In the ALICE experiment at Cern LHC, a set of hadron calorimeters will be used to determine the centrality of the Pb-Pb collision. The spectator protons and neutrons, will be separated from the ion beams, using the separator magnet (D1) of the LHC beam optics and respectively detected by a proton (ZP) and a neutron (ZN) "Zero-degree Calorimeter" (ZDC). The detectors will be placed in front of the separator D2 magnet, 115 meters away from the beam intersection point. The ZDCs are quartz-fiber spaghetti calorimeters that exploit the Cherenkov light produced by the shower particles in silica optical fibers.This technique offers the advantages of high radiation hardness (up to several Grad), fast response and reduced lateral dimension of the detectable shower. In addition, quartz-fiber calorimeters are intrinsically insensitive to radio-activation background, which produces particles below the Cherenkov threshold.The ALICE ZDC should have an energy resolution comparable with the intrinsic energy fluctuations, which range from about 20 0.000000or central events to about 5 0.000000or peripheral ones, according to simulations that use HIJING as event generator. The fiber-to-absorber filling ratio must be chosen as a good compromise between the required energy resolution and the fiber cost.The design of the proposed calorimeter will be discussed, together with the expected performances. Whenever possible, the simulated results will be compared with the experimental ones, obtained with the built prototypes and with the NA50 ZDC, which can be considered as a working prototype for the ALICE neutron calorimeter

Arnaldi, R

Chiavassa, E

Cicalò, C

Cortese, P

De Falco, A

De Marco, N

Dellacasa, G

Ferretti, A

Gallio, M

Macciotta, P

Masoni, A

Mauro, S

Mereu, P

Musso, A

Oppedisano, C

Piccotti, A

Puddu, G

Randaccio, P

Scalas, E

Scomparin, E

Serci, S

Siddi, E

Soave, C

Usai, G L

Vercellin, Ermanno

CERN Document Server

THE ALICE ZERO DEGREE CALORIMETERSR. ARNALDI1, E. CHIAVASSA1, C. CICALO2, P. CORTESE1, A. DE FALCO2,G. DELLACASA3, N. DE MARCO1, A. FERRETTI1, M. GALLIO1,P. MACCIOTTA2, A. MASONI2, A. MUSSO1, C. OPPEDISANO1,A. PICCOTTI1, G. PUDDU2, E. SCALAS3, E. SCOMPARIN1, S. SERCI2,E. SIDDI2, G. USAI2AND E. VERCELLIN1FOR THE ALICE COLLABORATION1Universita and I.N.F.N. Sezione di Torino, via P. Giuria 1, 10125 Torino, Italy.2Universita and I.N.F.N. Sezione di Cagliari, Cittadella Universitaria diMonserrato, 09042 Monserrato (Ca) , Italy.3Universita del Piemonte Orientale, Alessandria and I.N.F.N. Sezione di Torino,ItalyIn the Alice heavy-ion experiment at LHC a set of Zero Degree Calorimeters (ZDC)has been proposed to determine the centrality of the nucleus-nucleus collisions,measuring the energy carried away by the non-interacting nucleons. A series ofprototypes have been built and tested; the preliminary results concerning the lin-earity, the resolution, the uniformity of the response as a function of the beamimpact point are shown1 IntroductionThe ALICE experiment1is dedicated to the study of ultrarelativistic heavyion collisions where nuclear matter at high temperatures and energy densi-ties can be produced. ALICE is planned to start taking data in 2005, at theLHC facility at CERN, with two colliding lead ion beams at a c.m. energy ofps = 5:5 TeV per nucleon.In heavy-ion collisions, the event by event determination of the centralityplays a basic role; it is used at the trigger level to enhance the sample ofcentral collisions, and, more generally, to estimate the energy density reachedin the interaction. The energy EScarried away by the non-interacting nu-cleons (spectators)is the measurable quantity most directly related with thecentrality of the collision.2 The ZDCs at the LHC colliderAt an ion collider experiment, like ALICE, the design of devices for the detec-tion of spectator nucleons diers signicantly with respect to a xed targetone. As can be seen in Fig. 1, where a conceptual sketch of the LHC beam lineclose to the ALICE intersection point (IP) is presented, the colliding beamstext5: submitted to World Scientic on September 1, 1999 1are deected by means of the separation dipole D1 at about 50 m from theIP in order to reach the nominal distance (188 mm for the LHC2) betweenthe circulating beams. The D1 magnet will also deect the spectator protons,separating them from the spectator neutrons, which y away at zero degrees.It is therefore conceptually possible to place in front of the dipole D2, at 115 m from the intersection point, on the two sides of the IP, a set of twocalorimeters : one of them, positioned between the two beam pipes, to in-tercept the spectator neutrons, and the other one, external to the outgoingbeam, to collect the spectator protons5.Figure 1. Line sketch from the intersection point IP2 to D2 magnet.The transverse available space between the outgoing and incoming beampipes, where the neutron calorimeter (ZN) should be placed, is less than9 cm. However, the size of the spectator neutron spot on the front face of theneutron calorimeter, essentially due to the Fermi momentum distribution, isquite small (0:6 0:6 cm2at 1  level), and we expect no losses of neutronsalong the beam line. On the contrary the spatial distribution of the spectatorprotons on a plane normal to the LHC beams axis, located at 115 m fromthe intersection point, has been found to depend strongly on both the Fermimomentum of the nucleons and on the parameters of the LHC optics. Theacceptance of the proton calorimeter (ZP) closely depends on the apertureof the separation magnet D1. Assuming an inner diameter of 73 mm for thebeam pipe inside the coils of D1, a Monte Carlo simulation shows that 13%of the protons interact with the beam pipe at the exit of D1 before reachingthe ZDCs.3 Detection techniqueThe design of ZDCs in the LHC environment has to satisfy various technicalissues. As we have seen, the horizontal transverse dimension of the neutrondevice in any case must not exceed the distance between the two beam pipes intext5: submitted to World Scientic on September 1, 1999 2that region ( 8 cm), meaning that an extremely compact detector is needed.Furthermore, the ZDCs, even if they do not intercept the beams as at xedtarget experiments, will operate at a high radiation level; the deposited dose inthe neutron calorimeter is estimated to be  1 Mrad/day, at a LHC luminosityL=1027cm 2s 1.Because of these constraints, we choose as detection technique for theALICE ZDCs the quartz bre calorimetry, which allows to build compactand radiation hard devices. After an exploratory R&D phase3, quartz -bre calorimetry has been successfully used for the ZDC of the NA50 experi-ment4, where a device made of quartz bres embedded in a tantalum matrix,of dimensions 5565 cm3, has worked in an environment 10 times morecompelling than the ALICE one in terms of radiation levels.In addition, quartz bre calorimeters are intrinsically insensitive to radio-activation background, which produces particles below the Cherenkov thresh-old; moreover, their response is very fast, the signal duration being almostcompletely dominated by the readout electronics and the cables.Small dimensions (8  8  100 cm3) and the use of a dense material,like tantalum, are required for the neutron calorimeter (ZN). The transversedimensions of the proton calorimeter (ZP) should be chosen in order to ensurethat most of the spectator protons are collected; the relative big dimensions(of the order of tens of cm) suggest the choice of brass as a matrix material.The bre-to-absorber lling ratio has to be chosen as a good compromisebetween the required energy resolution and the bre cost, after simulationsand experimental tests.4 The NA50 calorimeter as the ALICE ZN prototypeThe ZDC calorimeter, now in use in the NA50 experiment, can be consideredas the ALICE neutron calorimeter prototype. In fact, the NA50 calorime-ter has tantalum as the absorber material and has similar dimensions (5 5  65 cm3); the active part of the detector is made of 900 quartz breswith silica uorinated cladding, uniformly distributed with a pitch of 1.5 mmcorresponding to a quartz to tantalum volume ratio of 1/17. The bres areoriented at 0with respect to the beam direction. A complete description ofthe calorimeter together with the obtained results can be found in Ref. 4.A light yield of about 0.5 photoelectron per GeV was observed using aphotomultiplier with a bialcaline photocatode and a borosilicate window.The resolution of the calorimeter for the Pb ion beam of 158 AGeV is 6%,obtained with a gate width of 12 ns.The ZDC, tested with electron and proton beams, resulted highly nontext5: submitted to World Scientic on September 1, 1999 3compensated giving an e/p ratio of 2.4. However, this fact does not give anyproblem when the calorimeter is used as centrality detector, since it measuresthe number of residual nucleons after the interaction of a Pb nucleus with thetarget nuclei. In this case the ZDC detects mainly hadrons, all having thesame xed energy per nucleon as the beam.5 Proton calorimeter prototypesTwo ZP prototypes (named ZP2 and ZP7) have been constructed with dier-ent geometrical and mechanical characteristics, summarized in Table 1. Theyare both equipped with PMMA bres. In fact, the use of the more expen-sive quartz bres is not necessary for the prototype tests, since the radiationdamage will be negligible.The detector conguration is similar to the NA50 ZDC, with the bresparallel to the beam axis. The photomultipliers are connected to the bres insuch a way that each PMT collects the light from a subset of bres uniformlydistributed in the passive material (see Fig. 2); in this way, considering oneor more PMTs, we obtain dierent quartz/absorber volume ratio. The ZP7calorimeter is equipped with 2 Philips XP2020 PMTs, while the Cerenkovlight from the ZP2 is read by 4 XP2020 PMTs. The two calorimeters haveapproximately the same lling ratio (1/80).Table 1. Proton ZDC prototype parameters.ZP2 ZP7Absorber copper brassNumber of plates 40 26Plate thickness 4 mm 6 mmFibres material PMMA PMMAFibre diameter (m) 500 750Total number of bres 1600 676Fibre pitch 4 mm 6 mmPM used 4 2The performances of the two prototypes were studied at the H6 test beamof the CERN SPS with hadron beams of 50, 100, 120, 150 and 180 GeV/c,and with positron beams of 50, 75, 100, 120 and 150 GeV/c.The experimental setup, shown in Fig. 3, consists of a plastic scintillatorhodoscope, used as trigger system, two small and movable scintillator-sticksused to localize the beam position on the calorimeter surface with an accuracytext5: submitted to World Scientic on September 1, 1999 4PM3 PM4 PM3 PM4PM1 PM2 PM1 PM2PM3 PM4 PM3 PM4PM1 PM2 PM1 PM2ZP2PM1 PM2 PM1PM2 PM1 PM2PM1 PM2 PM1ZP7Figure 2. The connection of the bres to the photomultipliers for ZP2 and ZP7.of 1 mm in x and y direction, a MWPC with delay line readout, allowing a spa-tial resolution of 200 m, installed in front of the calorimeter. The calorimeteritself is placed on a movable platform; a couple of plastic scintillation countersbeyond an iron wall detects muons.S1S2S3S4S5D1VD2HMWPCZDC Iron wallMU1vMU2hFigure 3. Experimental setup of the test. S1, S2, S3, S4, S5: trigger scintillators. D1V,D2H: scintillator sticks. MU1v, MU2h: scintillators for muon detection.For both prototypes we measured the linearity of the detector response asa function of the energy, the energy resolution for dierent bre to absorberlling ratios and for dierent bre spacings, the shower transverse size andthe uniformity of the response as a function of the beam impact point on thefront face of the calorimeter.The bres connected to each PMT were uniformly distributed in the pas-sive matrix, so that each channel could detect the same amount of energy. Arst equalization of the PMT high voltages was done at the beginning of thetest with a hadron beam of 100 GeV/c. A ne tuning of the ADC signals fortext5: submitted to World Scientic on September 1, 1999 5each channel was made oine to compensate possible small dierences.0501001502002503000 50 100 150 200E(GeV)PhotoelectronsZP2 HadronsDATA:    phe = -9 + 0.878*E0501001502002503000 50 100 150 200E(GeV)PhotoelectronsZP7 HadronsDATA:    phe = -5 + 0.888*E0501001502002503000 50 100 150 200E(GeV)PhotoelectronsZP2 PositronsDATA:    phe = 1.197*E0501001502002503000 50 100 150 200E(GeV)PhotoelectronsZP7 PositronsDATA:    phe = 4 + 1.2*EFigure 4. Top: response as a function of the hadron beam energy E for the ZP2 (left) andZP7 (right) prototypes. Bottom: same for positron beams.The response as a function of the beam energy (Fig. 4) is expressed innumber of photoelectrons; the results show a good linearity. An energy thresh-old at few GeV can be noticed.In Fig. 5 the resolution is plotted for dierent lling ratios, obtainedselecting the signals from 1, 2 or 4 PMTs in ZP2 (the lling ratios are respec-tively 1/325, 1/162, 1/80), and 1 or 2 PMTs in ZP7 (with lling ratios equalto 1/170 and 1/80). The experimental points are tted with the functiona=pE(GeV). The determination of a (small) constant term doesn't appearreliable at these energies. However, we can exclude, with a condence level of99%, a value of the constant term greater than 10%.In order to estimate the hadronic shower transverse size, we studied theresponse of the calorimeter as a function of the horizontal beam impact co-text5: submitted to World Scientic on September 1, 1999 60102030405060700 0.05 0.1 0.15 0.21/√E(GeV)Resolution(%)Filling ratio1/3251/1621/800102030405060700 0.05 0.1 0.15 0.21/√E(GeV)Resolution(%)Filling ratio1/3251/1621/800102030405060700 0.05 0.1 0.15 0.21/√E(GeV)Resolution(%)Filling ratio1/1701/850102030405060700 0.05 0.1 0.15 0.21/√E(GeV)Resolution(%)Filling ratio1/1701/85Figure 5. Top: resolution ((E)E) as a function of 1=pE for the ZP2 prototype in hadrons(left) and positrons (right) considering dierent lling ratios. Bottom: same for ZP7.ordinate. The information about the beam position is given either by theMWPC or by the 1 1 mm2scintillators. Making the derivative of the curveof the response of the ZDC at the edges of the calorimeter, the hadron showertransverse size could be evaluated; we found a value of 2.2 cm (FWHM).The detector response for hadrons should not be sensitive to the beamimpact point with respect to the bre position. As a result of the test, wefound that the response for hadrons can already be considered uniform for alling ratio of 1/162,when the bre pitch is 4 mm, as in ZP2, approximatelyequal to half the radiation length.6 The proposed ALICE calorimetersThe neutron ZDC will be placed at 116.1 m from the IP. A device, made ofquartz bres embedded in a tantalum matrix, of dimensions 77100 cm3,allows to contain 80% of the shower generated by the spectator neutrons. Thetext5: submitted to World Scientic on September 1, 1999 7bres with a core diameter of 365 m will be oriented at 0with respect tothe beam axis. The foreseen quartz to absorber volume ratio will be 1/22.The expected resolution for a 2.7 TeV neutron is  10%.The proton ZDC will be placed at 115.6 m from the IP. To collect mostof the spectator protons after the y path along the magnetic line and toobtain a shower containment and an energy resolution similar to the one ofthe neutron ZDC, a device of 20.812150 cm3, centered at 19 cm from thebeam axis, made of quartz bres embedded in a brass matrix, will be used.The bres will have a core diameter of 550 m and will be oriented at 0withrespect to the beam axis. The foreseen quartz to brass volume ratio will be1/65.Two identical sets of calorimeters will be placed on both sides of the IPto improve the resolution of the impact parameter; moreover, the correlatedinformation will be used to remove the background from beam-gas interac-tions.7 ConclusionsWe have discussed the characteristics of the proposed ZDCs for the ALICEheavy-ion experiment at the LHC collider. The aim of these calorimeters isthe determination of the centrality of the Pb-Pb collision, through the mea-surement of the energies carried out by the spectator nucleons. The basic re-quirements of these calorimeters are the radiation hardness and the necessityof having a small shower transverse size. An extremely compact calorimeter,made of tantalum and quartz bres, is proposed to detect the spectator neu-trons, whilst the energy of the spectator protons will be measured by meansof a brass-quartz bre calorimeter. The resolution of both calorimeters is ex-pexted to be of the order of 10% for a 2.7 TeV spectator nucleon, accordingto simulation. The proposed design for the ALICE ZDCs should ensure anestimation of the impact parameter of the collision within an uncertainty of10% for all but the most central events.References1. "ALICE Technical Proposal" CERN/LHCC/95-712. "LHC Conceptual Design" CERN/AC/95-05 (LHC)3. P. Gorodetsky et al., NIM, A 361, (1995 1614. R. Arnaldi et al., NIM A 411 (1998) 15. "Zero Degree Calorimeter ZDC" CERN/LHCC/99-5text5: submitted to World Scientic on September 1, 1999 8

The ALICE Zero Degree Calorimeters

The ALICE Zero Degree Calorimeters

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