The TRD of the CBM experiment

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1 ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 563 (2006) The TRD of the CBM experiment A. Andronic Gesellschaft für Schwerionenforschung, Darmstadt, Germany for the CBM Collaboration Available online 22 March 2006 Abstract We introduce the TRD of the CBM experiment at the planned FAIR facility of GSI. A study on the optimization of the detector parameters in what concerns the electron/pion identification performance is presented. Results on tracking performance as well as on rate capability are shown. r 2006 Elsevier B.V. All rights reserved. PACS: Cs Keywords: Transition radiation detector; Electron/pion identification; Wire chambers 1. The CBM experiment at FAIR The planned accelerator facility at GSI [1], FAIR (Facility for Antiprotons and Ion Research) will provide high intensity beams of protons and antiprotons, nuclei up to uranium as well as radioactive beams. The main goal for the study of nucleus nucleus collisions is the exploration of the phase diagram of Quantum Chromo-Dynamics (QCD), shown in Fig. 1 [2]. Lattice QCD (LQCD) calculations predict that, at high energy densities, the quarks and gluons are not anymore confined in ordinary hadrons, but are in a phase called Quark Gluon Plasma (QGP), a state of matter which was the substance of our Universe in the first microseconds after the Big Bang. For the energies available at FAIR, namely up to 35 AGeV uranium beams on fixed target, the region of the phase diagram explored corresponds to the intervals in temperatures of T MeV and baryo-chemical potential m b MeV. Based on existing measurements and on LQCD calculations (see Ref. [2] and references therein), it appears that in this domain the phase boundary between hadrons and QGP is approached [2]. Moreover, the critical point (corresponding to the termination of the first-order phase transition line) may be Tel.: ; fax: address: A.Andronic@gsi.de. approached as well, with characteristic imprints on physics observables (event-by-event fluctuations). The CBM (Compressed Baryonic Matter) experiment [1] at FAIR will address these issues based on precision, large acceptance and high-statistics measurements of common hadrons (p, K, p) as well as of so-called rare probes, like low-mass dileptons (r, f), charmed hadrons (D mesons, J/c) or multistrange baryons (X, O). Many of these observables will be measured for the first time in heavyion collisions at these energies. The proposed setup of the experiment is shown in Fig. 2. It consists of: (i) a silicon tracking system (STS) composed of seven layers of pixel and strip detectors, placed in a dipole magnet (1 Tm), a RICH detector, a TRD, a time-of-flight (ToF) wall based on resistive plate chambers and an electromagnetic calorimeter (ECAL). These systems allow precision tracking in high-multiplicity events, momentum measurement and particle identification. Ongoing simulations aim at optimizing these various subdetectors in the CBM setup and at providing the choice for other detection techniques (like muons). 2. The TRD of CBM The TRD will provide electron identification and tracking of all charged particles. It has to provide, in conjunction with the RICH detector and the ECAL, /$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi: /j.nima

2 350 ARTICLE IN PRESS A. Andronic / Nuclear Instruments and Methods in Physics Research A 563 (2006) T (MeV) QGP hadrons Data (fits) LQCD hadron gas n b =0.12 fm -3 ε=500 MeV/fm µ b (MeV) 1 st order crossover critical point Fig. 1. The phase diagram of QCD [2]. The points represent results of thermal fits to experimental data, the lines are calculations. absorption of transition radiation (TR) produced by the radiator. Because of the high rate environment expected in the CBM experiment (interaction rates up to 10 MHz are envisaged), a fast readout detector has to be used. To ensure the speed and also to minimize possible space charge effects expected at high rates, it is clear that the detector has to have a thickness smaller than about 1 cm. Two solutions exist for such a detector: a multiwire proportional chamber (MWPC) with pad readout or straw tubes. Both are currently being investigated as alternative designs. A combination of the two is feasible and is a possible solution. For the radiator there are two possibilities: regular and irregular. The regular radiator, composed of foils (polypropylene) and gaps of equal size, is obviously the choice which provides the highest TR yield. However, the costs may be high due to a complicated construction procedure. The irregular radiator, composed of fibers and/or foams has a reduced TR yield compared to a regular radiator of the same material budget [3], but the advantage is ease of manufacturing. The final choice of the radiator type in CBM TRD will be established after the completion of prototypes tests. The main characteristics of the TRD are: cell sizes: 1 10 cm 2 (depending on the polar angle, tuned for occupancy o10%), material budget: X=X %, rates: up to 100 khz=cm 2, doses (charged particles): up to 16 krad/year, corresponding to mc/cm/year charge on chamber wires. Fig. 2. The setup of the CBM experiment. sufficient electron identification capability for the measurements of charmonium and of low-mass vector mesons. The required pion suppression is a factor of about 100 and the position resolution has to be of the order of mm. In order to fulfill these tasks, in the context of the high rates and high particle multiplicities in CBM, a careful optimization of the detector is required. Currently, the whole detector is envisaged to be divided into three stations, positioned at distances of 4, 6 and 8 m from the target, each composed of at least three layers. A detailed study of the tracking performance in combination with all the CBM subdetectors is ongoing to optimize this segmentation, as well as for establishing the final requirements on position resolution within each of the planes. The total thickness of the detector in terms of radiation length has to be kept as small as possible to minimize multiple scattering and conversions. The gas mixture of the readout detectors has to be based on Xe, to maximize the For a MWPC-type of TRD, we envisage 9 12 layers, leading to a total area of detectors in the range m 2. The total number of electronic channels is between 562 and 749 thousands. 3. Simulations of e=p identification A standalone Monte Carlo C þþbased simulation code was developed to perform the simulations [4]. One layer of the TRD consists of a radiator, composed of polypropylene foils with air gaps, and a readout chamber filled with a Xe CO 2 [85 15] mixture. A mylar foil of 25 mm thickness acts as a detector gas barrier. We focus here on the MWPC-based TRD design. The following processes are considered in the simulations: (i) energy loss of electrons and pions in the gas detector, done following the procedure described in Ref. [5]. Primary electrons with energies above 10 kev are treated as delta electrons whose trajectory is collinear with the one of the primary particle; (ii) for electrons, production and absorption of TR in the radiator, absorption of TR in the mylar foil and absorption of TR in the active gas volume. Computation of the transition radiation spectra in the radiator is done with a simplified formula [6] (see also

3 ARTICLE IN PRESS A. Andronic / Nuclear Instruments and Methods in Physics Research A 563 (2006) Ref. [3]) for a regular radiator. In our case the nominal radiator parameters are foil thickness d 1 ¼ 15 mm, air gap d 2 ¼ 200 mm, number of foils N f ¼ 150. The baseline detector gas thickness is considered here to be 6 mm. For this configuration, electrons of 2 GeV=c produce on average 1.8 TR photons (at the end of the radiator), of which 0.85 are detected. We have not considered the propagation of unabsorbed TR to further layers. Based on the energy deposit in one layer of the detector, shown in Fig. 3 for pions and electrons, the likelihood (to be an electron) distributions are calculated [7]. The pion efficiency at 90% electron efficiency is used to quantify the e=p identification performance. The results shown are for a momentum of 2 GeV=c. In Fig. 4 we present the pion efficiency as a function of number of layers for different radiator thicknesses (number of foils) and for different thicknesses of the gas readout chamber. The required pion efficiency (we consider at this stage a safety factor of 10) can be reached with a 9 layer TRD with a radiator of 150 foils and a 6 mm thick detector. Different detector configurations are equivalent in this respect. A lighter radiator (which is more favorable for tracking) implies more layers (at higher costs for the extra electronics channels). A thicker readout chamber implies the benefit of fewer layers, but needs to be tested concerning the rate capability and the position resolution performance. The dependence of the pion efficiency on the foil thickness and on the gap width is shown in Fig. 5. Results for a TRD configuration with 9 and 12 layers are shown. In the case of a 9 layer TRD, the number of foils has to be increased to N f ¼ 150 in order to obtain the same pion rejection as for 12 layers with N f ¼ 90. Increasing the thickness of the foils from 10 to 15 mm improves the pion rejection by a factor of about 2. Even more remarkable is the effect of the gap width: an increase from 100 to 200 mm pion efficiency [%] pion efficiency [%] foils 90 foils 110 foils 130 foils 150 foils foils 190 foils 210 foils No. of layers 1 4 mm 5 mm 6 mm 7 mm mm No. of layers Fig. 4. Pion efficiency as a function of the number of layers for different thicknesses of the radiator (upper panel) and of the detector (lower panel). counts el. with TR pion Energy deposit [kev] Fig. 3. Spectra of energy deposit of pions and electrons of 2 GeV=c in 6 mm Xe CO 2 [85 15] mixture of the detector. leads to an improvement of the pion rejection by a factor of about 6. As increasing the gap does not imply any negative impact on the material budget, it is clear that a regular radiator with a large foil gap would lead to a significantly better electron/pion identification than an irregular radiator (which corresponds to small gap values) of the same material budget. The trends seen in Fig. 5 are known to result from the formation zone effect [8]. It interesting to note that increasing the foil thickness beyond the optimum value of about 16 mm, the effect of absorption of TR within the radiator leads to a degrading performance. In Fig. 6 we present the dependence of the pion efficiency on the Xe concentration in the detector mixture, for two detector configurations ðd 1 =d 2 =N f Þ, with 9 and 12 layers, respectively. As expected, the pion rejection shows a strong dependence on the Xe content, arising from the efficiency of TR absorption in the active detector gas. For instance, about a factor of 2 better rejection is achieved for each extra 10% Xe. However, the amount of CO 2 is dictated by

4 352 ARTICLE IN PRESS A. Andronic / Nuclear Instruments and Methods in Physics Research A 563 (2006) pion eff. [%] layers (150 foils) 12 layers (90 foils) comparable rejection is achieved with 9 and 12 layers for N f ¼ 150 of 90, respectively. The results of our simulations demonstrate that a TRD with 9 to 12 layers can fulfill the required electron/pion identification performance in CBM. From the multidimensional parameter space of the detector which we have explored, a configuration will be chosen based on further studies with simulations and detector prototypes. Similar performance in e=p identification as shown here for the MWPC-based TRD design have been demonstrated in simulations for the straw-based TRD option. pion eff. [%] foil thickness [µm] gap thickness [µm] 9 layers (150 foils) 12 layers (90 foils) Fig. 5. Pion efficiency as a function of foil (upper panel) or gap thickness (lower panel) for 9 and 12 layers TRD. pion efficiency [%] layers - 15/200/90 9 layers - 15/200/ Xe content [%] Fig. 6. Pion efficiency as a function of the concentration of Xe in the detector. detector consideration (stability, gas gain) and we expect to be able to operate our detectors at a CO 2 content of 10 15%. For the two configurations that we investigate, 4. Tracking performance The tracking performance of TRD is important in two aspects: (i) matching of the identified electrons to the track segment in the STS, where the momentum measurement is performed; (ii) matching of the hits in ToF, which identifies hadrons up to several GeV/c, to the STS track segment. To assess point (i) we have performed a study of the tracking performance in TRD assuming that the resolution in the 3 TRD stations are 300, 400, 500 mm, respectively, across the pads (wire direction) and from 3 mm to 3 cm along pads (depending on the pad length, which varies as a function of the polar angle). We have assumed alternating layers with these resolutions. A solution involving tilted pads, which will result in significantly better resolution along pads, is under consideration. The input for the complete Geant simulation of the detector, including secondary particle production, is a transport model calculation for central 25 AGeV Au+Au collisions with a primary charged particle multiplicity of about 900 particles (mainly pions, kaons and protons) in full phase space. The average momentum in the laboratory frame is about 3 GeV=c. The results for tracking (including matching with STS) efficiency are shown in Fig. 7 as a function of (laboratory) momentum for a nine layer TRD. The efficiency to find good tracks (tracks with at least eight good hits) is above 90% for most of the momentum range, while the fake tracks are below 10% (o5% for p43 GeV=c). This performance gives good confidence that the present choice of the TRD setup in CBM, despite being a starting point in the optimization procedure, is already very good. Note that for these simulations we have used a realistic material budget (2.2% X 0 per layer) for the active area of the chambers, but no detector frames and support structures. A segmentation with realistic chamber dimensions and frames is presently under evaluation. 5. Results from prototype tests The first exploratory beam measurements of detector prototypes have started in 2004 [9]. Due to limitations in beam intensity, no e=p performance could be studied, but we have investigated basic rate behavior of the detectors. The rate dependence of position resolution is presented in

5 ARTICLE IN PRESS A. Andronic / Nuclear Instruments and Methods in Physics Research A 563 (2006) URQMD, 3x3 layers, 8/9 hits, 2.2% efficiency Efficiency : % Found tracks Good tracks Fake tracks Resolution (µm) momentum [GeV/c] Fig. 7. Tracking efficiency as a function of momentum Rate (khz/cm 2 ) Fig. 8. Position resolution as a function of rate. Fig. 8. Resolutions of about 310 mm are achieved at low rates, although the geometry of the chambers was not yet optimized. A slight degradation of the resolution is observed for higher rates, amounting to about 20 mm at 100 khz=cm 2, which is the maximum rate envisaged in CBM. A new version of detector prototypes and of frontend electronics is being prepared for the next measurement campaign in beam. Further studies of detector performance at high rate are presently performed using a generator of X-rays with an average energy of about 8 kev. Results on gas gain (G) dependence as a function of rate (R) are presented in Fig. 9. The gain reduction due to space charge is very well described by the formula ln ðg=g 0 Þ¼kRG [10] with the same k parameter for all sets of nominal gain. Moderate gain drop is seen, even for rates as high as 200 khz=cm 2 (the dashed lines mark the loci of constant gain drop). An Ar-based mixture with 10% CO 2 has been used here, but comprehensive measurements on Xe-based mixtures are under way. 6. Summary The main requirements and the basic detector concept for the TRD in the CBM experiment have been presented. Results on simulations show that the e=p identification and the tracking performance within the current design are good and already meet the requirements. The first exploratory measurements at high rates, both with beams and with X-rays, indicate that the required performance for the CBM TRD can be achieved with MWPCs with pad readout. More quantitative statements will be derived from new measurements, which we foresee to realize in the course of year Gain Rate (khz/cm 2 ) Acknowledgements I am indebted to the people who have directly contributed to the results shown here: M. Al-Turany, D. Bertini, C. Garabatos, E. Jimenez, M. Kalisky, I. Kisel, D. Kresan, F. Uhlig, Yu.Vasiliev. This research is partially funded by the EU Integrated Infrastructure Initiative Project HADRON PHYSICS under Contract no. RII3-CT References 1% 2% 5% Ar-CO 2 [90-10] 10% gain drop Fig. 9. Gas gain as a function of rate for the mixture Ar CO 2 [90 10%]. [1] FAIR, hhttp:// CBM, hhttp:// experiments/cbm/i.

6 354 ARTICLE IN PRESS A. Andronic / Nuclear Instruments and Methods in Physics Research A 563 (2006) [2] A. Andronic, et al., nucl-th/ [3] A. Andronic, et al., Nucl. Instr. and Meth. A 558 (2006) 516. [4] A. Andronic, et al., GSI Scientific Report 2004, p. 11. [5] A. Andronic, et al., Nucl. Instr. and Meth. A 519 (2004) 508 [physics/ ]. [6] C.W. Fabjan, W. Struczinski, Phys. Lett. B 57 (1975) 483. [7] M.L. Cherry, et al., Nucl. Instr. and Meth. 115 (1974) 141. [8] B. Dolgoshein, Nucl. Instr. and Meth. A 326 (1993) 434. [9] A. Andronic, et al., GSI Scientific Report 2004, p [10] E. Mathieson, G.C. Smith, Nucl. Instr. and Meth. A 316 (1992) 246.