and Meth. A252 (1986) 423. [27] T. Zimmerman, Internal Report, FNAL (1986). [28] T. Fukuda et al., Nucl. Instr. and Meth. A 361 (1995) 485. [29] K. Shigaki et al., E866 Memo 5, BNL (1992). [30] A. Kumagai, Master's Thesis (in Japanese), University of Tsukuba (1994). [31] H. Sako, Doctor Thesis, University of Tokyo INS-J-186 (1997). [32] K. Shigaki, Doctor Thesis, University of Tokyo UTPN224 (1995).
A forward magnetic spectrometer system for high-energy heavy-ion experiments Forward Spectrometer Team of BNL-AGS E802/866 Collaboration K. Shigaki!,*,1, Y. Akiba",1, C. Chasman#, Z. Chen#, H. Hamagaki",2, A. Kumagai$, K. Kurita$,3, Y. Miake$, H. Sako",4, O. Sasaki%, S. Ueno-Hayashi$, H.E. Wegner#,5, F. Zhu# !Department of Physics, University of Tokyo, Hongo, Tokyo 113-0033, Japan "Institute for Nuclear Study, University of Tokyo, Tanashi, Tokyo 188-0002, Japan #Physics Department, Brookhaven National Laboratory, Upton, NY 11973-5000, USA $Institute of Physics, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan %KEK, Tsukuba, Ibaraki 305-0801, Japan Received 19 April 1999; accepted 10 June 1999
Abstract A small aperture magnetic spectrometer has been built to study hadron production in 197Au#197Au collisions at the AGS energy of 11.6A GeV/c. It operates in the forward angular range from 6 to 303 with respect to the incident beam axis and covers the mid-rapidity region for heavy particles such as protons. The detector components of the spectrometer system include two time projection chambers, four drift chamber modules and a time-of-#ight scintillation counter wall. A few new technologies are implemented in the design of the system to achieve the performance goals. The spectrometer has proved to function properly under the high particle-density environment encountered in experiments with the heavy-ion colliding system. The achieved momentum resolution is 1.3% in r.m.s. for pions at 1 GeV/c and 1.6% for protons at the same momentum. With the time-of-#ight resolution of 76 ps in r.m.s., the particle identi"cation momentum limit extends to 4 GeV/c for pions, 3 GeV/c for kaons, 5 GeV/c for protons, and 4.5 GeV/c for anti-protons. The tracking e$ciency stays above 86% for tracks up to 5 GeV/c with as many as 10 tracks in the spectrometer aperture. ( 1999 Elsevier Science B.V. All rights reserved. Keywords: Magnetic spectrometer; Relativistic heavy-ion collision; Tracking chambers; Particle identi"cation
* Corresponding author. Tel.:#1-516-344-7801; fax: #1516-344-7841. E-mail address: [email protected] (K. Shigaki) 1 Now at KEK, Tsukuba, Ibaraki 305-0801, Japan. 2 Now at Center for Nuclear Study, University of Tokyo, Tanashi, Tokyo 188-0002, Japan. 3 Now at Radiation Laboratory, RIKEN, Wako, Saitama 351-0198, Japan. 4 Now at GSI, Darmstadt D-64291, Germany. 5 Deceased.
1. Introduction In studies of relativistic nucleus}nucleus collisions, the measurement of semi-inclusive spectra of identi"ed particles (pions, kaons, protons, anti-protons and so forth) is essential to the understanding of reaction dynamics and particle production
0168-9002/99/$ - see front matter ( 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 6 8 0 - 4
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mechanisms. For example, the rapidity distribution of protons provides a measure of baryon stopping power, and it can be used to estimate the baryon density of the matter formed in the colliding system. Anti-proton yields can provide further insight into the baryon density because of the large annihilation cross section. Strangeness production, expected to be a signal for exotic states of matter, can be studied through measurement of charged kaons. The primary purpose of the E866 experiment at the AGS is to study particle production in 197Au#197Au collisions at 11.6 A GeV/c [1}5]. The experiment measures semi-inclusive spectra of pions, kaons, protons, anti-protons and even heavier hadrons over a wide kinematic region with global event characterization detectors establishing the collision geometry. The AGS-E802/859 experiments have previously measured those spectra in reactions induced by proton and lighter ion beams up to 28Si [6}9]. In 197Au#197Au collisions, however, a much higher baryon density is predicted in the reaction system [10,11] with consequently increased interest in the particle spectra. The major experimental challenge with the heavier colliding system is the higher spatial density of produced particles in the forward angular region. The spatial density of charged particles produced in a typical central 197Au#197Au collision at 11.6 A GeV/c is more than 4 times higher compared to a 28Si#197Au case at 14.5 A GeV/c. With this higher particle density, the E802 spectrometer [12] performs well only in the angular region larger than 203 in the laboratory frame which means that spectra of particles heavier than pions at mid-rapidity (y&y "1.6) are not covered by the spec#.4 trometer. Measurement of particle production is of particular importance at mid-rapidity, where the baryon rapidity density should peak in central 197Au#197Au collisions. A small aperture magnetic spectrometer has hence been built to complement the E802 spectrometer by covering the forward angular range down to 63. The angular region covered by the spectrometer corresponds to the mid-rapidity region of protons and kaons. The forward spectrometer was "rst commissioned in 1993, and operated through 1995 in 197Au beam runs at the AGS.
283
This paper describes the design, characteristics, and performance of the forward spectrometer system. An overview of the design of the spectrometer is given in Section 2. Section 3 describes the characteristics of each component of the spectrometer. The performance of the spectrometer is discussed in Section 4, and the conclusions are given in Section 5.
2. Design concept 2.1. Acceptance of the spectrometer system Fig. 1. shows the kinematic coverage of the forward spectrometer and the E802 spectrometer for pions, kaons and protons at the AGS energies. The angular coverage and ranges of measurable momentum are converted onto the phase space of rapidity and transverse momentum. The E802 spectrometer, which properly functions with the particle density up to 0.3}0.5/msr, does not cover the mid-rapidity region for particles heavier than pions. The forward spectrometer deals with about 10 times higher particle density to cover the most interesting kinematic region. The useable momentum range of the forward spectrometer is limited on the high side by its particle identi"cation capability. The lowest measurable momentum is determined by the "eld strength of the two bending magnets and is 0.4 A GeV/c for the nominal settings. With a twodimensional particle identi"cation method based on momentum and time-of-#ight, the particle identi"cation limit reaches 4 GeV/c for pions, 3 GeV/c for kaons, 5 GeV/c for protons, and 4.5 GeV/c for anti-protons. See Section 4.3 for details. 2.2. Design principles and basic layout The con"guration of the forward spectrometer is schematically shown in Fig. 2, in which the spectrometer is placed at its most forward angle, i.e. 63 with respect to the incident beam axis. The spectrometer is placed on the opposite side to the E802 spectrometer in the AGS-B1 experimental area. The spectrometer is designed to sweep a region from 63 to about 303 in the laboratory frame with
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Fig. 1. The coverages in the rapidity and transverse-momentum space of the two arms of the spectrometer; for pions (top), kaons (middle), and protons (bottom). Solid lines: the forward spectrometer (FS); broken lines: the E802 spectrometer (HH). The forward spectrometer covers an angular range from 63 to 283 and a momentum range from 0.4 to 4.0 GeV/c for pions, to 3.0 GeV/c for kaons and to 5.0 GeV/c for protons; the E802 spectrometer covers from 203 to 563 and from 0.5 to 5.5 GeV/c for pions, to 3.6 GeV/c for kaons and to 5.8 GeV/c for protons.
respect to the incident beam, covering about 33 in polar angle at one time.6 The design emphasis is on its capability to deal with high particle-density events. Its solid angle is rather small (4.7 msr) so
6 The aperture is about 43.
that the multiplicity of particles entering the aperture is moderate, 6}10 in average per central 197Au#197Au collision at the most forward angle. The solid angle is still su$cient for measurement of single-particle spectra, even for particles with small production cross sections such as anti-protons, and of closely correlated particle pairs. The distance between the target and the center of the "rst chamber station is 2.5 m to reduce the spatial particle density at the detectors. Shielding made of heavy metal is placed limiting the acceptance to reduce the background rate in the tracking chambers and hence to make the spectrometer system function properly even at the most forward angles. E!ects of the shielding are discussed in Section 3.1. A twomagnet arrangement is adopted to reduce background from the shielding. A dipole magnet (FM1) is placed in front of the "rst tracking chambers to curl up secondary particles with low momenta, which are generated in various material in the experimental apparatus including the target, the beam pipe and the shielding. It also serves to reduce the spatial particle density at the chambers by de#ecting the detected particles. The momenta of the detected particles are determined by the second magnet (FM2), with more bending power, placed between the two tracking stations. Speci"cations of the bending magnets are in Section 3.2. The forward spectrometer is equipped with two tracking chamber stations in front of and behind the analyzing magnet, FM2. The "rst station consists of a time projection chamber (TPC1) and two drift chamber modules (FT1, FT2) sandwiching TPC1; the second likewise contains a time projection chamber (TPC2) and two drift chamber modules (FT3, FT4). The time projection chambers are the primary devices to "nd and de"ne tracks, making the best use of their high capability of threedimensional pattern recognition with good twoparticle separation. The drift chambers are to improve the spatial resolution of the tracking system by "xing the endpoints of the track segments found in the time projection chambers. The three-dimensional tracking capability of the time projection chambers not only reduces the number of incorrectly identi"ed tracks but also diminishes the requirements on the computational power needed for track reconstruction. Characteristics of the
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Fig. 2. A schematic view of the experimental setup of the AGS-E866. The lower half shows the forward spetrometer built for the E866 experiment, at the most forward angle setting at 63. FM1, FM2: dipole magnets; TPC1, TPC2: time projection chambers; FT1}4: drift chamber modules; FTOF: a time-of-#ight wall. The upper half shows the E802 spectrometer.
tracking chambers are described in Sections 3.4 and 3.5. A high-segmentation time-of-#ight scintillation counter wall (FTOF) is used for particle identi"cation. An on-tube discriminator technique is developed to improve its timing resolution. The characteristics and performance of FTOF, as well as the technique, are described in Section 3.6. A photograph of the forward spectrometer is shown in Fig. 3.
3. Characterization of the individual subsystems In this section, we describe characteristics of each subsystem of the forward spectrometer, i.e. detectors and auxiliary components. We de"ne a coordinate system on the spectrometer as follows: the origin is at the target position; the z-axis is a line perpendicular to the surfaces of the magnets and the tracking chambers as shown in
Fig. 2; the y-axis is vertically upward; the x-axis is taken to form a right-handed rectangular system. The spectrometer angle is de"ned as the angle between the incident beam line and the z-axis of the spectrometer coordinates. 3.1. Collimator A collimator made of heavy metal (90% tungsten, 6% nickel and 4% copper) is placed in front of FM1, i.e. at the most upstream of the spectrometer system. Its front surface with a square window of 52 mm high and 52 mm wide is placed 76 cm away from the target. It de"nes the opening angles of the spectrometer aperture to be 4.03 in the horizontal direction and $2.03 in the vertical, leading to the solid angle of 4.7 msr. The thickness of the collimator is 56 cm, corresponding to 5.6 nuclear interaction length (1.5]102 radiation length). It prevents charged particles emitted outside the detector acceptance bump into surrounding
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3.2. Dipole magnets The forward spectrometer has two dipole magnets: FM1 is placed upstream of the "rst tracking chamber station to sweep out background particles with low momenta; FM2 is an analyzing magnet with an e!ective length of 94 cm and bending power of about 0.9 T m at its full strength. Both of the magnets are asymmetric window-frame type with iron yokes and "eld clumps at their entrances and exits. Their characteristic parameters are summarized in Table 1. Field maps of FM1 and FM2 have been measured with Hall probes at 0.6 and 0.4 T settings, respectively [15]. The magnets are placed so that the z-axis of the spectrometer coordinate system is perpendicular to the entrance surfaces and 10 mm inside of the edges of the gaps. The distances from the target to the centers of FM1 and FM2 are 175 and 351 cm, respectively. 3.3. Trigger Fig. 3. A photograph of the forward spectrometer viewed from the upstream of the beamline. The spectrometer is seen on the right side of the beam pipe.
materials, e.g. the return yoke of the "rst magnet and frames of the tracking chambers, and create background into the tracking system. The e!ect of the collimator was studied extensively using the GEANT simulation packages7 [13] with the GHEISHA hadron shower generator [14]. For central 197Au#197Au collisions, the average number of good tracks from the interaction point is 6}7 per event at the entrance of the "rst tracking station. Number of background particles, most of which are hadrons from secondary production, is as large as 12}13 without any shielding, but can be reduced by a factor of 3}4 with a lead collimator of the particular geometry and by a factor of 8}10 with a tungsten one. The heavy metal is selected as the shielding material due to the space limitation.
7 Versions 3.13 and 3.14 of the simulation code were used for the Monte Carlo study.
3.3.1. External trigger The readout electronics system of the forward spectrometer is provided with three levels of external triggers by the data acquisition system of the E866 experiment. A fast-timing pre-trigger8 (LVL0) signals an interaction of a beam particle in the target region. A trigger accepted by the data acquisition system is called a "rst-level trigger (LVL1). The experiment also has a second-level trigger9 provided to the readout electronics as a fast clear signal (LVL2). The LVL0 and the LVL1 signals are available on the spectrometer platform, where most readout electronics for the tracking chambers are placed, approximately 200 and 800 ns after the interaction, respectively. The LVL2 comes 60 ls later if it applies.
8 The pre-trigger is implemented as de"ned only with the beam counters and the beam-fragments counter, and is created near the experimental apparatus for the fast timing. 9 The E802 spectrometer is capable of particle-identi"cation triggers and provides a second-level trigger to both the spectrometer arms when they run in parallel. The forward spectrometer is not equipped with its own second-level trigger.
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Table 1 Characteristic parameters of the dipole magnets Magnet Gap size (w]h]d) E!ective "eld length Maximum "eld
(mm3) (cm) (T)
FM1
FM2
229]159]508 62 &1.0
597]283]762 94 &1.0
3.3.2. Spectrometer trigger To e!ectively collect minimum bias data with the small-aperture spectrometer, a hardware trigger is formed to signal that at least one particle passes through it. The forward spectrometer trigger (FS) is formed on the spectrometer platform using the "rst two x-planes of FT2 drift chamber module and the "rst two x-planes of FT3, as FS,MFT2(x)XFT2(x@)NWMFT3(x)XFT3(x@)N
(1)
where FTn(w) stands for that as least one wire is "red on the w-plane of the n-th drift chamber module. See Section 3.5 for descriptions of the drift chambers. Requiring at least one particle behind FM2 e!ectively triggers on a particle "ring a chamber in front of the analyzing magnet (FMz) and one particle passing through the whole forward spectrometer. The OR of two planes in each chamber module is to minimize ine$ciency of the tracking chambers. The enhancement factor of the spectrometer trigger over the simple interaction trigger is approximately 2 at the most forward angle of 63 but as large as 4}9 at backward angles of 14}243, where the average multiplicity in the solid angle of the forward spectrometer is much less than 1 per interaction. In the E866 experiment, the FS trigger signal is sent to the data acquisition system and handled as one of the inputs to the LVL1 trigger. 3.4. Time projection chambers At the center of each tracking chamber station, a time projection chamber is set as a main tracking device. The two time projection chambers are designed for operation under a high particle-density condition [16,17] adopting the segmented anode wire readout scheme. Conceptual schematics of the
Fig. 4. A schematic view of segmented anode wires of a time projection chamber. A particle trajectory and drift path of electrons are also shown.
scheme, which has been developed for relativistic heavy-ion collision experiments [18}20], is shown in Fig. 4. The most important advantage of the scheme over the commonly-used cathode pad scheme is better two-particle separation. While the separation with cathode pads is primarily limited by the distribution of the induced electric charge on the pads, that with the segmented anode wire readout scheme is determined dominantly by the di!usion of the drift electrons which can be kept reasonably small by choosing an appropriate gas mixture. Other advantages of the scheme include the smaller amount of the data and availability of pipeline time-to-digital converter (TDC) modules with a lower cost compared to modules needed to read out cathode pads.
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Both chambers are positioned normal to the zaxis of the spectrometer coordinates; the distances from the target to their centers are 250 and 458 cm, respectively. 3.4.1. Mechanical structure Each of the time projection chambers consists of three major parts as shown in Fig. 5, i.e. an aluminum cage holding the gas, a "eld cage to supply the drift "eld for electrons and an end-cap multi-wire proportional chamber (MWPC) with a gating grid [21]. Characteristic parameters of the time projection chambers are summarized in Table 2. To keep the electric "eld in the sensitive volumes uniform, the "eld cage of each chamber has a top electrode with a high negative voltage and side strips and wires to keep a uniform gradient of the potential. The side walls of the cage are made of G10. Copper "eld strips on one side of each wall are staggered to strips on the other side for better "eld uniformity. Since the amount of material is another concern with the front and back sides of the cage, a 6 lm thick copper foil (1.0]10~4 radiation
Fig. 5. A schematic structure of the upstream time projection chamber, TPC1.
Table 2 Characteristic parameters of the time projection chambers Chamber Sensitive volume (w]h]d) Lever arm Number of rows Row-to-row pitch Chamber gas Drift "eld Maximum drift length
TPCI
TPC2
(mm)
29]20]28 230 6 46
77]35]33 270 6 54
(V/cm) (cm)
Ar/iso-C H (75 : 25) 4 10 800 26
Ar/CH (80 : 20) 4 160}220 42
(cm3) (mm)
Number of anode wires Total number of readout
(/row)
96 576
192 1152
Anode-to-anode pitch Anode-to-cathode gap E!ective anode wire length Anode wire diameter Operation voltage
(mm) (mm) (mm) (lm) (kV)
3.0 3.0 7 15 1.85}1.95
4.0 4.0 7 15 2.10}2.20
Gating grid wire pitch Grid-to-cathode distance Gating grid operation voltage
(mm) (mm) (V)
1.0 8.0 $80
1.0 8.0 $50
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Fig. 6. A cross sectional view of a part of the end cap wire chamber of the upstream time projection chamber, TPC1.
length) pasted on a 25 lm kapton "lm (4.2]10~4 radiation length) is used for each of the entrance and exit windows of the "eld cage to shield the outer electric "eld, with a help of another row of wires (0.9]10~4 radiation length in average) inside each window. 3.4.2. Gas and drift xeld Each chamber has several gas inlets distributed over its bottom surface, through its end-cap MWPC as shown in Fig. 6, and two outlets on its side wall for e!ective gas circulation. The upstream chamber, TPC1, is operated with a gas mixture of argon (75%) and isobutane (25%) to obtain a small electron di!usion size10 and, consequently, a small pixel size. It is an essential requirement for the chambers in the "rst tracking station where the particle density is higher. As TPC2 has a longer drift length and its pixel size requirement is less stringent, it is operated with a mixture of argon (80%) and methane (20%) which requires a less strong electric "eld for stable operation. The drift "eld for TPC1 is set at
10 The di!usion size of electrons for 1 cm drift in the gas mixture is 210 lm in r.m.s. [22]
800 V/cm, slightly below the plateau region of the drift velocity of electrons. That for TPC2 is 160}220 V/cm which is on the plateau. 3.4.3. End-cap multi-wire proportional chamber The end-cap chamber is composed of four layers of wires or pads, i.e. a gating grid, a cathode grid, segmented anode wires and a cathode plane from the top to the bottom, on "ve layers of G10 boards with single-sided printed circuits as shown in Fig. 6. A connector board for the pre-ampli"ers, which also has connectors for high voltages and the gas inlet, is at the bottom of the layers. The anode wires are 15 lm diameter goldplated tungsten, and the cathode grid wires are 100 lm diameter gold-plated Cu}Be. See Table 2 for more characteristic parameters of the MWPCs. 3.4.4. Gating grid system Each of the time projection chambers is equipped with a gating grid system to suppress distortion of its drift "eld caused by positive ions drifting back from the MWPC ampli"cation part to the sensitive gas volume [23,24]. Fig. 7 shows the schematics of the gating grid in its open and closed states. The gating grid system is designed as to have the
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Fig. 7. The open (the top two plots) and closed (the bottom two plots) states of the gating grid of TPC2, simulated with the GARFIELD [25]. The left two plots show trajectories of electrons from the sensitive region (upside); the right two plots show trajectories of positive ions drifting back from the anode wires.
two sets of grid wires with operation voltages of opposite polarities placed symmetric with respect to every sense wire of the chamber; the induced pulses on the sense wires are canceled out with the con"guration to minimize the dead time of the readout electronic modules. The gating grid plane has 100 lm diameter Cu}Be wires in 1 mm pitch, and is 8 mm above the cathode grid.
Voltages on the gating grid wires of each time projection chamber are controlled by a gating grid driver with an external trigger (see Section 3.3). Based on the design of drivers for the PEP-4 experiment at Stanford Linear Accelerator Center and the TOPAZ experiment [26] at KEK, Japan, it has been upgraded to operate with a higher o!set voltage up to 3 kV. A schematic circuit diagram of the gating grid driver is shown in Fig. 8.
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Fig. 8. A schematic circuit diagram of the gating grid driver for the time projection chambers. < is the o!set voltage and < is the 3%& * operation voltage. Three optical couplings are used for electric isolation.
3.4.5. Readout electronics The readout electronics for the time projection chambers uses a current sensitive pre-ampli"er of Redeka type [27] and an ampli"er/ discriminator. They are designed by the KEK electronics group originally for experiments at the Superconducting Kaon Spectrometer [28] at KEK. The pre-ampli"er boards contain 16 each hybrid ampli"er chips, and are directly mounted on the bottom planes of the chambers. The quasi-differential output of the pre-ampli"er is sent to the ampli"er/discriminator card. The cards are arranged in 6-unit VME crates on the spectrometer platform to minimize the transmission length of the analog signal and reduce the electric noise. A common threshold voltage is provided to the cards in each crate. FASTBUS pipeline TDCs (LeCroy model 1879) are used to record the timing of the leading and trailing edges of the discriminated signals. Each pipeline TDC module contains 96 bu!er channels 1024 bins deep. The widths of the recorded pulses re#ect the energy deposit of the particles and provide supplementary information for identifying composite particles with the electric charge larger than one.
The data acquisition trigger, LVL1, signal is used with a delay to stop the pipeline TDCs through FASTBUS calibration and trigger (CAT) modules (LeCroy model 1810). The readout system of each of the two time projection chambers has a CAT module which is also used to program the full range of the TDCs to cover maximum drift time of the chamber, i.e. about 6 ls for TPC1 and 8 ls for TPC2. The stop signal is accordingly delayed by 7 and 9 ls for TPC1 and TPC2, respectively. The TDC time binning (smaller than 10 ns) is "ne enough considering the cluster/pixel size of the chambers in the drift (y-) direction. A fast clear signal from the second level trigger, LVL2, is provided to the TDCs through the CAT modules to stop data conversion for unwanted events and to reduce the dead time. The gating grid drivers on the other hand require fast triggering, since it takes 0.5}1.0 ls to fully open the gating grids. They are hence triggered by the pre-trigger, LVL0, which comes within 200 ns after the interaction. The grids are closed again in 700 ns unless the corresponding LVL1 signal is provided during the period, in which case the grids are kept open for the time duration covering the maximum drift time of the each time projection chamber.
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K. Shigaki et al. / Nuclear Instruments and Methods in Physics Research A 438 (1999) 282}301 Table 3 Performance of the upstream time projection chamber, TPC1, measured in test beams. Variation of the values represent measurements with di!erent drift lengths. x- and y-directions are transverse and parallel to the drift direction, respectively. Cluster size per row
(x) (y)
(mm) (mm)
4.0}4.6 2.4}2.6
Position resolution per row
(x) (y) (x) (y)
(lm) (lm) (mrad) (mrad)
560}660 285}330 2.7}3.3 1.4}1.6
(%)
96.7}99.4
Angular resolution with 6 rows
E$ciency per row
The cluster size11 listed in Table 3 represents the e!ective pixel size of the chamber in the x}y plane. It is a measure of the two-particle separation capability, one of the most important performance parameters for chambers under high particle-density conditions. While the cluster size depends on several factors including the applied clustering algorithm and correlated to the chamber gain and threshold, i.e. the detection e$ciency, the main factor governing the two-particle separation is the di!usion size of electrons on the drift path. Note that the di!usion size of TPC1 is 800 lm in r.m.s. averaged over its sensitive volume. That of TPC2 is larger due to the di!erent gas mixture and the longer drift length, leading to larger clusters. Fig. 9. A schematic logic diagram of the readout system for the time projection chambers (TPC), the drift chambers (FT) and the time-of-#ight wall (FTOF). Boxes lableled G.G. stand for gate generators. See text for descriptions.
Fig. 9 shows a schematic logic diagram of readout electronics for the time projection chambers, the drift chambers and the time-of-#ight wall. 3.4.6. Performance The performance of TPC1 was tested with beams at the BNL AGS and the KEK PS in 1992 [29] and is summarized in Table 3.
3.5. Drift chambers Four projective drift chamber modules, FT1}4, are utilized for tracking, complimentary to the time projection chambers, to improve spatial resolution of the tracking system. All the drift chamber modules are positioned normal to the z-axis of the spectrometer coordinates. The distances from the target to their centers are 225, 276, 427 and 489 cm, respectively.
11 Hits created by a particle on a row of segmented sense wires form a cluster in the two-dimensional space of wire position and drift time. The size of the cluster is measured in the converted geometric space, i.e. in the x}y plane.
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3.5.1. Mechanical structure Characteristic parameters of the drift chambers are summarized in Table 4. In order to handle high multiplicity events, the chambers are highly segmented, with the maximum drift length ranging from 2 mm for FT1 to 5 mm for FT4. Each chamber module has alternating layers of foil planes and wire planes. Aluminized mylar foils of 25 lm thick are stretched and glued to the frame boards made of G10 with printed circuits. Proper electric contacts between both sides of the foils and the detector frames are ensured by conducting glue. The wires of the chambers are wound controlling the pitch of the wires with grooved bars. The wires are gold-plated tungsten, and their diameters are listed in Table 4. Wires are glued onto aligned frame boards and subsequently soldered. The tension on the wires is controlled by adjustable weights during the wiring procedure. After chamber fabrication the tension and uniformity of the wires are checked by observing the resonant frequency of the forced vibration induced by applying electric pulses of about 200 V to the sense wires on the frame. 3.5.2. Gas and drift xeld The drift chambers are operated with a gas mixture of argon (50%) and isobutane (50%). The
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applied drift voltages for the chambers are listed in Table 4 along with the cell sizes. 3.5.3. Readout electronics The basic structure of the readout electronics for the drift chambers consists of a current sensitive pre-ampli"er followed by a combination of ampli"er and discriminator. FASTBUS pipeline TDCs (LeCroy model 1879) are used to read out the drift time. The dynamic range of the pipeline TDCs is set to be the minimum for the highest time resolution, but is still large (1 ls) compared to the maximum drift time of all the chambers which is within 150 ns. That allows reducing the required number of TDC channels (and the cost) by half, by multiplexing two wire channels into one TDC channel after delaying one of the signals by an appropriate amount of time. Fig. 10 illustrates the design schematically. The high segmentation of the chambers implies a small probability of having more than one particle in a single cell, and relieves the need for the electronics to distinguish two pulses from the same wire. Grounded-base pre-ampli"ers with an impulse response of 2 ns are plugged on the chambers in groups of 16 channels each, except at some edges where only 8 channels are left to group. The
Table 4 Characteristic parameters of the drift chambers. The x-planes have vertical wires, and wire orientations in y-, u- and v-planes are 903, !303 and 303, respectively, measured counterclockwise from the vertical, looking into the beam Chamber module Sensitive area (w]h) Number of wire planes Plane order Wire-to-foil gap Number of wires per plane
FT1
FT2
FT3
FT4
(mm2)
238]174
353]233
644]320
803]360
(mm)
10 xx@uu@yy@vv@xx@ 1.6
10 xx@uu@yy@vv@xx@ 1.6
8 xx@uu@vv@xx@ 2.4
8 xx@uu@vv@xx@ 2.4
x y u v
56 40 64 64 560
56 36 64 64 552
78 * 80 80 632
78 * 80 80 632
(mm) (lm) (kV)
2.0 10.0 2.40
3.0 12. 2.55
4.0 15. 2.75
5.0 15. 2.80
Total number of wires Maximum drift length Anode wire diameter Operation voltage
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Fig. 10. Schematics of the drift chamber readout electronics which combines two adjacent wire channels into one TDC channel with delaying one of the signals.
pre-ampli"er12 is developed at BNL for applications requiring a fast response and short pulse shaping. The unit is tested to ensure required timing, hence position, resolution of the chambers. Besides the electronic properties, the compact design, 19 mm]13 mm with hybrid technology, allows dense packing of the chips in a small board which can be plugged directly onto the chambers. A resistor of 62 ) before the output is used to eliminate cross talks between neighboring channels. Ribbon twisted pair cables of 6 m in length are used to propagate analog signals to the ampli"erdiscriminator boards. The signal from the pre-ampli"er is further ampli"ed by a fast ECL line receiver MC10115 with a typical rise time of 2 ns. The ampli"ed signal is fed into the discriminator circuit. The discriminator consists of three stages of di!erential ampli"cation with a feedback, which stretches the output logic signal to ensure proper functioning of the following ECL delay chip. Since the delay chip requires the minimum width of 100 ns for the input pulse, longer pulses of about 200 ns wide are produced for
12 The pre-ampli"er is code-named BNL IO-354.
the channels intended for the delay lines, while the pulse width for the other channels is set at 120 ns. The delay time is about 400 ns with a jitter smaller than 1 ns. Two channels from neighboring wires, one without delay and one with, are joined together with a wired-OR. The logic signals are regenerated and propagated in twisted pair cables to the FASTBUS TDCs. The ampli"er and discriminator circuit is fabricated on a 4-layer printed circuit board and housed in a 9-unit VME crate. Each board comprises 64 input channels and 32 output channels. A single threshold voltage is supplied to each board, and a strobe signal is provided to cut o! pulses with unwanted timing. One of the concerns in the physical layout of the readout electronics boards is possible cross talk between neighboring channels sharing the same printed circuit board and twisted pair cable. At the nominal threshold, no cross talk is observed in pulser tests up to the point where the output of the pre-ampli"er saturates. An upper limit of electronic cross talk is also found to be about 1.5% from the probability of two neighboring wires "red simultaneously in the real data. The LeCroy 1879 pipeline TDCs receive the LVL1 signal as the common stop signal directly from their front panels, without using a CAT module. Their full range is set at the minimum, i.e. 1 ls, to obtain the best time binning of 1 ns. Since the LVL1 arrives at the modules on the spectrometer platform approximately 800 ns after the particles pass through the chambers, no additional delay is needed for the stop timing. The TDCs receive the LVL2 signal as a fast clear. 3.5.4. Performance Each pair of planes of the drift chambers is staggered with "eld wires of one plane aligned on top of the sense wires of the other plane. The sum of the two TDC values from the two staggered cells should hence be a constant value, corresponding to the size of the cells assuming a constant drift velocity. The distribution of this timing sum is used to evaluate the timing resolution of the chamber. Resolution of 3 ns in r.m.s., corresponding to spatial resolution of about 150 lm, is achieved in a beam test at the BNL AGS with the operating
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voltages listed in Table 4 and with e$ciencies higher than 95% per plane. A consistent resolution is also obtained with pulser signals. 3.6. Time-of-yight wall High capability of particle identi"cation is required for systematic studies of semi-inclusive spectra of mesons and baryons produced in relativistic heavy-ion collisions. A time-of-#ight scintillation counter wall (FTOF), located at the most downstream end of the spectrometer system, is adopted as the particle-identi"cation device. It is tilted on the x}z plane clockwise by 63 with respect to the x-axis, as seen in Fig. 2, in order to maximize its e!ective segmentation. The distance from the target to the beam-side edge of FTOF is 600 cm. 3.6.1. Mechanical structure The time-of-#ight wall has 100 segments. Each segment, called a slat, is a rod of plastic scintillator with square cross section made of Bicron BC404. Fig. 11 shows a sketch of FTOF, and Table 5 summarizes its geometric characteristic parameters. Each slat of FTOF is read out with two photomultiplier tubes (Hamamatsu model R3478S), through light guides on its ends. The tubes are operated at !1.80 kV. Since the photo-multiplier tube is wider than the scintillator rod, the light guide is bent by 303 and arranged alternatively out of the FTOF plane as shown in Fig. 11 to accommodate all the tubes. A newly developed on-tube
Table 5 Geometric characteristic parameters of the time-of-#ight wall Sensitive area (w]h) Number of segments Segment size (w]h]d)
(cm2) (mm3)
126]42 100 12.4]419]12.5
discriminator board with two outputs, analog and discriminated, is mounted on each tube [30]. 3.6.2. On-tube discriminator The main factor which determines the timing resolution of a photo-multiplier tube is the number of photo-electrons. Assuming that N photo-electrons are emitted at the same time on a photocathode of a photo-multiplier tube whose transit time spread is p , the best timing obtainable with TTS the tube is p /JN. TTS Time-of-#ight resolution in real experiments is worse than the pure photo-multiplier tube timing due to many factors such as electric noise, spread of the photon arrival time onto the photo-cathode and so forth. There is hence room to improve the actual time-of-#ight resolution for a given tube by reducing those factors. One of the attempts made in this system is to improve the time-of-#ight robustness against electric noise. It is achieved by making the signal rise time as short as possible at discrimination. When a leading edge discriminator is used, the detected arrival time jitters due to the electric noise superimposed on the signal shape as indicated in Fig. 12. It is a random noise and cannot be corrected for. The time jitter p introduced by this noise is 5*.% p p " /0*4% 5*.% Dd
Fig. 11. A sketch of the time-of-#ight wall, FTOF.
295
(2)
where p is the noise level and d
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Fig. 12. How a higher voltage gradient d
therefore introduced what we call on-tube discriminator to the time-of-#ight system to eliminate the necessity of signal transmission through cables. It is on a 5 cm]20 cm circuit card which is directly mounted onto a photo-multiplier tube as shown in Fig. 11. The high-voltage bleeder and the discriminator circuit are implemented on the card sharing the same ground. This feature is another advantage in reducing the electric noise. The signal from the tube is "rst passively split into two. One side of the split signal is discriminated by the on-board circuit to send out a NIM timing signal; the other side is directly sent as an analog output. 3.6.3. Readout electronics After the discriminated timing signal is sent from the experimental area to the counting house though a 68 m long foam-polyethylene RG58 A/U coaxial cable (Belden type 8219), it is discriminated again with a NIM discriminator (Phillips model 710). The discriminator output is then sent to a CAMAC TDC (Phillips model 7186). Each TDC module contains 16 channels of time digitizers with the minimum binning of 25 ps and full range of 100 ns. The TDCs are operated in common start mode with the LVL0 signal as the start. Unless the LVL1
signal is received within 200 ns, a fast clear signal is generated and all the TDC modules get cleared together. The recovery time is 850 ns. When the LVL1 signal is received in time, the hit register bits for the modules whose LAM is set13 are read out "rst. Only the corresponding channels which got hit are read out. The LVL2 signal is also provided to the TDCs as a fast clear. On the other hand, the analog output of the on-tube discriminator is put into a FASTBUS analog-to-digital converter (ADC) module (LeCroy model 1882N) in the counting house, with an additional delay of 250 ns. The module has 96 channels each of charge sensitive ADC, with minimum binning of 50 fc and the full range of 200 pc. The ADC information is used to correct for the timing shift due to the pulse height variation (the `slewing correctiona) to improve the time-of-#ight measurement, as well as for a charge cut to discriminate hadrons from photons and d-rays. The ADC gate pulse is locally generated from the LVL1 signal and fed into the ADC modules through a CAT module (LeCroy model 1810). The CAT also distributes the LVL2 signal as a fast clear to the ADCs. 3.6.4. Performance A bench test and a beam test were performed to evaluate the on-tube discriminator. At the bench test, a pulsed laser (Hamamatsu PLP-02) was used as the light source with a time jitter less than 10 ps/3C. Fig. 13 shows the output voltage gradient d
13 LAM is set if the hit register of the TDC module is non-zero.
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are evaluated from the residual distribution with the track projection onto the detector. Fully reconstructed p~ tracks within the momentum range from 1.0 to 3.0 GeV/c are selected for the evaluation to minimize the e!ect of multiple scattering and of contamination by other particle species. Table 6 summarizes the performance of the detectors in the 197Au beam runs in 1994. All the values listed are averaged over the sensitive areas, while the detection e$ciency of the time projection chambers is found to have a slight position dependence. Note that all the listed values of resolution are in r.m.s.. 4.2. Tracking performance Fig. 13. Observed voltage gradient d
4. Spectrometer performance 4.1. Detector performance The e$ciency and the spatial and timing resolutions of each detector in the forward spectrometer
The e$ciency of the tracking system is a function of particle multiplicity in the spectrometer. Under high multiplicity conditions, the hardware ine$ciency of each detector increases due to "nite granularity and resolution. Tracking algorithms can also be confused by misassociation of hits and lose an increasing fraction of tracks. With the smallaperture spectrometer, the e!ect is more apparent for particles with higher momenta since they pass through the detectors almost parallel to other tracks, while particles with lower momenta are swept to regions where local particle density is lower.
Table 6 A summary of the performance of the detectors in the forward spectrometer in the 197Au beam runs. Variation of TPC performance represents di!erent spectrometer settings. All the listed values are in r.m.s. Time projection chamber E$ciency per row Resolution per row Cluster size
(x) (y) (x) (y)
(%) (mm) (mm) (mm) (mm)
Drift chamber E$ciency per plane Resolution per plane
(%) (mm)
Time-of-#ight wall Timing resolution Position resolution
TPC1
TPC2
87}97 0.47}0.65 0.28}0.37 4.6}5.0 1.9}2.3
83}94 0.81}1.20 0.56}0.85 7.1}7.6 4.3}5.3
FT1
FT2
FT3
FT4
97 0.25
96 0.30
97 0.34
93 0.37
FTOF
(y)
(ps) (mm)
76 8.5
298
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Fig. 14. Estimated tracking e$ciency as a function of total multiplicity and momentum of the track.
The e$ciency of the tracking system is evaluated by merging a Monte Carlo track into real events. Fig. 14 shows the result as a function of particle multiplicity in the spectrometer for merged tracks at di!erent momenta. The tracking e$ciency stays above 86% for tracks up to 5 GeV/c with a total multiplicity as high as 10. In an extreme condition with 20 particles in the spectrometer, the e$ciency goes down to 92% at 1 GeV/c and 76% at 3 GeV/c. 4.3. Particle identixcation capability Fig. 15 shows a scatter plot of measured squared mass versus signed momentum in a 197Au beam run with the spectrometer at 143 and a magnet setting favoring negative particles. The cuts used for particle identi"cation are also shown in the "gure. Pions, kaons and protons are identi"ed on a particle-by-particle basis from the measured momentum and time of #ight. Misidenti"cation of other particle species sets the upper limit of the identi"able momentum range, especially for particles with relatively low yields. An asymmetric cut on the squared mass is applied to suppress the contamination and extend the high momentum limit. Additional cuts are applied to identify anti-
Fig. 15. A scatter plot of measured squared mass versus signed momentum in a 143 run with a magnet setting favoring negative particles. See text for a description on the particle identi"cation cuts indicated by the solid lines.
protons whose yield is more than three orders of magnitude lower than protons. The optional cuts include energy deposit and isolation on the timeof-#ight wall [31]. The particle identi"cation limit reaches 4 GeV/c for pions, 3 GeV/c for kaons and 5 GeV/c for protons with less than 1% contamination from other particle species, and 4.5 GeV/c for anti-protons with a systematic error from contamination estimated to be less than 5%. The e$ciency of particle identi"cation is limited by the time-of-#ight wall segmentation. Timing information of second or later hits on each FTOF slat is lost since the readout electronics lacks multiple hit capability. Even the timing and position information of the "rst hit can be distorted if more than one particle hits a slat with a time interval shorter than the light propagation time in the scintillator. The particle identi"cation e$ciency is evaluated from real data as a function of spectrometer setting, trigger type, FTOF slat number and time of #ight. Fig. 16 shows the e$ciency plots for minimum-bias and central 197Au#197Au events with the spectrometer at 63 and 243.
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Fig. 16. Particle identi"cation e$ciency due to the "nite segmentation of FTOF. The e$ciency is plotted as a function of the FTOF slat number and time of #ight. A smaller slat number corresponds to the beam pipe side. The top two plots are for minimum-bias 197Au#197Au events; the bottom two are for central 197Au#197Au events. The left two plots are with the spectrometer at 63; the right two plots are at 243.
4.4. Momentum resolution The momentum resolution of the forward spectrometer is evaluated from the experimental mass resolution for pions, kaons and protons. Its dependence on the momentum and particle species can be deconvoluted into three contributing factors: the intrinsic angular resolution of the tracking system, the timing resolution of the time-of#ight system, and the e!ect of multiple scattering [32]. Fig. 17 shows the result of the momentum resolution evaluation by the method for pions and protons in a 143 run as a function of momentum. Resolution of 1.3% in r.m.s. is achieved for pions at 1.0 GeV/c, and 1.6% for protons at the same momentum. The time-of-#ight resolution evaluated by this method is 76$4 ps in r.m.s. and is consistent with the evaluation using high momentum p~ tracks also 76 ps.
Fig. 17. The experimental resolution of momentum for pions (solid line) and protons (dotted line) in a 143 run as a function of momentum.
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Fig. 18. A typical particle spectrum measured with the forward spectrometer [5]. Invariant cross section divided by the trigger cross section for identi"ed protons in central 197Au#197Au collisions at 11.6 A GeV/c, in di!erent rapidity intervals as a function of transverse kinetic energy m !m . The solid points 5 0 are data measured with the forward spectrometer (FS); the open ones are with the E802 spectrometer (HH). The bin width of the rapidity interval is 0.1, and *y denotes the absolute di!erence of the measured rapidity of each spectrum from the centre-of-mass rapidity y "1.6. The spectra are scaled down successively by #.4 a factor of 10. The error bars are statistical only, either shown or smaller than the data point.
4.5. Particle spectra Fig. 18 shows a typical particle spectrum measured with the forward spectrometer, invariant cross section divided by the trigger cross section for protons as a function of transverse kinetic energy. The data were collected in the 1994 runs of the E866 experiment from central 197Au#197Au collisions at 11.6 A GeV/c, with event selection in use of the zero-degree calorimeter. Single particle spectra measured with the spectrometer are presented in Ref. [4,5]. 5. Conclusions A small aperture magnetic spectrometer has been built to study hadron production in 197Au#197Au
collisions at the AGS energies. It operates in the forward angular range down to 63 with respect to the incident beam axis and covers the mid-rapidity region for heavy particles such as protons. The detector components of the forward spectrometer include two time projection chambers, four drift chamber modules and a time-of-#ight scintillation counter wall. A few new technologies are implemented in the design of the system to achieve the performance goals. The spectrometer has proved to function properly under the high particle-density environment encountered in experiments with the heavy colliding system. The achieved momentum resolution is 1.3% in r.m.s. for pions at 1 GeV/c and 1.6% for protons at the same momentum. With time-of-#ight resolution of 76 ps in r.m.s., the particle identi"cation limit reaches 4 GeV/c for pions, 3 GeV/c for kaons, 5 GeV/c for protons, and 4.5 GeV/c for antiprotons. The tracking e$ciency stays above 86% for tracks up to 5 GeV/c with as many as 10 tracks in the spectrometer aperture.
Acknowledgements We acknowledge Mr. K. Asselta, Mr. E. Baker, Mr. H. Diaz, Mr. J. Dioguardi, Mr. S. Kato, Mr. Y. Matsuyama and Mr. R.A. Scheetz for their technical supports. This work is supported in part by the US Department of Energy under contract DE-AC02-98CH10886 with BNL and in part by Japanese Ministry of Education, Science, Sports and Culture.
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