The ALICE electromagnetic calorimeter

ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 617 (2010) 5–8

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Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima

The ALICE electromagnetic calorimeter Terry C. Awes Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

For the ALICE Collaboration a r t i c l e in fo

abstract

Available online 7 October 2009

ALICE is the general purpose experiment at the LHC dedicated to the study of heavy-ion collisions. The electromagnetic calorimeter (EMCal) is a late addition to the ALICE suite of detectors with first modules installed in ALICE this year. The EMCal is designed to trigger on high energy gamma-rays and jets, and to enhance the capabilities of ALICE for these measurements. The EMCal is a Pb/scintillator sampling shish-kebab type calorimeter. The EMCal construction, readout, and performance in beam tests at the CERN SPS and PS are described. Published by Elsevier B.V.

Keywords: Electromagnetic calorimeters Heavy-ion collisions ALICE experiment

1. Introduction The ALICE [1] experiment at the LHC has been designed to carry out comprehensive measurements of nucleus–nucleus collisions with the goal to study matter under extreme conditions. In particular, a major goal is to explore the phase transition between confined matter and QCD matter [2,3]. For this purpose, ALICE contains a wide variety of detector systems for measurements of hadrons, leptons, and photons in the high multiplicity environment of heavy-ion collisions. The EMCal is a large acceptance electromagnetic calorimeter that complements the high resolution, but limited acceptance, PHOS electromagnetic calorimeter of ALICE. With primary goal to improve the capabilities of the ALICE experiment for jet measurements, the main EMCal design criterion was to provide as much electromagnetic calorimetry coverage as possible within the constraints of the existing ALICE detector pffiffiffi systems, with moderate energy resolution (s=E  15%= E). The space available to the EMCal was limited to a cylindrical volume of 110 cm radial depth between the ALICE space-frame, supporting the ALICE inner detectors, including the TPC, and the ALICE solenoidal coils and magnet at the outer radius. Due to the presence of the PHOS electromagnetic calorimeter below the TPC, and the High Momentum Particle Identification Detector (HMPID) RICH detector, the EMCal was limited to about 1103 of azimuthal coverage. The EMCal is a Pb/scintillator sampling calorimeter of shishkebab design, with wavelength-shifting (WLS) fiber readout [4]. The physical parameters of the EMCal are summarized in Table 1. The EMCal is constructed of 3072 identical individual modules.

E-mail address: [email protected] 0168-9002/$ - see front matter Published by Elsevier B.V. doi:10.1016/j.nima.2009.10.010

Each module consists of 77 layers of Pb and scintillator with the scintillators of each layer composed of four individual optically isolated tiles, to provide four independent readout towers per module. Twelve modules are attached to a strongback to form a strip module, with 24 strip modules inserted into a crate to form a supermodule. A full super module subtends 203 in f and 0.7 in Z. Two supermodules installed end-to-end provide the full 0:7 o Z o 0:7 acceptance of the ALICE TPC. The modules have a 1:53 taper in the Z direction to provide an approximately projective geometry in Z.

2. EMCal readout and trigger The light collected by the WLS fiber of each tower is read out with an avalanche photodiode (APD) and charge sensitive preamplifier located within the strongback of the strip module. Due to radial space limitations the APD signals are routed to the Front End Electronics (FEE) crates located at the large Z end of each supermodule. The EMCal readout and trigger electronics follow closely those developed for the ALICE PHOS detector [5–9], which in turn make use of elements of the ALICE TPC readout electronics [10]. The EMCal readout is shown schematically in Fig. 1. Similar to a PHOS FEE card [5,6] a single EMCal FEE card provides readout of 32 EMCal towers. The signals are shaped with 100 ns shaping time (cf. 1 ms for PHOS) in dual shaper channels differing by a factor of 16 in gain, and sampled at 10 MHz with the 10-bit ALICE TPC Readout (ALTRO) chip [10] for 14-bit effective dynamic range. The APDs are operated at a nominal gain of M ¼ 30 which gives a full scale range of 250 GeV, with a least significant bit of 16 MeV. The FEE also provides individual bias control between 210 and 415 V for each of the 32 APD readout by the FEE. The setup,

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control, and readout of the FEE cards occurs over a custom GTL bus, developed for the ALICE TPC. The Readout and Control Unit (RCU) provides control of the GTL bus and carries a Detector Control System (DCS) daughter card which provides control via a linux operating system by ethernet connection, and provides the interface to the LHC trigger and timing control system. The RCU carries a standard ALICE Detector Data Link (DDL) daughter card for transmission of data via optical fiber to the ALICE DAQ and high level trigger. The EMCal provides input to the Level 0 (L0) and Level 1 (L1) trigger decisions in ALICE. The signals of 2  2 groups of towers (corresponding to one EMCal module) are analog summed in the FEE boards and transmitted to a local Trigger Region Unit (TRU) [7,8] board where the 2  2 tower sums of 12 FEE cards (96 2  2 sums) are digitized at the LHC clock frequency of 40 MHz. The digitized 2  2 tower sums are summed over time samples with pre-sample pedestal subtraction to provide an integral energy measurement. Finally, overlapping 4  4 tower digital sums are formed within each TRU. Each 4  4 sum is then compared against a threshold to provide a L0 trigger input to indicate a shower in the EMCal. The L0 trigger inputs from each of the TRUs is passed to a Summary Trigger Unit (STU) [11] which performs the logical OR of the L0 outputs from all TRUs to provide a single EMCal L0 input to the ALICE Central Trigger Processor (CTP). Upon receipt of an accepted L0 trigger from the CTP, the digitized time-summed 2  2 tower sums from each TRU are passed to the STU over two LVDS serial data lines operating at 400 Mbits/s. In the STU the 4  4 overlapping tower sums are formed again, but across TRU boundaries over the full EMCal acceptance to provide an L1 high energy shower trigger. At the same time, sums over large N  N overlapping tower regions are

Table 1 The EMCal physical parameters. Quantity Tower size (at Z ¼ 0) Tower size Sampling ratio Number of layers Effective radiation length X0 Effective moliere radius RM Effective density Sampling fraction Number of radiation lengths Number of towers Number of modules Number of super modules Weight of super module Total coverage

Value  6:0   6:0  24:6 cm3

Df  DZ ¼ 0:0143  0:0143 1.44 mm Pb/1.76 mm scint. 77 12.3 mm 3.20 cm 5.68 g=cm3 10.5 20.1 12,288 3072 10 full size, 2 13 size  7:7 metric tons (full size) Df ¼ 1073 ; 0:7o Z o 0:7

also formed to provide an L1 jet trigger. The STU simultaneously receives event multiplicity information from the ALICE V0 detector which can be used to provide multiplicity dependent (impact parameter dependent) thresholds for the L1 singleshower and jet energy sum triggers. Since the EMCal will be operated at ambient temperature inside the ALICE magnet, and due to the known temperature dependence of the APD gains (1:7%=3 C at M ¼ 30), the EMCal temperature is monitored across the supermodule. In addition, the time dependence of the APD gain is monitored using a custom built LED system. The light from a single ultra-bright blue LED is transmitted to an EMCal strip module via a 3 mm optical fiber. At the strip module it is split into 12 500 mm fibers that bring the light to a hole between the four towers at the back of each module. A small diffuser reflects the LED light back up into the WLS of each tower. The LED is triggered by an avalanche pulser system to provide an intense pulse of several ns duration [5]. The 24 LEDs are themselves monitored by 24 unity-gain photodiodes that are read out with an extra FEE card. The LED system is located between FEE crates at the end of the supermodule.

3. EMCal test beam performance During a period of five weeks in the autumn of 2007, pre-production EMCal modules were tested in the H6 beam line at the SPS, and at the T10 beam line at the PS. The purpose of the beam tests was to validate the design of the EMCal module and readout electronics; to determine the momentum dependence of the energy and position resolution; to investigate the uniformity of the response across the EMCal module and across module boundaries, and as a function of angle of incidence; to investigate the EMCal capabilities to discriminate between electromagnetic and hadronic showers; and to validate the planned initial EMCal calibration procedure based on measurements with cosmics. Preliminary versions of the EMCal modules and electronics were tested at the Meson Test Facility at Fermilab in autumn 2005. The tests were performed on an array of 4  4 modules (8  8 towers) assembled with the same components and structure as used for the production of EMCal modules installed in ALICE. The modules were read out with pre-production versions of the FEE boards. The readout, including the GTL bus and RCU þ DCSþ DDL link, used final components. The ALICE DATE [12] data acquisition system was used to record the EMCal data. A prototype of the LED gain-monitoring system was used to track the variation of the tower gains, and correlate with temperature measurements. A set of three Multi-Wire Proportional Chambers, each with x2y position readout, were located upstream of the EMCal to provide

Fig. 1. The EMCal readout electronics organization.

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Fig. 2. The EMCal energy resolution. The solid line is a fit to the data. The dashed line is the prediction of a GEANT3 simulation.

measurement of the incident position of each beam trigger particle to better than 1 mm resolution. In the H6 beam line of the SPS the EMCal modules were scanned with electron and hadron beams of 5–100 GeV/c at trigger rates up to 1500 triggers per 4 s spill, depending on momenta. In the H6 beam line electron beams of better than 99% purity are obtained by converting photons from secondary neutral pions, and hadron beams are produced by use of appropriate Pb absorbers after the production target and momentum selection. At the T10 beam line of the PS the EMCal was scanned with mixed beams of electrons and hadrons of 0.5–6.5 GeV/c. At the PS electrons were identified by a threshold Cherenkov counter. The EMCal tower gains were initially matched at the SPS by scanning the 80 GeV/c electron beam across all towers. The tower gains were checked again at the PS with 3 GeV/c electron scanned across all towers. During the test beam periods the temperature in the test area varied by as much as 3 3 C and the associated gain variations were monitored and corrected for based on the LED calibration system. The energy resolution of the EMCal is shown in Fig. 2 and can be parameterized as

sðEÞ E

11:17 0:4 ð%Þ ¼ ð1:7 7 0:3Þ  pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi : EðGeVÞ

ð1Þ

The measured energy resolution is slightly worse than preliminary GEANT3 simulations, also shown. However, the simulations shown do not yet include the effects of light collection and attenuation in the WLS fibers. The measured dependence of the EMCal position resolution on incident momenta is shown in Fig. 3. The position resolution can be parameterized as

sðxÞ x

5:3 ðmmÞ ¼ 1:5  pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi : EDeposit ðGeVÞ

ð2Þ

The x and y positions are calculated based on the energy deposit in the individual towers of the shower cluster based on a logarithmic weighting of the tower energies [13].

4. EMCal initial calibration Since it is not practical to calibrate all EMCal electrons in a test beam, each supermodule is calibrated using the peak of the energy deposit cosmic muons traversing the calorimeter. The goal

towers with individually spectrum of of the initial

7

Fig. 3. The EMCal position resolution from measurements performed at Fermilab.

calibration with cosmics is to obtain an initial tower energy calibration with rms variation of the tower-to-tower gain variation of less than 10%. This is required so that the tower signals summed in the EMCal triggers will all be on the same relative energy scale, to within this variation. After the EMCal supermodules are assembled, cabled, and instrumented with readout electronics, they are tested with the LED system to insure that all 1152 readout channels of the supermodule are functioning properly. Prior to installation in the supermodule, all APD+preamplifiers are tested in a temperature controlled test station built for that purpose. All APDs have their gain vs voltage dependence measured, and a subset of APDs also have their gain vs temperature dependence measured. For initial operation in the supermodule each APD has its individual bias voltage set to the voltage corresponding to gain M ¼ 30 at 25 3 C. Cosmics data are accumulated in an overnight run with cosmics particles triggered externally by an array of scintillators. The scintillators are deployed with a scintillator slat above and below each EMCal strip in the supermodule, with each scintillator readout by a photomultiplier at each end. Each scintillator pair is operated in coincidence, and a set of eight scintillator pairs is deployed together and logically ORed to provide cosmics triggers for one-third of the supermodule. The time relative to the trigger is recorded for all photomultipliers. The scintillators are moved to calibrate the different regions of the supermodule. The scintillator slat pair coincidence selects cosmic particles that pass completely through the associated strip module. In addition, the time difference between photomultiplier time measurements provides a position measurement along the slat with a resolution of about one EMCal module. Further offline analysis cuts to require no signal in the surrounding towers allows to accumulate an energy spectrum for cosmics passing roughly perpendicularly through each tower. Fig. 4 shows the distribution of the measured peak positions in the energy deposit spectrum of cosmic muons for each tower, after application of the tower isolation cuts. The light solid curve shows the distribution at the initial M ¼ 30 gains. It gives an indication of the uniformity of the light collection/MeV of the modules tested. After the cosmic data is taken, the measured cosmic peaks are used to adjust the APD biases to compensate for differences in the light collection. The heavy solid histogram shows the distribution of cosmic peaks after two iterations, where it is seen that the cosmic peaks have an rms variation of less than 3%.

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higher pT , complementing the better low pT performance of the ALICE PHOS electromagnetic calorimeter. The EMCal will also improve the capabilities of ALICE for jet measurements by measuring the neutral energy component of jets, and allowing to trigger on jets. The EMCal will provide trigger input at L0 to indicate energy deposit in the EMCal, and at L1 to trigger on high energy showers (photons, p0 ’s, and electrons) and on jets. Test beam measurements pffiffiffi have demonstrated an energy resolution of sðEÞ=Eð%Þ ¼ 11:1= E  1:7. Prior to installation in ALICE the EMCal towers are calibrated with cosmics muons with a towerby-tower gain variation of about 3%. Final EMCal calibrations will be based on p0 mass measurements. As of this time, four of 10 (plus two-third) supermodules have been installed in ALICE.

References Fig. 4. The distribution of peak positions of cosmic muons in the EMCal at initial M ¼ 30 gains (light solid curve) and after 2-iterations (heavy solid curve) with Gaussian fit (dashed curve).

The EMCal absolute and tower-by-tower energy calibration will be improved after installation in ALICE based on measurements of the p0 mass peak in the two-gamma invariant mass spectrum, where one of the showers is centered on the tower to be calibrated.

5. Conclusion A large acceptance sampling electromagnetic calorimeter, EMCal, is being added to the ALICE suite of detectors. By virtue of its significantly larger acceptance, the EMCal will extend the capabilities of ALICE for photon, p0 , and electron measurements to

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