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RICH detectors in heavy ion experiments
Nuclear Instruments and Methods in Physics Research A 371 ( 1996) 275-284
NUCLEAN INSTNlWNNTS a YETNOOS IN PNVSICS
__ __ !!!8 EISEVIER
RUsz”
RICH detectors in heavy ion experiments Eugenio Nappi INFN. Se:. Bari, Bari, Italy
Abstract The latest design choices and performance for the major RICH detectors employed in ultrarelativistic heavy ion experiments are reviewed in the light of their potential physics programmes. Perspectives in the planned experiment ALICE at LHC. with a RICH detector based on recent progress in highly efficient solid CsI photocathodes. are also discussed.
1. Introduction The aim of this paper is twofold: we give a survey of RICH detectors that serve as particle identifiers in experiments where ultrarelativistic heavy ion collisions are investigated and, in particular, devices not covered by other papers in this workshop will be extensively detailed; second we discuss in depth the potentialities of the proposed RICH array for the experiment ALICE at LHC, this being the largest detector system ever designed with outstanding features envisaged to cope with the huge particle density expected. The expectation of unveiling the quark-gluon plasma (QGP) makes heavy-ion collisions a constant ingredient in the physics programme of major accelerator facilities, tirst at the CERN SPS, later at RHIC and LHC. The very high multiplicity of final state particles in central ultrarelativistic nucleus-nucleus collisions presents a serious challenge for particle identification systems (PID). Indeed, especially when a RICH device is used, the total hit density could rise to such a level as to spoil the pattern recognition. Nevertheless, the presence of a PID in heavy-ion experiments is often mandatory and techniques have been developed to perform the pattern recognition in such an environment, In the framework of statistical quantum chromodynamics, the phase transition from the ordinary nuclear matter to the QGP, where the quarks are deconfined. is accompanied by chiral symmetry restoration resulting in abnormal nuclear states and excitation of the vacuum. Since there is no single definitive signature for the QGP, many experimental observables must be measured to test unambiguously plasma formation. In the following, an accent will be put on signatures that require particle identification. It is well known that the most valuable tool for understanding hadronic reaction is the measurement of
inclusive transverse momentum distribution, this is certainly true even in the study of heavy ion collisions where a dramatic increase of (p,) could be the signal of the phase transition from ordinary matter to QGP. Indeed, efficient particle identification is necessary to allow a precise determination of the temperature as deduced from the transverse momentum spectra and of the mean transverse mass for different particle species. Furthermore a comparison of particle spectra at high p, allows us to disentangle possible phase transition effects from nuclear or hadronic medium effects: in fact, large differences in the high pt tails of spectra are predicted depending upon various medium effects. Moreover, PID enables the measurement of the ratio of strange to nonstrange particles and antistrange to strange particles since equilibration of strange and light quark flavors is expected to proceed much faster in QGP phase than by rescattering in a hadron gas. Thus a strong enhancement of strangeness production in heavy ion reactions is one of the expected observables of QGP formation. Particle identification is also important in the study of &meson production and the two particle Bose-Einstein correlations of identical charged pion and kaon mesons. The #-meson abundance, in fact, could serve as a probe of QGP formation because the &meson mass and width are expected to be sensitive to changes in the quark masses, if chiral symmetry were partially restored or medium effects were strong. Precision correlations of charged kaons or pions are predicted to provide helpful information on the space-time evolution of the boson emitting source. Last but not least. electron identification is needed to investigate the thermal emission of lepton pairs and photons carrying information on the hot and early state of the collisions due to the absence of final state interactions. There are five experimental groups actively working on Cherenkov ring imaging systems employed in heavy ion experiments: two identification systems have already been built for fixed target experiments at CERN-SPS and three
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are planned for colliding beam experiments at RHIC and LHC. A description of each, performance for those already built and future perspectives for those planned, are given in the following paragraphs.
2. RICH
detectors in present experiments
So far only two RICH devices have been specifically designed and built for identifying debris from heavy ion collisions. They are employed in the experiments NA35 and NA45, both at CERN-SPS. The first, although operated in the real experiment and allowed to acquire data for physics analysis, remained at the prototype level, while the second is still operational in NA45 and after minor technical changes from its first design, mainly on the photodetector part, run in the high density environment of Pb-Pb collisions in 1994 and will take data in 1995 and 1996. For completeness, among the RICH detectors in operating heavy-ion experiment, one must mention the attempt to use the OMEGA RICH [I] in the experiment WA94 [2].
sion in a two-step parallel plate chamber (MSAC) equipped with an optical readout. In fact, the total gas amplification (>lOh) was sufficient to produce enough visible photons via the TMAE de-excitation so that, through the pirex exit window of the MSAC, photons were viewed from 1 m distance with a CCD camera (576 X 576 pixels) equipped with a three stage light amplification chain (Fig. 2). A gate electrode was placed in the MSAC transfer region to enable the electron drift to the second amplification gap only when triggered, reducing in this way the overall detector noise. The photodetector operated at atmospheric pressure with a gas mixture consisting of 97% helium, 3% methane and TMAE vapour circulated at 48°C. Data were acquired with the RICH placed 4m downstream from the target at an angle of 22.5” on the side where positively charged particles were bent by the vertex magnet. The momenta of the particles reaching the RICH
MSAC
1.1. NA35 The NA35 experiment was run from 1987 to 1992 at the CERN SPS, employing a large acceptance charged hadron spectrometer to study particles produced near rapidity in collisions of a 200 A GeV ‘*S beam on nuclear targets [3]. The hadron spectrometer consisted of a I.5 T superconducting vertex magnet equipped with a large volume streamer chamber to perform tracking. Particles emitted at forward rapidities were collected by a calorimeter complex used for triggering purpose. In 1989 the NA35 experiment was upgraded with a time projection chamber (TPC) positioned downstream of the magnet and a 50 X 50 cm’ RICH designed to identify pions and kaons in the backward hemisphere at midrapidity corresponding to the momentum range 1.5-4 GeV/c (Fig. I ). C,F,, was selected as radiator (IO mm thick) and a proximity focussing geometry was chosen. The Cherenkov photon
detection
Fig. I. Schematic
was achieved
view of the upgrade
via TMAE
photoconver-
of the NA35 experiment.
Fig. 2. Schematic of the MSAC optical readout system: a mirror, placed at 45” with respect to the detector exit window, reflects the produced light to the imaging system. A Leitz-Noctihtx objective focalizes the image on the image intensifier complex that consists of three stages. The CCD management is handled by the driver Thomson-CSF TH7966-3 whose analog output corresponding to each CCD pixel is converted to an I-bit digital value by a VME-resident digitizer board.
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were determined by tracking from the streamer chamber. A more detailed description of NA35 RICH can be found in Ref. [4]. Particle density of the order of 20-30 mm’ were analyzed with the described layout and physics results published in Ref. [5]. As final comment, one has to say that the technique to detect Cherenkov photons via the de-excitation line of TMAE results to be extremely unefficient since, at least in the operation at atmospheric pressure, a very high charge gain is required causing frequent sparking. Actually, such problems seem intrinsically related to the operational principle of the MSAC and they were met also in a similar device built for the experiment CERES in the first running period [6,7]. In the NA35 detector, losses of the order of 15% were incurred due to breakdowns and long recovery time, consequently the average number of photoelectrons per ring resulted to be less than predicted, thus restricting the ?r/K separation to 2.5 GeVlc. Nevertheless, the NA35 prototype represents the largest device ever operated with an optical readout.
The RICH UV detectors are multi-step gas chambers operated with TMAE as photoconverter. A silicon drift counter placed downstream the target determines the collision impact parameter by measuring the charged particle multiplicity. The very peculiar geometry of the two RICHs allows the discrimination of the origin of electron pairs. In fact. electron pairs from photon conversion in the target show up as two overlapped rings in the first RICH while they appear as distinct rings in the second RICH due to the separation effect of the existing magnetic field, vice versa electron pairs from virtual photons are characterized by two well separated rings in both RICHs. The electron momenta are reconstructed from the ring position and the deflection angle. A remarkable presentation of the CERES RlCHs features including the technical problems met to build and operate such devices can be found in Refs. 18.9). while interesting pattern recognition methods developed for CERES data analysis were discussed by Ullrich at this workshop [ 1O].
2.2. CERESINA45
3. RICH detectors in planned experiments
CERES (ChErenkov Ring Electron Spectrometer) is the first experiment dedicated to the measurement of low mass electron-positron pairs and direct photons produced in hadron and nuclear collisions, at CERN SPS energies. Its main goal is to systematically study the pair continuum in the mass region from 100 MeV/c’ to almost 3 GeV/c*, and real photons by external pair conversion in the target [8]. To achieve these goals, a nonconventional spectrometer has been realized based on two azimuthally symmetric RICH devices (Fig. 3). Since the aim is to identify electrons, a CH, gas radiator at atmospheric pressure has been selected making the spectrometer “hadron blind”.
Despite the increasing energy available in the future heavy ion accelerators and therefore the higher multiplicity expected, the largest experiments planned so far still envisage the implementation of RICH arrays for the particle identification. Three experiments are reviewed in the following: BRAHMS and PHENIX at RHIC and ALICE at LHC. A fourth one, SQUASH, which has been proposed at CERN-SPS for the search of strange matter has been stepped out by the SPSC a few months ago, and the R&D on its RICH has been extensively presented at this workshop [ 1 I] and therefore will not be mentioned further in this paper. 3.1. BRAHMS
CERES Spectrometer
-1
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Fig. 3. Layout of the experiment CERES at CERWSPS. field lines are shown in the lower part.
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Magnetic
The BRAHMS (Broad RAnge Hadron Magnetic Spectrometer) experiment at RHIC is designed to measure charged hadrons over a wide range of rapidity and transverse momentum for central and peripheral collisions for all available beams and energies [ 121. Physics goals are achieved using two small solid angle spectrometers which operate from 2 to 20” and from 20 to 90”, respectively (Fig. 4). The rapidity range will be from 0 to 4, and the p, up to 2.5 GeVlc for most of this y range. The expected multiplicity through the PID devices is quite low, the typical number of charged particles being 0.5-2 [12]. A RICH detector is under study for particle identification in the Forward Spectrometer arm. The aim is to identify pions at the 3a level for the momentum range from 4 to 20 GeV/c, kaons from 9 to 20 GeVlc and protons from 17 to 35 GeVIc, therefore a radiator of gaseous
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Mid rapldlty spectrometer
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Forward spectrometer Fig. 4. Layout of the experiment BRAHMS at RHIC. Dl , D2, D3, D4: dipole magnets: Tl, T2, T3, T4, T5, T6: tracking detectors; HI, H2: high resolution TOF hodoscopes; Cl, GASC: segmented gas Cherenkov counters; TPCl, TPC2: mid-rapidity TPCs; TOFW: mid-rapidity TOF-wall; MULT: multiplicity detector; DX: RHIC beam line magnet.
(C,F,,) at 1.25 b, absolute pressure, has been selected. The photodetector system is based on the new-generation multianode photomultiplier R4549 from Hamamatsu whose characteristics are the following: i) 20 electron amplification stages; ii) 10 X 10 cm* photocathode area corresponding to 100 output anode channels (10 X 10); iii) internal focussing electrodes in front of the first dynode to reduce the cross-talk; iv) fast photomultiplier response for first-level triggering. A successful prototype test has been performed at AGS in a beam of momentum-selected negative secondaries
perfluorobutane
V31.
3.2. PHENIX The PHENIX experiment (Pioneering High Energy Nuclear Interaction Experiment) at RHIC is designed to perform a systematic investigation of both leptonic and hadronic signatures and to look for simultaneous anomalies attributable to QGP formation [14]. Since electrons, muons and charged hadrons must be measured simultaneously, various PID arrays are planned: RICH for electrons, a time expansion chamber (TEC) to measure dE/dx, a time of flight (TOF) for hadrons and an electromagnetic calorimeter (EMCAL). PHENIX basic approach is to.attain a very good particle identification for a limited number of particles (300-500 charged particles per event), although the detectors are optimized to handle at least 1000 particles per event. The experimental layout is an axial-filled spectrometer covering the mid-rapidity region in addition to a forward magnet muon spectrometer. Most of the tracking chambers are installed outside the magnetic field, a silicon vertex
Fig, 5. Cut through view of the PHENIX experiment at RHIC, showing the two large magnets and the detector subsystems that serve as trackers and particle identifiers.
detector is installed close to the interaction area. A cut through view of the PHENIX experiment is shown in Fig. 5. 3.2.1. The RICH detector The RICH detector is designed to be the primary subsystem for electron identification and it consists of two arms located outside the central magnet at a distance of 2.6 m from the collision point (Fig. 6). Each arm covers 90” in azimuth and 0.35 units of rapidity. The radiator is selected with the aim to make the RICH “hadron-blind”, i.e. hadrons are below the Cherenkov threshold and electrons produce rings of asymptotic radius. Therefore either ethane (n - 1.0008) or methane (n 1.00044), at atmospheric pressure, are possible candidates. The Cherenkov photons after reflection from spherical mirrors with a radius of curvature of 4 m, are registered on a highly segmented array of 6400 photomultipliers arranged into 256 sets of 25 tubes each, mounted on the
Fig. 6. Two side views of the PHENIX RICH complex. An electron is shown crossing the detector and emitting Cherenkov light reflected and focused by the mirror.
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3.3. ALICE
Fig. 7. Principle of Cherenkov multiplier array of the PHENIX
photon detection in the photoRICH. Four electron tracks and the Cherenkov light cones focused by the mirror are shown. Note the photomultipliers placed above the magnet pole tip.
mirror focal plane, behind the central magnet (Fig. 7). As a benefit of this geometry, the pole tips of the magnet shield the photodetectors from the incoming hadrons. Winston cones are envisaged to be attached on the entrance window of each phototube for reducing dead space on the focal plane.
3.2.3. R&D results Results from a beam test at KEK have been obtained using a module of 25 Hamamatsu H3 171 phototubes and few gas radiators: nitrogen, methane and ethane [14]. Quite a large number of photoelectrons have been measured bringing the N, value to 110. The phototubes have been selected with a short transit time spread for the twofold aim of: i) providing a trigger signal from local clusters of phototubes above threshold; ii) reducing background hits and random noise at the level of 2 X 10 -4 corresponding to one spurious hit per 6400 phototubes per event both for the high time resolution (u = 250 ps) and for the cuts on the arrival time of the Cherenkov photons. Applying timing cuts in the offline analysis, only three events had more than four photomultipliers hit above pedestal out of 69 K pion triggers recorded. Therefore, a pion suppression of 2.3 X lo4 was achieved [14].
ALICE (A Large Ion Collider Experiment) is the only dedicated experiment planned for the CERN LHC to investigate heavy ion collisions at centre-of-mass energy of -6TeV per nucleon. It is meant as a general purpose experiment to study the physics of the nuclear matter at high densities and temperatures through the systematic study of a number of specific signals, together with a global survey of the events [ 151. It is designed to cope with the highest particle density anticipated for Pb-Pb collisions at the LHC (theoretical models predict up to 8000 charged particles per unit of rapidity for central Pb on Pb collisions). The ALICE layout presently includes a silicon inner tracking system (ITS), a large volume time projection chamber (TPC), a highly segmented particle identification array (TOF or RICH) and a calorimetric system. The listed detectors are embedded in the 0.2 T uniform magnetic field provided by a large L3-like solenoid magnet to perform momentum analysis of charged particles over two units of pseudo-rapidity with full azimuthal coverage (Fig. 8). The ITS is located as close as possible to the beam axis (r = 7.5 cm) and has the task of tracking particles of very low momentum, identifying short-lived particles (mostly hyperons) decaying before they reach the TPC, and improving the momentum resolution of the tracked particles. The TPC is designed to provide tracking and dEldx measurements for charged particles with pseudo-rapidities in the range - 1 < r] < + 1. It has an active volume defined by the two cylindrical surfaces at r = 100 cm and r = 250 cm. A number of options are under study for the hadron identification system: three based on the TOF technique (namely Pestov spark tubes, parallel plate gas chambers and a scintillator array) and the fourth alternative based on the RICH technique. A separation power greater than 3a is required on a track-by-track basis to keep the contamination below a 10% level in the presence of a huge bulk of hadrons at low momentum, moreover a reliable pattern recognition is mandatory due to the high track density. Depending on the performance achieved by the most suitable technology, the PID system will be placed at radii between 3 and 4.5 m. Since LHC operations with lead beams is likely to reach a luminosity of 2 X lo*’ cm-’ s ’ , an interaction rate of about lo4 Hz is expected, but fortunately only a small fraction, approximately 2-3%, corresponds to the most interesting central collisions with maximum particle production. It follows that the requirements on the PID frontend electronics are much less stringent than those in LHC pp experiments, but as already pointed out the high multiplicity represents in this case a very demanding task when it combines with the large rapidity range to be covered. Moreover the realization of such ambitious
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ALICE Fig. 8. The baseline ALICE layout, showing the large L3 magnet and the location of the tracking detectors and the PID barrel. detector complex must be based on a cost effective production of the system elements with standard techniques, with reliability and durability as key factors. 3.3. I. RICH system layout Presently, the RICH system for ALICE represents the largest scale application of such a technique. The layout is governed by the need to optimize the detector performance in terms of angle resolution and efficient pattern recognition: since the best angle resolution is achieved for particles with normal incidence to the radiator, it is necessary to shape the detector in a way to minimize the angular dispersion of incoming particles. An efficient way to fulfill the above requirements is to design the RICH barrel based on modular construction with elements of the largest technically feasible area, in order to reduce the total number needed to cover the acceptance region. The RICH array is conceived as a barrel of 3.6 m radius and 7.2 m length, composed of 60 modules on a supporting structure that allows the tilting of each module independently. The detector is placed inside the magnetic field with a segmentation of 12 modules in azimuth and 5 along the beam axis. The barrel can be moved sideways independently to give free access for installation and survey of major detector components.
The RICH array is planned to provide the following tasks: 1) supplement the PID obtained by dEldx method in the earlier parts of the detector (ITS and TPC) in the range where they are not operational, namely at the minimum of ionisation (e.g. 0X-2.5 GeV/c for pion-kaon identification); 2) provide an additional tracking plane in the detector with good position determination and multitrack separation capabilities. The tasks described will be covered by a RICH counter of the proximity focussing type with a liquid freon (C,F,,) radiator and a UV detector based on the use of a low gain multiwire proportional chamber (MWPC) with a pad readout for a two-dimensional determination of the positions of the ionizing particles and of the Cherenkov photons. The UV photoconversion into electrons is achieved by using a solid photocathode consisting of a thin layer of CsI evaporated onto the pad plane. The schematic of the proposed RICH is shown in Fig. 9. 3.3.2. The UV detector The operation of a RICH counter is dictated primarily by the performance of the UV detector. The MWPC will consist of a cathode mesh, or wires (for larger photon transparency), a wire anode plane with wires
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Fig. 9. Cut away view of the ALICE RICH. The MWPC has 20 pm diameter anode wires at 4 mm pitch with the two cathode planes equidistant from the anode plane at a distance of 2 mm. The Csl cathode plane is segmented in pads of size 8 X 8 mm’. Each pad is viewed by two anode wires. The proximity gap is defined by two electrodes, the first of them, made of 100 pm diameter wires of 4 mm pitch, is located close to the quartz window and is positively polarized with respect to the cathode mesh. so as to collect the primary ionization deposited by the charged particles in the proximity gap.
spaced 4 mm, and a pad readout cathode plane with pads of 8 X 8 mm* in size. The pad size is chosen in function of the position accuracy wanted, and of the optimum anode coupling for detection of the signal induced by the electron avalanche. The total material thickness of the UV detector. including the readout electronics, is 3% X,,. The present project, that makes use of the existing experience in the field of fast RICH detectors coupled to the R&D with solid photocathodes, provides a detector of outstanding features that we summarize as follows: i) the thickness of the MWPC (4 mm) allows for positive identification of charged particle impact on the detector (a clear discrimination against photon clusters is possible), keeping at the same time the active gap small enough for the operation of the detector in a high density environment with inclined tracks without deterioration of the localization characteristics; ii) photoconversion is achieved in a single layer thus eliminating an importance source of parallax error present in detectors where a photosensitive gas is used instead of a solid photocathode; iii) the use of a solid photocathode allows for a substantial saving in terms of material thickness. In fact the gap between the detector and the radiator may be filled with the same gas as the detector thus eliminating the need for a quartz window:
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iv) the analog multiplexing readout electronics and the intrinsic characteristics of a MWPC of the present design enable the operation at interaction rates 10J- 10‘ Hz with acquisition speeds of 10’ Hz; v) high segmentation flexibility and low cost per channel dictated by the need to cover large surfaces. In 1992, a large detector development effort was started in view of the above very ambitious technological aspects to be fulfilled. A specific R&D was approved by the DRDC at CERN under the name RD26 [ 161. The RICH design for ALICE has largely benefited from activities carried on in RD26 and the on-going researches will very likely still contribute in assessing the final performance. An updated RD26 status report has been presented by Piuz at this workshop [17]. Some of the RD26 outcomes are really fundamental to a successful realization of a well performing CsI-RICH for ALICE. For instance, the technology of evaporating large photocathodes has been successfully implemented and tested with photocathodes up to 50 X SOcm’. The method is inexpensive and works without the use of time-consuming masking techniques. ALICE people are confident that the same technique may be applied for larger sizes. 3.3.3. The radiator The use of a liquid fluorocarbon as a radiator is with an index of envisaged. The C,F ,., characteristics, of n=1.2834 at A= l75nm and low refraction chromaticity is adequate for the range of energies under study. The radiator vessel represents the most critical part in the detector design [ 181. The rather high freon density, 1.68 g/cm’, and the need to avoid pollution from the material in contact with the liquid ,radiator that would affect the transparency in the 160-220 nm band, require a careful investigation and optimization. The liquid radiator container, which is currently under study, consists of a tray made of composite material closed by 5-6 mm thick UV transparent quartz window. An aramide-fibre/epoxy honeycomb material (NOMEX) has been chosen as inner layer because its thermal expansion coefficient is very small and its very low density (40 kg/m’) makes it almost transparent to the incoming particles. A 100 p,rn carbon fiber layer is used for the skin surfaces because of its high modulus of elasticity. The inner surface of the vessel is covered with 50 pm thick aluminum foil to prevent the liquid C,F,, to come into contact with the carbon fiber tissue (Fig. 10). All the elements are glued with Araldite AWlO6. The liquid radiator inlet and outlet are performed in the honeycomb volume inserting two stainless steel pipes that run lengthwise along the container wall on opposite vessel comers, the outlet always being at the highest location. Given the quantum efficiency achieved so far, the radiator thickness is chosen 10 mm for an optimal Cherenkov angle resolution. The vessel exit windows consist of a UV-grade fused silica plates whose thickness and size must be carefully optimized investigating the best compromise
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o-ring freon
Llquld /Honeycomb
Inlet
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1 i
2500-
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-
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Fig. 11. Reconstructed single photon Cherenkov angle distribution corresponding to the analog center of gravity of the cluster (dashed line) and to the center of each hit pad (solid line).
Aluminum Carbon
ah In tlber
ahln
Honeycomb
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Fig. IO. Cut away view of the radiator vessel structure.
between the detector total radiation length and the freon hydrostatic pressure. A possible solution is to segment each module in four or six independent sections with reinforcement ribs of composite material. The thin quartz window will be supported in the central region by cylindrical spacers connected to the back wall of the radiator. A reduced scale prototype is being realized to gain useful information on its mechanical and chemical stability. 3.3.4. Performance of the detector The performance of the detector in a high density environment depends on two factors: 1) the ability to distinguish the clusters from charged particles from those belonging to photons. In the present prototype design a clear distinction between these two types of clusters is clearly possible: the mean cluster size for a minimum ionization particle is 4 pads while for photon is 1.2 pads. This feature is particularly important because apart from distinction from photon clusters, the larger size allows very good spatial precision on the impact point of the particle on the radiator, which is of paramount importance for the tracking precision, since with more pads the centroid position may be better determined. The coordinates of impact of the charged particles have been measured in the proposed setup (8 X 8 mm2 pads and anode wire distance 4 mm) with a precision better than 400 km. The two track separation has been estimated by simulations to be better than 2 mm. Accuracy on the photon centroid position is less demanding for as long as it is lower than the error on the localization of the photon due to chromatic aberration and geometrical effects (proximity gap). In fact, as shown in Fig. 11, the Cherenkov
angle resolution does ot change if pads are not associated to a cluster; 2) the number of photons registered per event. This number is a function of the quantum efficiency of the photocathode, the beta value of the particle and finally the thickness of the radiator. Bench tests of the Csl quantum efficiency and single-electron detection sensitivity performed in the framework of the R&D programme RD26 have provided useful information on how to optimize these parameters [ 191. 3.3.5. Particle identijcation in high multiplicity environments At the foreseen LHC luminosity of -2 X 10” cm-’ s- ’ in ion mode operation and for central collisions the number of particles reaching the RICH is estimated to be -80 at the center of the detector and -50 particles/m’ particles/m* at the outer edge of the detector, taking into account also the secondary particles created. Pattern recognition studies have been carried out with real data from 3 GeV pion and proton beam tests of a prototype with the presently investigated geometry: 1.0 cm freon thickness and 7.0 cm proximity gap width. Multi-ring events have been simulated by translating and superimposing single-ring images randomly over the area of an imaginary detector thus creating an image that corresponds to a high-density event. Four events with a multiplicity of 80 particles/m” are shown in Fig. 12. It is important to note that the generated images have a density higher than the expected from the actual Pb-Pb collision due to the fact that some particles will not generate rings (because they are below the Cherenkov threshold) and that the is peaked at low values momentum distribution (-350 MeVlc) thus generating rings with smaller radii and smaller number of photons per ring. The pattern recognition procedure has been described in Ref. [20]. The coordinates of the impact point were determined from the center-of-gravity of the MIP ionization pattern on the pad chamber, in the real experiment they will be provided by the tracking detectors in the ALICE setup. In the pattern recognition phase, every simulated event was analyzed, including those containing possible interactions in the
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Overlaped raw events - density 80 park/m2 60 50 40 30 20
20
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Fig. 12. Four scatter plots, corresponding to events with 80 particles/m’, generated 3 GeV/c pions and protons in the 50 X 50 cm2 CsI RICH prototype (see text).
detector volume. ionizing particles passing across the detection volume, noise due to pedestal fluctuations, ring patterns with a small photon number. Nevertheless, only particles that fell more than 10 cm away from the detector edges were considered since the edge effects will later be studied more carefully. Results from the superimposed events are given in Fig. 13. Note again that the distributions are very clean although no cuts in the raw data were applied. The method allows a kaon/pion 3a separation at 2.6 GeV/c. The separation power is a function of pattern recognition criteria. With severe criteria (e.g. a cut on the ADC content
pad number
superimposing raw Cherenkov patterns produced by
of each pad) one may improve the particle separation at the expense of efficiency. With no cuts applied the inefficiency is below 5%.
4. Conclusions In summary, we should note that RICH technology is making rapid advances in identifying particles even in a high multiplicity environment, in fact it has already been successfully employed to exploit a significant physics programme in two forward spectrometer experiments at
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PHENIX and BRAHMS experiments. In addition he wishes to thank T. Ljubicic Jr. and A. Tricomi for providing and discussing results on the ALICE-RICH pattern recognition.
17.5 15 12.5 10
References
7.5 5 2.5 0
0
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Fig. 13. Angle distribution obtained counting all the hit pads, around each particle impact point, that fall within a scanning window of 0.1” width. The highest bin content is chosen as the Cherenkov angle. The bottom plot shows the improvement in the signal height over the background when pads with charge below 14 ADC channels are removed.
CERN-SPS. Furthermore, the three devices in development for ALICE at LHC, PHENIX and BRAHMS at RHIC have obtained satisfactory results from their R&D programme. Nevertheless, not all the existing problems have been solved so far and therefore more R&D efforts are required.
Acknowledgements The author is very grateful to H. Hamagaki, S. Nagamiya, F. Videbaek for having sent him material on the
[I] H.-W. Siebert et al., Nucl. Instr. and Meth. A 343 (1994) 60. [2] D. Elia, these Proceedings (1995 Int. Workshop on Ring Imaging Cherenkov RICH Detectors, Uppsala, Sweden) Nucl. Instr. and Meth. A 371 (1996) 22. [3] A. Sandoval et al., Nucl. Phys. A 461 (1987) 465~. [4] J. Bachler et al., NucI. Instr. and Meth. A 324 (1993) 449; Nucl. Ins&. and Meth. A 343 (1994) 213. [5] J. Bkhler et al., Nucl. Instr. and Meth. A 343 (1994) 288. [6] R. Baur et al., Nucl. Instr. and Meth. A 343 (1994) 231. [7] J. Va’vra, these Proceedings (1995 Int. Workshop on Ring Imaging Cherenkov Detectors, Uppsala, Sweden) Nucl. Instr. and Meth. A 371 ( 1996) 33. [8] R. Baur et al., Nucl. Instr. and Meth. A 343 (1994) 87. [9] R. Baur, these Proceedings (1995 Int. Workshop on Ring Imaging Cherenkov Detectors, Uppsala, Sweden) Nucl. Instr. and Meth. A 371 (1996) 16. [IO] T. Ulhich, ibid., p. 243. [l I] R. Nobrega, ibid., p. 285. [I21 BRAHMS Conceptual Design Report, October 1994. [13] R. Debbe, these Proceedings (1995 Int. Workshop on Ring Imaging Cherenkov Detectors, Uppsala. Sweden) Nucl. Instr. and Meth. A 371 (1966) 327. [ 141 PHENIX Conceptual desing report, January 1993. 1151 ALICE Letter of Intent, CERN/LHCC/93-16. [16] E. Nappi et al., CERN/DRDC/92-16. [17] F. Piuz, these Proceedings (1995 Int. Workshop on Ring Imaging Cherenkov Detectors, Uppsala, Sweden, 1995) Nucl. Instr. and Meth. A 371 (1996) 96. [I81 G. Demelio et al., presented at this Workshop (1995 Int. Workshop on Ring Imaging Cherenkov Detectors, Uppsala. Sweden). [I91 A. Di Mauro et al., RD-26 Status Report, CERN/DRDC/9449. [20] A. Di Mauro et al., Nucl. Instr. and Meth. A 343 (1994) 284.
NUCLEAN INSTNlWNNTS a YETNOOS IN PNVSICS
__ __ !!!8 EISEVIER
RUsz”
RICH detectors in heavy ion experiments Eugenio Nappi INFN. Se:. Bari, Bari, Italy
Abstract The latest design choices and performance for the major RICH detectors employed in ultrarelativistic heavy ion experiments are reviewed in the light of their potential physics programmes. Perspectives in the planned experiment ALICE at LHC. with a RICH detector based on recent progress in highly efficient solid CsI photocathodes. are also discussed.
1. Introduction The aim of this paper is twofold: we give a survey of RICH detectors that serve as particle identifiers in experiments where ultrarelativistic heavy ion collisions are investigated and, in particular, devices not covered by other papers in this workshop will be extensively detailed; second we discuss in depth the potentialities of the proposed RICH array for the experiment ALICE at LHC, this being the largest detector system ever designed with outstanding features envisaged to cope with the huge particle density expected. The expectation of unveiling the quark-gluon plasma (QGP) makes heavy-ion collisions a constant ingredient in the physics programme of major accelerator facilities, tirst at the CERN SPS, later at RHIC and LHC. The very high multiplicity of final state particles in central ultrarelativistic nucleus-nucleus collisions presents a serious challenge for particle identification systems (PID). Indeed, especially when a RICH device is used, the total hit density could rise to such a level as to spoil the pattern recognition. Nevertheless, the presence of a PID in heavy-ion experiments is often mandatory and techniques have been developed to perform the pattern recognition in such an environment, In the framework of statistical quantum chromodynamics, the phase transition from the ordinary nuclear matter to the QGP, where the quarks are deconfined. is accompanied by chiral symmetry restoration resulting in abnormal nuclear states and excitation of the vacuum. Since there is no single definitive signature for the QGP, many experimental observables must be measured to test unambiguously plasma formation. In the following, an accent will be put on signatures that require particle identification. It is well known that the most valuable tool for understanding hadronic reaction is the measurement of
inclusive transverse momentum distribution, this is certainly true even in the study of heavy ion collisions where a dramatic increase of (p,) could be the signal of the phase transition from ordinary matter to QGP. Indeed, efficient particle identification is necessary to allow a precise determination of the temperature as deduced from the transverse momentum spectra and of the mean transverse mass for different particle species. Furthermore a comparison of particle spectra at high p, allows us to disentangle possible phase transition effects from nuclear or hadronic medium effects: in fact, large differences in the high pt tails of spectra are predicted depending upon various medium effects. Moreover, PID enables the measurement of the ratio of strange to nonstrange particles and antistrange to strange particles since equilibration of strange and light quark flavors is expected to proceed much faster in QGP phase than by rescattering in a hadron gas. Thus a strong enhancement of strangeness production in heavy ion reactions is one of the expected observables of QGP formation. Particle identification is also important in the study of &meson production and the two particle Bose-Einstein correlations of identical charged pion and kaon mesons. The #-meson abundance, in fact, could serve as a probe of QGP formation because the &meson mass and width are expected to be sensitive to changes in the quark masses, if chiral symmetry were partially restored or medium effects were strong. Precision correlations of charged kaons or pions are predicted to provide helpful information on the space-time evolution of the boson emitting source. Last but not least. electron identification is needed to investigate the thermal emission of lepton pairs and photons carrying information on the hot and early state of the collisions due to the absence of final state interactions. There are five experimental groups actively working on Cherenkov ring imaging systems employed in heavy ion experiments: two identification systems have already been built for fixed target experiments at CERN-SPS and three
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are planned for colliding beam experiments at RHIC and LHC. A description of each, performance for those already built and future perspectives for those planned, are given in the following paragraphs.
2. RICH
detectors in present experiments
So far only two RICH devices have been specifically designed and built for identifying debris from heavy ion collisions. They are employed in the experiments NA35 and NA45, both at CERN-SPS. The first, although operated in the real experiment and allowed to acquire data for physics analysis, remained at the prototype level, while the second is still operational in NA45 and after minor technical changes from its first design, mainly on the photodetector part, run in the high density environment of Pb-Pb collisions in 1994 and will take data in 1995 and 1996. For completeness, among the RICH detectors in operating heavy-ion experiment, one must mention the attempt to use the OMEGA RICH [I] in the experiment WA94 [2].
sion in a two-step parallel plate chamber (MSAC) equipped with an optical readout. In fact, the total gas amplification (>lOh) was sufficient to produce enough visible photons via the TMAE de-excitation so that, through the pirex exit window of the MSAC, photons were viewed from 1 m distance with a CCD camera (576 X 576 pixels) equipped with a three stage light amplification chain (Fig. 2). A gate electrode was placed in the MSAC transfer region to enable the electron drift to the second amplification gap only when triggered, reducing in this way the overall detector noise. The photodetector operated at atmospheric pressure with a gas mixture consisting of 97% helium, 3% methane and TMAE vapour circulated at 48°C. Data were acquired with the RICH placed 4m downstream from the target at an angle of 22.5” on the side where positively charged particles were bent by the vertex magnet. The momenta of the particles reaching the RICH
MSAC
1.1. NA35 The NA35 experiment was run from 1987 to 1992 at the CERN SPS, employing a large acceptance charged hadron spectrometer to study particles produced near rapidity in collisions of a 200 A GeV ‘*S beam on nuclear targets [3]. The hadron spectrometer consisted of a I.5 T superconducting vertex magnet equipped with a large volume streamer chamber to perform tracking. Particles emitted at forward rapidities were collected by a calorimeter complex used for triggering purpose. In 1989 the NA35 experiment was upgraded with a time projection chamber (TPC) positioned downstream of the magnet and a 50 X 50 cm’ RICH designed to identify pions and kaons in the backward hemisphere at midrapidity corresponding to the momentum range 1.5-4 GeV/c (Fig. I ). C,F,, was selected as radiator (IO mm thick) and a proximity focussing geometry was chosen. The Cherenkov photon
detection
Fig. I. Schematic
was achieved
view of the upgrade
via TMAE
photoconver-
of the NA35 experiment.
Fig. 2. Schematic of the MSAC optical readout system: a mirror, placed at 45” with respect to the detector exit window, reflects the produced light to the imaging system. A Leitz-Noctihtx objective focalizes the image on the image intensifier complex that consists of three stages. The CCD management is handled by the driver Thomson-CSF TH7966-3 whose analog output corresponding to each CCD pixel is converted to an I-bit digital value by a VME-resident digitizer board.
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were determined by tracking from the streamer chamber. A more detailed description of NA35 RICH can be found in Ref. [4]. Particle density of the order of 20-30 mm’ were analyzed with the described layout and physics results published in Ref. [5]. As final comment, one has to say that the technique to detect Cherenkov photons via the de-excitation line of TMAE results to be extremely unefficient since, at least in the operation at atmospheric pressure, a very high charge gain is required causing frequent sparking. Actually, such problems seem intrinsically related to the operational principle of the MSAC and they were met also in a similar device built for the experiment CERES in the first running period [6,7]. In the NA35 detector, losses of the order of 15% were incurred due to breakdowns and long recovery time, consequently the average number of photoelectrons per ring resulted to be less than predicted, thus restricting the ?r/K separation to 2.5 GeVlc. Nevertheless, the NA35 prototype represents the largest device ever operated with an optical readout.
The RICH UV detectors are multi-step gas chambers operated with TMAE as photoconverter. A silicon drift counter placed downstream the target determines the collision impact parameter by measuring the charged particle multiplicity. The very peculiar geometry of the two RICHs allows the discrimination of the origin of electron pairs. In fact. electron pairs from photon conversion in the target show up as two overlapped rings in the first RICH while they appear as distinct rings in the second RICH due to the separation effect of the existing magnetic field, vice versa electron pairs from virtual photons are characterized by two well separated rings in both RICHs. The electron momenta are reconstructed from the ring position and the deflection angle. A remarkable presentation of the CERES RlCHs features including the technical problems met to build and operate such devices can be found in Refs. 18.9). while interesting pattern recognition methods developed for CERES data analysis were discussed by Ullrich at this workshop [ 1O].
2.2. CERESINA45
3. RICH detectors in planned experiments
CERES (ChErenkov Ring Electron Spectrometer) is the first experiment dedicated to the measurement of low mass electron-positron pairs and direct photons produced in hadron and nuclear collisions, at CERN SPS energies. Its main goal is to systematically study the pair continuum in the mass region from 100 MeV/c’ to almost 3 GeV/c*, and real photons by external pair conversion in the target [8]. To achieve these goals, a nonconventional spectrometer has been realized based on two azimuthally symmetric RICH devices (Fig. 3). Since the aim is to identify electrons, a CH, gas radiator at atmospheric pressure has been selected making the spectrometer “hadron blind”.
Despite the increasing energy available in the future heavy ion accelerators and therefore the higher multiplicity expected, the largest experiments planned so far still envisage the implementation of RICH arrays for the particle identification. Three experiments are reviewed in the following: BRAHMS and PHENIX at RHIC and ALICE at LHC. A fourth one, SQUASH, which has been proposed at CERN-SPS for the search of strange matter has been stepped out by the SPSC a few months ago, and the R&D on its RICH has been extensively presented at this workshop [ 1 I] and therefore will not be mentioned further in this paper. 3.1. BRAHMS
CERES Spectrometer
-1
I
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Fig. 3. Layout of the experiment CERES at CERWSPS. field lines are shown in the lower part.
3
Magnetic
The BRAHMS (Broad RAnge Hadron Magnetic Spectrometer) experiment at RHIC is designed to measure charged hadrons over a wide range of rapidity and transverse momentum for central and peripheral collisions for all available beams and energies [ 121. Physics goals are achieved using two small solid angle spectrometers which operate from 2 to 20” and from 20 to 90”, respectively (Fig. 4). The rapidity range will be from 0 to 4, and the p, up to 2.5 GeVlc for most of this y range. The expected multiplicity through the PID devices is quite low, the typical number of charged particles being 0.5-2 [12]. A RICH detector is under study for particle identification in the Forward Spectrometer arm. The aim is to identify pions at the 3a level for the momentum range from 4 to 20 GeV/c, kaons from 9 to 20 GeVlc and protons from 17 to 35 GeVIc, therefore a radiator of gaseous
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Mid rapldlty spectrometer
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Forward spectrometer Fig. 4. Layout of the experiment BRAHMS at RHIC. Dl , D2, D3, D4: dipole magnets: Tl, T2, T3, T4, T5, T6: tracking detectors; HI, H2: high resolution TOF hodoscopes; Cl, GASC: segmented gas Cherenkov counters; TPCl, TPC2: mid-rapidity TPCs; TOFW: mid-rapidity TOF-wall; MULT: multiplicity detector; DX: RHIC beam line magnet.
(C,F,,) at 1.25 b, absolute pressure, has been selected. The photodetector system is based on the new-generation multianode photomultiplier R4549 from Hamamatsu whose characteristics are the following: i) 20 electron amplification stages; ii) 10 X 10 cm* photocathode area corresponding to 100 output anode channels (10 X 10); iii) internal focussing electrodes in front of the first dynode to reduce the cross-talk; iv) fast photomultiplier response for first-level triggering. A successful prototype test has been performed at AGS in a beam of momentum-selected negative secondaries
perfluorobutane
V31.
3.2. PHENIX The PHENIX experiment (Pioneering High Energy Nuclear Interaction Experiment) at RHIC is designed to perform a systematic investigation of both leptonic and hadronic signatures and to look for simultaneous anomalies attributable to QGP formation [14]. Since electrons, muons and charged hadrons must be measured simultaneously, various PID arrays are planned: RICH for electrons, a time expansion chamber (TEC) to measure dE/dx, a time of flight (TOF) for hadrons and an electromagnetic calorimeter (EMCAL). PHENIX basic approach is to.attain a very good particle identification for a limited number of particles (300-500 charged particles per event), although the detectors are optimized to handle at least 1000 particles per event. The experimental layout is an axial-filled spectrometer covering the mid-rapidity region in addition to a forward magnet muon spectrometer. Most of the tracking chambers are installed outside the magnetic field, a silicon vertex
Fig, 5. Cut through view of the PHENIX experiment at RHIC, showing the two large magnets and the detector subsystems that serve as trackers and particle identifiers.
detector is installed close to the interaction area. A cut through view of the PHENIX experiment is shown in Fig. 5. 3.2.1. The RICH detector The RICH detector is designed to be the primary subsystem for electron identification and it consists of two arms located outside the central magnet at a distance of 2.6 m from the collision point (Fig. 6). Each arm covers 90” in azimuth and 0.35 units of rapidity. The radiator is selected with the aim to make the RICH “hadron-blind”, i.e. hadrons are below the Cherenkov threshold and electrons produce rings of asymptotic radius. Therefore either ethane (n - 1.0008) or methane (n 1.00044), at atmospheric pressure, are possible candidates. The Cherenkov photons after reflection from spherical mirrors with a radius of curvature of 4 m, are registered on a highly segmented array of 6400 photomultipliers arranged into 256 sets of 25 tubes each, mounted on the
Fig. 6. Two side views of the PHENIX RICH complex. An electron is shown crossing the detector and emitting Cherenkov light reflected and focused by the mirror.
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3.3. ALICE
Fig. 7. Principle of Cherenkov multiplier array of the PHENIX
photon detection in the photoRICH. Four electron tracks and the Cherenkov light cones focused by the mirror are shown. Note the photomultipliers placed above the magnet pole tip.
mirror focal plane, behind the central magnet (Fig. 7). As a benefit of this geometry, the pole tips of the magnet shield the photodetectors from the incoming hadrons. Winston cones are envisaged to be attached on the entrance window of each phototube for reducing dead space on the focal plane.
3.2.3. R&D results Results from a beam test at KEK have been obtained using a module of 25 Hamamatsu H3 171 phototubes and few gas radiators: nitrogen, methane and ethane [14]. Quite a large number of photoelectrons have been measured bringing the N, value to 110. The phototubes have been selected with a short transit time spread for the twofold aim of: i) providing a trigger signal from local clusters of phototubes above threshold; ii) reducing background hits and random noise at the level of 2 X 10 -4 corresponding to one spurious hit per 6400 phototubes per event both for the high time resolution (u = 250 ps) and for the cuts on the arrival time of the Cherenkov photons. Applying timing cuts in the offline analysis, only three events had more than four photomultipliers hit above pedestal out of 69 K pion triggers recorded. Therefore, a pion suppression of 2.3 X lo4 was achieved [14].
ALICE (A Large Ion Collider Experiment) is the only dedicated experiment planned for the CERN LHC to investigate heavy ion collisions at centre-of-mass energy of -6TeV per nucleon. It is meant as a general purpose experiment to study the physics of the nuclear matter at high densities and temperatures through the systematic study of a number of specific signals, together with a global survey of the events [ 151. It is designed to cope with the highest particle density anticipated for Pb-Pb collisions at the LHC (theoretical models predict up to 8000 charged particles per unit of rapidity for central Pb on Pb collisions). The ALICE layout presently includes a silicon inner tracking system (ITS), a large volume time projection chamber (TPC), a highly segmented particle identification array (TOF or RICH) and a calorimetric system. The listed detectors are embedded in the 0.2 T uniform magnetic field provided by a large L3-like solenoid magnet to perform momentum analysis of charged particles over two units of pseudo-rapidity with full azimuthal coverage (Fig. 8). The ITS is located as close as possible to the beam axis (r = 7.5 cm) and has the task of tracking particles of very low momentum, identifying short-lived particles (mostly hyperons) decaying before they reach the TPC, and improving the momentum resolution of the tracked particles. The TPC is designed to provide tracking and dEldx measurements for charged particles with pseudo-rapidities in the range - 1 < r] < + 1. It has an active volume defined by the two cylindrical surfaces at r = 100 cm and r = 250 cm. A number of options are under study for the hadron identification system: three based on the TOF technique (namely Pestov spark tubes, parallel plate gas chambers and a scintillator array) and the fourth alternative based on the RICH technique. A separation power greater than 3a is required on a track-by-track basis to keep the contamination below a 10% level in the presence of a huge bulk of hadrons at low momentum, moreover a reliable pattern recognition is mandatory due to the high track density. Depending on the performance achieved by the most suitable technology, the PID system will be placed at radii between 3 and 4.5 m. Since LHC operations with lead beams is likely to reach a luminosity of 2 X lo*’ cm-’ s ’ , an interaction rate of about lo4 Hz is expected, but fortunately only a small fraction, approximately 2-3%, corresponds to the most interesting central collisions with maximum particle production. It follows that the requirements on the PID frontend electronics are much less stringent than those in LHC pp experiments, but as already pointed out the high multiplicity represents in this case a very demanding task when it combines with the large rapidity range to be covered. Moreover the realization of such ambitious
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ALICE Fig. 8. The baseline ALICE layout, showing the large L3 magnet and the location of the tracking detectors and the PID barrel. detector complex must be based on a cost effective production of the system elements with standard techniques, with reliability and durability as key factors. 3.3. I. RICH system layout Presently, the RICH system for ALICE represents the largest scale application of such a technique. The layout is governed by the need to optimize the detector performance in terms of angle resolution and efficient pattern recognition: since the best angle resolution is achieved for particles with normal incidence to the radiator, it is necessary to shape the detector in a way to minimize the angular dispersion of incoming particles. An efficient way to fulfill the above requirements is to design the RICH barrel based on modular construction with elements of the largest technically feasible area, in order to reduce the total number needed to cover the acceptance region. The RICH array is conceived as a barrel of 3.6 m radius and 7.2 m length, composed of 60 modules on a supporting structure that allows the tilting of each module independently. The detector is placed inside the magnetic field with a segmentation of 12 modules in azimuth and 5 along the beam axis. The barrel can be moved sideways independently to give free access for installation and survey of major detector components.
The RICH array is planned to provide the following tasks: 1) supplement the PID obtained by dEldx method in the earlier parts of the detector (ITS and TPC) in the range where they are not operational, namely at the minimum of ionisation (e.g. 0X-2.5 GeV/c for pion-kaon identification); 2) provide an additional tracking plane in the detector with good position determination and multitrack separation capabilities. The tasks described will be covered by a RICH counter of the proximity focussing type with a liquid freon (C,F,,) radiator and a UV detector based on the use of a low gain multiwire proportional chamber (MWPC) with a pad readout for a two-dimensional determination of the positions of the ionizing particles and of the Cherenkov photons. The UV photoconversion into electrons is achieved by using a solid photocathode consisting of a thin layer of CsI evaporated onto the pad plane. The schematic of the proposed RICH is shown in Fig. 9. 3.3.2. The UV detector The operation of a RICH counter is dictated primarily by the performance of the UV detector. The MWPC will consist of a cathode mesh, or wires (for larger photon transparency), a wire anode plane with wires
E. Nappi
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Fig. 9. Cut away view of the ALICE RICH. The MWPC has 20 pm diameter anode wires at 4 mm pitch with the two cathode planes equidistant from the anode plane at a distance of 2 mm. The Csl cathode plane is segmented in pads of size 8 X 8 mm’. Each pad is viewed by two anode wires. The proximity gap is defined by two electrodes, the first of them, made of 100 pm diameter wires of 4 mm pitch, is located close to the quartz window and is positively polarized with respect to the cathode mesh. so as to collect the primary ionization deposited by the charged particles in the proximity gap.
spaced 4 mm, and a pad readout cathode plane with pads of 8 X 8 mm* in size. The pad size is chosen in function of the position accuracy wanted, and of the optimum anode coupling for detection of the signal induced by the electron avalanche. The total material thickness of the UV detector. including the readout electronics, is 3% X,,. The present project, that makes use of the existing experience in the field of fast RICH detectors coupled to the R&D with solid photocathodes, provides a detector of outstanding features that we summarize as follows: i) the thickness of the MWPC (4 mm) allows for positive identification of charged particle impact on the detector (a clear discrimination against photon clusters is possible), keeping at the same time the active gap small enough for the operation of the detector in a high density environment with inclined tracks without deterioration of the localization characteristics; ii) photoconversion is achieved in a single layer thus eliminating an importance source of parallax error present in detectors where a photosensitive gas is used instead of a solid photocathode; iii) the use of a solid photocathode allows for a substantial saving in terms of material thickness. In fact the gap between the detector and the radiator may be filled with the same gas as the detector thus eliminating the need for a quartz window:
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iv) the analog multiplexing readout electronics and the intrinsic characteristics of a MWPC of the present design enable the operation at interaction rates 10J- 10‘ Hz with acquisition speeds of 10’ Hz; v) high segmentation flexibility and low cost per channel dictated by the need to cover large surfaces. In 1992, a large detector development effort was started in view of the above very ambitious technological aspects to be fulfilled. A specific R&D was approved by the DRDC at CERN under the name RD26 [ 161. The RICH design for ALICE has largely benefited from activities carried on in RD26 and the on-going researches will very likely still contribute in assessing the final performance. An updated RD26 status report has been presented by Piuz at this workshop [17]. Some of the RD26 outcomes are really fundamental to a successful realization of a well performing CsI-RICH for ALICE. For instance, the technology of evaporating large photocathodes has been successfully implemented and tested with photocathodes up to 50 X SOcm’. The method is inexpensive and works without the use of time-consuming masking techniques. ALICE people are confident that the same technique may be applied for larger sizes. 3.3.3. The radiator The use of a liquid fluorocarbon as a radiator is with an index of envisaged. The C,F ,., characteristics, of n=1.2834 at A= l75nm and low refraction chromaticity is adequate for the range of energies under study. The radiator vessel represents the most critical part in the detector design [ 181. The rather high freon density, 1.68 g/cm’, and the need to avoid pollution from the material in contact with the liquid ,radiator that would affect the transparency in the 160-220 nm band, require a careful investigation and optimization. The liquid radiator container, which is currently under study, consists of a tray made of composite material closed by 5-6 mm thick UV transparent quartz window. An aramide-fibre/epoxy honeycomb material (NOMEX) has been chosen as inner layer because its thermal expansion coefficient is very small and its very low density (40 kg/m’) makes it almost transparent to the incoming particles. A 100 p,rn carbon fiber layer is used for the skin surfaces because of its high modulus of elasticity. The inner surface of the vessel is covered with 50 pm thick aluminum foil to prevent the liquid C,F,, to come into contact with the carbon fiber tissue (Fig. 10). All the elements are glued with Araldite AWlO6. The liquid radiator inlet and outlet are performed in the honeycomb volume inserting two stainless steel pipes that run lengthwise along the container wall on opposite vessel comers, the outlet always being at the highest location. Given the quantum efficiency achieved so far, the radiator thickness is chosen 10 mm for an optimal Cherenkov angle resolution. The vessel exit windows consist of a UV-grade fused silica plates whose thickness and size must be carefully optimized investigating the best compromise
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o-ring freon
Llquld /Honeycomb
Inlet
esndwloh
~I-oove
1 i
2500-
mco 1500
.. i :,
-
i
iooo -
:’ : !
MO0
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0
~~~~‘,,~~‘~,)~‘~~,.‘.~-~‘~ 0.1 0.2 0,3
0.4
0.5
: !
~I **-+I,,~ 0.7 0.6 Aqk (roi;
Fig. 11. Reconstructed single photon Cherenkov angle distribution corresponding to the analog center of gravity of the cluster (dashed line) and to the center of each hit pad (solid line).
Aluminum Carbon
ah In tlber
ahln
Honeycomb
J
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/
Fig. IO. Cut away view of the radiator vessel structure.
between the detector total radiation length and the freon hydrostatic pressure. A possible solution is to segment each module in four or six independent sections with reinforcement ribs of composite material. The thin quartz window will be supported in the central region by cylindrical spacers connected to the back wall of the radiator. A reduced scale prototype is being realized to gain useful information on its mechanical and chemical stability. 3.3.4. Performance of the detector The performance of the detector in a high density environment depends on two factors: 1) the ability to distinguish the clusters from charged particles from those belonging to photons. In the present prototype design a clear distinction between these two types of clusters is clearly possible: the mean cluster size for a minimum ionization particle is 4 pads while for photon is 1.2 pads. This feature is particularly important because apart from distinction from photon clusters, the larger size allows very good spatial precision on the impact point of the particle on the radiator, which is of paramount importance for the tracking precision, since with more pads the centroid position may be better determined. The coordinates of impact of the charged particles have been measured in the proposed setup (8 X 8 mm2 pads and anode wire distance 4 mm) with a precision better than 400 km. The two track separation has been estimated by simulations to be better than 2 mm. Accuracy on the photon centroid position is less demanding for as long as it is lower than the error on the localization of the photon due to chromatic aberration and geometrical effects (proximity gap). In fact, as shown in Fig. 11, the Cherenkov
angle resolution does ot change if pads are not associated to a cluster; 2) the number of photons registered per event. This number is a function of the quantum efficiency of the photocathode, the beta value of the particle and finally the thickness of the radiator. Bench tests of the Csl quantum efficiency and single-electron detection sensitivity performed in the framework of the R&D programme RD26 have provided useful information on how to optimize these parameters [ 191. 3.3.5. Particle identijcation in high multiplicity environments At the foreseen LHC luminosity of -2 X 10” cm-’ s- ’ in ion mode operation and for central collisions the number of particles reaching the RICH is estimated to be -80 at the center of the detector and -50 particles/m’ particles/m* at the outer edge of the detector, taking into account also the secondary particles created. Pattern recognition studies have been carried out with real data from 3 GeV pion and proton beam tests of a prototype with the presently investigated geometry: 1.0 cm freon thickness and 7.0 cm proximity gap width. Multi-ring events have been simulated by translating and superimposing single-ring images randomly over the area of an imaginary detector thus creating an image that corresponds to a high-density event. Four events with a multiplicity of 80 particles/m” are shown in Fig. 12. It is important to note that the generated images have a density higher than the expected from the actual Pb-Pb collision due to the fact that some particles will not generate rings (because they are below the Cherenkov threshold) and that the is peaked at low values momentum distribution (-350 MeVlc) thus generating rings with smaller radii and smaller number of photons per ring. The pattern recognition procedure has been described in Ref. [20]. The coordinates of the impact point were determined from the center-of-gravity of the MIP ionization pattern on the pad chamber, in the real experiment they will be provided by the tracking detectors in the ALICE setup. In the pattern recognition phase, every simulated event was analyzed, including those containing possible interactions in the
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Overlaped raw events - density 80 park/m2 60 50 40 30 20
20
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,I,
r 20
.a -= I
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I)
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60 X 60 pads
Fig. 12. Four scatter plots, corresponding to events with 80 particles/m’, generated 3 GeV/c pions and protons in the 50 X 50 cm2 CsI RICH prototype (see text).
detector volume. ionizing particles passing across the detection volume, noise due to pedestal fluctuations, ring patterns with a small photon number. Nevertheless, only particles that fell more than 10 cm away from the detector edges were considered since the edge effects will later be studied more carefully. Results from the superimposed events are given in Fig. 13. Note again that the distributions are very clean although no cuts in the raw data were applied. The method allows a kaon/pion 3a separation at 2.6 GeV/c. The separation power is a function of pattern recognition criteria. With severe criteria (e.g. a cut on the ADC content
pad number
superimposing raw Cherenkov patterns produced by
of each pad) one may improve the particle separation at the expense of efficiency. With no cuts applied the inefficiency is below 5%.
4. Conclusions In summary, we should note that RICH technology is making rapid advances in identifying particles even in a high multiplicity environment, in fact it has already been successfully employed to exploit a significant physics programme in two forward spectrometer experiments at
VIII. PROPOSED
EXPERIMENTS
E. Nappi I Nucl. Instr. and Meth. in Phys. Res. A 371 (1996) 275-284
284
PHENIX and BRAHMS experiments. In addition he wishes to thank T. Ljubicic Jr. and A. Tricomi for providing and discussing results on the ALICE-RICH pattern recognition.
17.5 15 12.5 10
References
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150
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300
350
400
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Fig. 13. Angle distribution obtained counting all the hit pads, around each particle impact point, that fall within a scanning window of 0.1” width. The highest bin content is chosen as the Cherenkov angle. The bottom plot shows the improvement in the signal height over the background when pads with charge below 14 ADC channels are removed.
CERN-SPS. Furthermore, the three devices in development for ALICE at LHC, PHENIX and BRAHMS at RHIC have obtained satisfactory results from their R&D programme. Nevertheless, not all the existing problems have been solved so far and therefore more R&D efforts are required.
Acknowledgements The author is very grateful to H. Hamagaki, S. Nagamiya, F. Videbaek for having sent him material on the
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