The ATLAS Transition Radiation Tracker - Springer Link

ISSN 00204412, Instruments and Experimental Techniques, 2012, Vol. 55, No. 3, pp. 323–334. © Pleiades Publishing, Ltd., 2012. Original Russian Text © A.S. Boldyrev, V.G. Bondarenko, V.N. Bychkov, B.A. Dolgoshein, O.L. Fedin, I.L. Gavrilenko, Yu.V. Gusakov, N. Grigalashvili, Ya.V. Grishkevich, V.A. Kantserov, S.V. Katunin, F.F. Kayumov, G.D. Kekelidze, E.M. Khabarova, N.V. Klopov, N.V. Kondratieva, S.P. Konovalov, N.A. Korotkova, S.N. Kovalenko, V.A. Krama renko, L.G. Kudin, I.A. Kudryashov, K.A. Levterov, V. M. Lysan, S.P. Lobastov, V. P. Maleev, R.Yu. Mashinistov, V.V. Mialkovskii, S.V. Morozov, S. V. Muraviev, A.V. Nadtochii, N.V. Nikitin, O.V. Novgorodova, E.G. Novodvorskii, S.B. Oleshko, S.K. Patrichev, V.D. Peshekhonov, A.S. Romanyuk, Yu.F. Ryabov, A.A. Savenkov, E.V. Sedykh, D.M. Seliver stov, A.P. Shmeleva, S.Yu. Sivoklokov, S.Yu. Smirnov, L.N. Smirnova, V.V. Sosnovtsev, S.I. Suchkov, V.V. Sulin, V.O. Tikhomirov, L.F. Vasilieva, K.I. Zhukov, 2012, published in Pribory i Tekhnika Eksperimenta, 2012, No. 3, pp. 27–39.

NUCLEAR EXPERIMENTAL TECHNIQUE In memory of Professor B. A. Dolgoshein

The ATLAS Transition Radiation Tracker A. S. Boldyreva, V. G. Bondarenkob, V. N. Bychkovc, B. A. Dolgoshein†b, O. L. Fedind, I. L. Gavrilenkoe, Yu. V. Gusakovc, N. Grigalashvilic, Ya. V. Grishkevicha, V. A. Kantserovb, S. V. Katunind, F. F. Kayumove, G. D. Kekelidzec, E. M. Khabarovac, N. V. Klopovd, N. V. Kondratievab, S. P. Konovalove, N. A. Korotkovaa, S. N. Kovalenkod, V. A. Kramarenkoa, L. G. Kudind, I. A. Kudryashova, K. A. Levterovc, V. M. Lysanc, S. P. Lobastovc, V. P. Maleevd, R. Yu. Mashinistove, V. V. Mialkovskiic, S. V. Morozovb, S. V. Muraviev†e, A. V. Nadtochiid, N. V. Nikitina, O. V. Novgorodovae, E. G. Novodvorskiid, S. B. Oleshkod, S. K. Patrichevd, V. D. Peshekhonovc, A. S. Romanyukb, Yu. F. Ryabovc, A. A. Savenkovc, E. V. Sedykhd, D. M. Seliverstovd, A. P. Shmelevae, S. Yu. Sivoklokova, S. Yu. Smirnovb, L. N. Smirnovaa, V. V. Sosnovtsevb, S. I. Suchkove, V. V. Suline, V. O. Tikhomirove, L. F. Vasilievae, and K. I. Zhukove a

Skobel’tsyn Institute of Nuclear Physics, Moscow State University, Moscow, 119991 Russia b National Research Nuclear University, Moscow Engineering Physics Institute (MEPhI), Kashirskoe sh. 31, Moscow, 115409 Russia c Joint Institute for Nuclear Research, ul. JoliotCurie 6, Dubna, Moscow oblast, 141980 Russia d Konstantinov Institute of Nuclear Physics, St. Petersburg, Orlova roshcha, Gatchina, Leningrad oblast, 188300 Russia e Lebedev Physical Institute, Russian Academy of Sciences, Leninskii pr. 53, Moscow, 119991 Russia Received July 27, 2011

Abstract—The transition radiation tracker of the ATLAS setup, which is one of the two multipurpose detec tors at the Large Hadron Collider (LHC), its design, and the tasks it performs are described. The tracker is fully assembled and commissioned. The first physical results obtained by the tracker in the ATLAS cosmic muon runs are presented. DOI: 10.1134/S0020441212010125 †

1. INTRODUCTION

The transition radiation tracker (TRT) is a part of the ATLAS Inner Detector [1], which is one of the two multipurpose detectors at the Large Hadron Collider (LHC). The Inner Detector is located at the center of the ATLAS setup in close vicinity of the region of highenergy proton collisions and is inserted into a solenoid with a magnetic field of 2 T. The detector is shaped as a barrel and is intended to precisely measure coordinates and momenta of charged particles, as well as vertices of primary and secondary interactions. The charged particle momenta are measured above a threshold of 0.5 GeV/c in the pseudorapidity region |η| < 2.5. For inelastic events with minimum selection conditions, it is possible to measure momenta at a lower threshold of 0.1 GeV/c. In addition, the TRT is used to identify electrons in the pseudorapidity region † Deceased.

|η| < 2.0 and in the momentum range of 0.5– 150 GeV/c. The idea of a combined TRT was proposed for the first time by a team of researchers at the Moscow Engineering Physics Institute (MEPhI) and the Lebe dev Physical Institute [2] and was based on the experi ence gained in designing and operation of the transi tion radiation detector (TRD) for the HELIOS exper iment [3]. Along with the institutes and research teams of CERN, Denmark, Poland, Sweden, Turkey, and the United States, Russian institutes—Konstantinov Institute of Nuclear Physics, MEPhI, Lebedev Physi cal Institute, Skobel’tsyn Institute of Nuclear Physics, and Joint Institute for Nuclear Research—contrib uted significantly to the development of the ATLAS TRT. The fundamental difference of the LHC from the earlier colliders and the machines that are currently in operation is in the fact that, at a designed luminosity of ≥2 × 1034 (cm–2 s–1), radiation background is gener

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ated by particles produced in interactions of acceler ated protons in the collision region. This radiation background presents the most serious danger to the ATLAS Inner Detector. It is expected that, owing to the use of the stateoftheart technologies in its design, the TRT will be capable of performing all its tasks at the highest LHC luminosity. In this paper, we present the general description and functions of the ATLAS Inner Detector. The TRT sensor element, the principles of transition radiation generation and detection, the TRT structure, the detector modules, and the electronics for data readout and transmission are described. Results of the first measurements of the TRT as a part the ATLAS setup are presented. The main conclusions drawn from the TRT development and the first experimental results are presented. 2. ATLAS INNER DETECTOR The ATLAS Inner Detector is housed inside the barrelshaped solenoid, which is symmetrical about the intersection point of the proton beam at the center of the detector. The Inner Detector covers the region with a length of ±3512 mm along the beam axis and has an outer radius of 1150 mm. It contains three comple mentary detectors of different types. The inner part adjacent to the interaction axis is occupied by the pre cision pixel and microstrip silicon detectors. In the barrel section, they are fixed in place on cylinders with axes aligned with the beam direction; at the end caps, they are mounted on disks, the planes of which are perpendicular to the beam. The detector design has been selected so that all particles escaping from the region of proton collisions intersect the planes of the detector at nearly right angles. The pixel and microstrip silicon detectors consti tute a discrete tracking system of the Inner Detector. It is complemented by a “continuous” tracking system, which is composed of closely spaced thinwalled drift tubes (or straws) 4 mm in diameter and allows detec tion of up to 36 coordinates of a particle crossing the straws. The spacing between the straws is filled with highly structured plastic materials, in which transition radiation is generated by charged particles as they pass through this combination of layers. Transition radia tion photons, as well as the signals from ionization losses, are detected by the straws; for this reason, the tracking system is called the Transition Radiation Tracker. Highefficiency transition radiation detec tion provides a means for separating the hadron and electron tracks. In the TRT barrel, the straws are 144 cm long and are aligned with the detector axis. At the center of each straw, there is a separator on its anode wire, which allows the signal to be read out from onehalf of the straw. Only the (R–ϕ) track coordinate can be deter mined in the TRT barrel, where ϕ is the azimuthal angle in the (x, y) plane perpendicular to axis z, and R

is the radius in this plane with reference to the detector axis. In the TRT endcap regions, the straws are 37 cm long and are radially located in the space between radii R = 644 and 1004 cm. The intrinsic coordinate resolu tion of each straw is 130 μm. The total number of read out channels for the detector signals is 350 848. The arrangement of the Inner Detector elements is shown in Fig. 1 [1]. The combination of the precision detectors at small radii and the straw tubes at large radii ensures reliable reconstruction of tracks and precision measurements of coordinates (R–ϕ) and z. The straw tube signals provide a means for substantially raising the coordi nate accuracy in the outer region of the Inner Detec tor. The worse spatial accuracy of the straws relative to the precision detectors is compensated for by their large quantity and the significant increase in the mea surable track length. The possibility of reconstructing closely spaced secondary vertices from decays of heavy particles is ensured primarily by the most interior layer of the pixel detectors located on the cylinder with a radius of 5 cm. The material budget in the TRT in terms of radia tion lengths Х0, which a particle travels after being emitted from the region of beam collisions at angles corresponding to pseudorapidities η = 0 and η = ±1.8, is estimated to be 0.264Х0 and 0.219Х0, respectively. 3. TRT SENSOR ELEMENT The active element (or sensor) of the TRT is a thin walled straw tube 4 mm in diameter. The design of the straw has been developed so that its walls have the minimum thickness while ensuring sufficient mechanical strength and electric conductivity [4]. A polyimide Dupond film 25 μm thick was the starting material for the straw walls. Three layers of coatings were applied to the film: one side of the film was coated with a 0.2μmthick aluminum layer and a 5– 6 μm thick protective coating of graphite mixed with polyimide, and the other side had a 5μmthick poly urethane coating. To form a tube, two film tapes were joined so as to match their polyurethanecoated sur faces and then were sintered at a temperature of 200– 250°C. The structure of the straw wall and the process of its production are shown in Figs. 2a and 2b [4]. To additionally enhance the mechanical strength, four blackreinforced plastic fibers were glued to the straw from the outside at equal angular distances. The fibers contain 1000 8μmdiameter threads and are aligned with the straw axis (Fig. 2c) [4]. The electric resistance of the straw walls that act as a cathode is 2 GeV is capable of generating 7–10 highthreshold signals on average due to transition radiation. The TRT barrel [7] has three ring layers of modules with 32 modules in each layer. Along the edges, they are supported by a frame that acts as the key element of the supporting structure of the Inner Detector bar rel. Each module has a blackreinforced plastic shell. The inner volume of the module is filled with straws fixed in place in the matrix of 19μmdiameter polypropylene fibers used to generate transition radia tion. The straws form an axially uniform space with an average gap of ~7 mm. To reduce the volume of the dead region for tracks with large transverse momenta, the shells of the modules have not been disposed in the projective geometry. The main parameters of the barrel are presented in Table 1. The tolerances for the mechanical constructions are determined by the intrinsic resolution of the straws in the (R–ϕ) plane, which is 130 μm and is based on the assumption that the position of the wires is known with an accuracy of ±50 μm. The shell of the barrel module, made from blackreinforced plastic with a high thermal conductivity and a thickness of 400 μm, has a flatness of 250 μm or less. According to the mea surements, this value corresponds to the maximum deformation of 99% of the active channels.

tions of the particle bunches and is used as a time stamp of the signal generated in the readout electron ics with a low noise level;

6. ELECTRONIC SYSTEM OF SIGNAL READOUT AND TRANSMISSION The TRT readout electronics has the following fea tures in common with the other subsystems of the Inner Detector: —obtaining of the timing signal with a frequency of 40.08 MHz, which is synchronized with intersec INSTRUMENTS AND EXPERIMENTAL TECHNIQUES

Fig. 8. TRT barrel and systems of the microstrip silicon detectors of the ATLAS setup in assembling process. Vol. 55

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Fig. 9. TRT end cap assembled. From left to right inside the end cap, there are 12 type A modules and 8 type B modules, all sur rounded by the service and mechanical support systems.

Highvoltage lines Eyelet Tapered crimping pin Frontend board

Gas fitting Capacitor assembly

Tension board Activegas manifold Highvoltage board Highvoltage lines Potting glue

Radiator

CO2 purging gas volume

Endplug/wire support Carbonfiber shell

Fig. 10. Design of the edge of the barrel module. The joint between the straw ends to the highvoltage board, the connection of the wires to the front panel via the tension boards, and the gas input into individual straws via the activegas manifold are shown.

—generation of the signal and its storing in a binary or digital buffer for a time of ~3.2 μs, which is comparable to the 2.5μs delay time of firstlevel trig ger L1; —depending on the signal of trigger L1, the next operation is the transmission of the buffer contents associated with a particular intersection of particle bunches or with a few intersections to the TRT read out driver (ROD). The data are read out into the communication line under control of the L1 trigger signal transmitted from the calorimeters or the muon detectors via the central trigger processor. The Inner Detector has not been

included in the system of trigger L1. The supply volt age is applied to the detector sensors and the readout electronics from external sources. Two types of specialized ASIC chips are used to process the analog signals and the threshold discrimi nators for detection of relativistic particles and transi tion radiation, as well as to perform subsequent time todigital conversion and signal transportation. Figure 13 [1] shows the TRT signal at each step of the readout sequence. The signal readout sequence includes: —an analog eightchannel ASIC chip [12], which is called ASDBLR and produced in the biCMOS

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—the next 16channel ASIC produced in the com mercial radiationhard 0.25μm CMOS technology [13]; the chip performs measurements of the drift time with a step of ~3 ns and includes a digital transmission line for holding the data for the delay time of trigger L1, a buffer, and a 40Mbit/s serial interface; in addi tion, the chip contains the interface for the timing sig nal, the trigger, and the control signal, as well as the digitaltoanalog converter for setting the discrimina tor thresholds of the analog ASIC and a control system for generation of the analog signals for entering the analog ASIC. Some operating parameters of the TRT are pre sented in Table 2. Both ASIC chips reside on the front panel con nected to the TRT. There exist 12 different panels for the TRT barrel and 3 panels for the TRT endcap sec tion. The electronics is cooled by the system based on monophase C6F14 liquid.

Fig. 11. Photos of the end cap module with four planes being assembled. One can see the inner and outer carbon rings, the first straw layer, and the first polypropylene block beneath them, as well as the plastic bushes used to position and fix the straws in place on the outer carbon ring.

radiationhard DMILL technology; the chip performs amplification, shaping, and baseline restoration of the signal and comprises two discriminators, one of which is operative at a low threshold (~250 eV) for detecting signals from minimum ionizing particles, and the other is operative at a high threshold (~6 keV) for tran sition radiation detection;

In the course of operation, the signals with the low threshold are used to reconstruct tracks. This corre sponds to a ~15% fraction of the mean signal expected from relativistic particles. The mean level of the counting rate due to the straw noise is ~2%. In this case, a small fraction (1%) of the channels has a noise rate of >10%, which is nevertheless small relative to the expected maximum counting rate of the straws (40%). The entire sequence of the readout electronics was tested using a neutron flux with a fluence of ~4 × 1014 cm–2 and a photon flux with a total dose of 80 kGy. As a result, the gain of the ASDBLR chip changed by ~25%; however, after the standard voltage compensation procedure, no changes were observed for the thresholds and the noise level.

Glass fiber plate Outer activegas manifold Crimping pin

End plugs

Inner activegas manifold

Radiator foil in the cooling CO2 gas

Rigid flexible printed circuit board

End pins

Carbonfiber rings Connector to frontend electronics board

Fig. 12. Construction of the edges of the inner and outer endcap modules, which shows the plastic bushes used to position and fix the straws in place on the inner and outer carbon rings, the crimping pins holding and positioning the anode wires, the inner and outer activegas manifolds, and the rigid–flexible printed circuit board used for highvoltage connection of the straws and the wire with the readout electronics. INSTRUMENTS AND EXPERIMENTAL TECHNIQUES

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Tale 2. Operating parameters of the TRT High voltage: maximum voltage

2000 V

operating voltage

1530 V

Maximum current

3 mA

Segmentation

One for ~200 straws 1984 channels

Voltage of the electronic system (low voltage): analog voltage

±3 V

digital voltage

2.5 V

Current

6.5 kA

Segmentation

1/32

Power supply: readout electronics

22 kW

cables and regulators

~23 kW

total power (initial stage of op eration)

~44 kW

7. TRT MEASUREMENTS OF TRANSITION RADIATION Readiness of the TRT for operation as a part of the ATLAS setup was demonstrated on the first LHC beams and in measurements of cosmic particles pene trating into the detector, which was located in the

underground mine, through a ~100mthick layer of soil and concrete constructions. The first physical results of the ATLAS experiment were obtained using the TRT. The probabilities of gen erating transition radiation photons were measured based on tracks of cosmic muons with different momenta. The results of these measurements are pre sented in Fig. 14 [14]. The probability that a signal with the high threshold corresponding to detection of a transition radiation photon will be produced is shown in this figure versus the Lorentz factor of muons. The experimental data are presented with dots. The fit of the experimental curve for muons is shown in Fig. 14 with a solid line. The cosmic muon measurements were carried out by the TRT barrel since its configuration is optimal for measuring the vertical flux of cosmic muons. The results of cosmic muon measurements comply with the data obtained in 2008–2010. The results of measurements on the accelerator beams agree with the results of the cosmic muon measurements at the ATLAS setup until the Lorentz factor reaches a value of ~103, i.e., in the whole range of the detector sensi tivity sufficient for the efficient electron identification. The results are in agreement both for the barrel and for the end caps, though the radiators in them differ in the structure. 8. CONCLUSIONS The largescale combined tracking detector of transition radiation has been developed and produced by the TRT international collaboration of the ATLAS experiment. This detector is a part of the ATLAS setup

Input signal with the “ion tail”

Particle track

Preamplifier

Gain ~2×104

Output signal after amplification and shaping 20 ns/division

Baseline restorer

Straw

Shaper tail cancellation

10 ns/division

Low threshold (tracking) High threshold (transition radiation)

Negative high voltage

Doublethreshold discriminator

20 ns/division

Fig. 13. Circuit diagram of the TRT readout electronics. The waveforms of the input signal are shown, as well as the waveforms of the signals after their amplification and shaping, the baseline restoration and doublethreshold discrimination providing the output signal corresponding either to the low threshold or to both the low and high thresholds. INSTRUMENTS AND EXPERIMENTAL TECHNIQUES

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Probability of triggerin by the high threshold

10

Muon momentum, GeV 10 102 103 1

0.16 0.12

Data on cosmic rays of 2008

0.08 0.04 0

101

102

103

104 γ factor

Fig. 14. Probability that a transition radiation photon will be produced in the ATLAS TRT vs. the Lorentz factor of the cosmic muon: the experimental data are shown with dots, and the fit is presented with a solid line.

on the LHC and acts as the element of the ATLAS Inner Detector. The TRT has been designed to mea sure charged particle tracks and identify electrons pro duced in the central region of interactions both of pro tons and heavy ions with teraelectronvolt energies at an accelerator luminosity of >2 × 1034 (cm–2 s–1). The large size is the distinctive feature of the TRT: the detector length is 5.5 m, and its outer diameter is 2.2 m. The sensor elements of the detector are 298304 thin drift tubes. The number of electronic channels is 350848. The TRT barrel section was manufactured in the United States, while the end cap modules and the mechanical constructions were produced in Russia. The check measurements performed after the TRT was assembled and integrated into the ATLAS setup have demonstrated that >99% of all electronic chan nels are in the operative state. The TRT has been commissioned, and, today, it takes part in the ATLAS experiment. The first physical results have been obtained for the probability that transition radiation photons will be produced in the detector by cosmic muons. These results comply with the data of the tests on the accelerator. The investigations conducted in the course of TRT development have shown that it will be capable of operating under hard experimental conditions without access to the detector for more than 10 years. ACKNOWLEDGMENTS This work was performed within the ATLAS col laboration. We are grateful to the collaborators from the other institutes participating in the ATLAS exper iment: Faculty of Sciences, Department of Physics, Bogazici University, Istanbul, Turkey; Brookhaven National Laboratory, Physics Department, Upton, United States of America; CERN, Geneva, Switzer INSTRUMENTS AND EXPERIMENTAL TECHNIQUES

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land; Niels Bohr Institute, University of Copenhagen, Kobenhavn, Denmark; Faculty of Physics and Applied Computer Science of the AGHUniversity of Science and Technology, Krakow, Poland; The Hen ryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Science, Krakow, Poland; Duke University, Department of Physics, Durham, United States of America; Universite de Geneve, Section de Physique, Geneva, Switzerland; Laboratoire de Phy sique Subatomique et de Cosmologie, CNRSIN2P3, Universite Joseph Fourier, Grenoble Cedex, France; Hampton University, Department of Physics, Hamp ton, United States of America; Indiana University, Department of Physics, Bloomington, United States of America; Lunds Universitet, Fysiska Institutionen, Lund, Sweden; INFN Milano and Universita di Mil ano, Dipartimento di Fisica, Milano, Italy; Max PlanckInstitut fuer Physik, Muenchen, Germany; LAL, University of ParisSud, Orsay, France; Depart ment of Physics, University of Oslo, Oslo, Norway; University of Pennsylvania, Department of Physics & Astronomy, Philadelphia, United States of America; University of Pittsburgh, Department of Physics and Astronomy, Pittsburgh, United States of America; Instituto de Fisica Corpuscular, Centro Mixto UVEGCSIC, Valencia, Spain; Departamento de Fisica Atomica, Mol. y Nuclear, University of Valen cia and Instituto de Microelectronica de Barcelona, Barcelona, Spain; University of British Columbia, Department of Physics, Vancouver, Canada; and Yale University, Department of Physics, New Haven CT, United States of America. The works presented in this publication were sup ported in part by the European Union (DGXII), the International Science Foundation, the Danish Natu ral Science Research Council, the Swedish Research Council, the Knut and Alice Wallenberg Foundation, the Polish Ministry of Science and Higher Education, the International Science and Technology Center, the United States Civilian Research and Development Foundation, the United States Department of Energy, the United States National Science Foundation, the Natural Sciences and Engineering Research Council of Canada, the Ministry of Education and Science of the Russian Federation, the President Program of Support for Leading Scientific Schools of Russia (grants NSh4142.2010.2, NSh3489.2008.2, and NSh4404.2010.2), the International Association for the Promotion of Cooperation with Scientists from the Independent States of the Former Soviet Union, and the Turkish Atomic Energy Authority. REFERENCES 1. Aad, G., Abat, E., Abdallah, J., et al., J. Instr. (JINST), 2008, vol. 3, p. S08003. 2. Dolgoshein, B., Proc. ECFA Study Week on Instrumen tation Technology for HighLuminosity Hadron Collid ers, Barcelona, 1989; Geneva: CERN 8910/ECFA 89 Vol. 55

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