Vichou, I

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The ACCORDION Calorimetry for LHCIrene VichouLAL OrsayUniversite Paris-Sud91405 Orsay, FRANCERepresenting the LARG Group of the ATLAS CollaborationABSTRACTOne of the basic elements of the ATLAS experiment for LHC is very good calorimetry.The liquid ionization and the `Accordion' technique is an adopted choice. A review ofrecent progress on building and testing the performance of such calorimeters is presentedhere, covering the em barrel sector prototype {standalone and equipped with a separate orintegrated preshower{, as well as the em endcap and hadronic barrel prototypes.1 IntroductionThe ACCORDION Calorimetry, which is too often men-tioned in this paper, stems out of the idea of usingaccordion-shaped electrodes in liquid ionization calorime-ters. Such an approach seemed attractive for LHC anda large R&D activity was launched giving nice resultsin beam tests, which in turn were exploited to design aperforming em calorimeter system for the ATLAS ex-periment.At LHC calorimetry is a critical issue due to strin-gent requirements dictated either by physics performanceor by dicult experimental conditions [1]. Concern-ing physics, not only a very good energy resolution isneeded, but also good position measurement and abilityto cover a wide dynamic range and geometrical accep-tance. On top of that LHC will oer a very high lu-minosity of 1034cm 2sec 1, implying a big amount ofpile-up (23 soft interactions per event) and high neutronand gamma uxes. Thus a calorimeter for LHC shouldbe highly performant, fast, ne-grained and resistant toradiation.A good choice for all the above is a liquid ioniza-tion calorimeter exploiting the novel idea of D.Fournierto use accordion shaped electrodes and absorber plateswith their zig-zags running parallel to the shower axis[2]. The use of liquid Ar as active medium and radi-ation resistant front-end electronics makes the systemradiation-hard. The high readout speed is achieved bythe use of only the fast rise of the signal pulse in theLAr and by the clever transmission of the signal bythe accordion electrodes themselves to the preampli-ers. The calorimeter cells are dened in one directionby etching strips on the electrodes and in the other bygrouping electrodes together. Thus there are no cracksbetween cells, high granularity (also low detector C andlow noise) is easily achieved and the number of connec-tions (also the inductance) are minimised.The performance of several prototypes having usedthe accordion technique will be presented in the follow-ing sections, showing that they also meet the physicsrequirements set for various channels, e.g. the H ! channel.2 Accordion EM BarrelA very good test of the above characteristics was car-ried out with the construction by the RD3 collabora-tion of a large scale LAr em calorimeter prototype witha fully projective structure which was envisaged as asector of the ATLAS barrel calorimeter [3]. Its lengthalong the z-axis (LHC beam axis) is 2m, correspondingto pseudo-rapidity range 0  jj  1:08, and it covers27oin azimuth (). The total thickness at  = 0 is25X0's, segmented longitudinally in three parts(9/9/7respectively). The Pb accordion-shaped absorbers are1.8 (1.2) mm thick for  < 0:7( > 0:7) to compen-sate for the lower sampling fraction at higher 's. Theelectrodes are copper-kapton boards separated by 1.9mm LAr gaps from the absorbers. The granularity is0.020 in , dened by grouping three kapton boards,and 0.018 in  which is the width of the etched elec-trode strips.The front-end/readout chain of the calorimeterconsisted of cold or warm preampliers followed by shaperswith 38 nsec shaping time (that value is a compromisebetween the electronics and pile-up noise for LHC). Thesignal is then sampled at the peak and digitised by a12bit ADC. The noise from the electronics for such afast shaping is  300 MeV for a region containing an emshower. Muons can be detected with a signal-to-noiseratio of 4 in a region of two em towers.The em barrel prototype showed the followingproperties when exposed and scanned with electron beamsFigure 1: The em barrel module prototype under con-struction.of various energies. The energy resolution at two dif-ferent  points corresponding to two dierent absorberthicknesses can be shown in gure 2. The samplingFigure 2: The energy resolution curve for the em barrelprototype.term (a=pE) is similar for the two positions. The con-stant term (c) of the resolution is due to local imper-fections in the cell. Its value of 0.3% is residual afterthe relevant geometrical corrections and the accordiongeometry does not introduce extra contributions. Thenoise term (b=E) is compatible with measurements ex-tracted from pedestal events. The position resolutionin the two coordinates is:= (0:210 0:015)4:70 0:05pEmm (1)= (0:186 0:021)3:87 0:05pEmm (2)The uniformity of the large scale prototype wasmeasured with a beam of 287 GeV electrons shot over123 spots. The resulting overall energy spectrum showsan rms of (0:69  0:05)% which is the global constantterm. This is a convolution of the local constant termof 0.35% mentioned above and of another 0.58% (0.37%from calibration and 0.45% frommechanical non-uniformities)due to response variation over the large area.3 Accordion Hadronic BarrelThe Accordion design was also used for the constructionof a hadronic calorimeter prototype which was testedtogether with the em barrel prototype described in theprevious section [4]. Two modules were tested, seg-mented longitudinally in two parts of 1.2 and 1.6 Irespectively. It consisted of 8  9 towers and its gran-ularity was    = 0:045  0:05. The converterplates were made from stainless steel 9.8 mm thick andthe LAr gap was 3 mm.A novel technique was used for the readout, whichwas the EST (ElectroStatic Transformer) scheme. Thissignies the fact that while ganging several LAr gapsto form a cell, a fraction of the gaps are connected inseries. This is really helpful in reducing the resultinglarge detector capacitance, which is more critical heresince the gaps are wider than the em case.The pion data collected with beams of energies20 to 400 GeV were analysed by applying a weightingmethod. The energy resolution found was:EE=(51:6 2:6)%pE (3:3 0:1)%2:5 0:1E(3)The measured uniformity over 4 cells was 1%.4 Accordion EM EndcapWhile the accordion design ts well radially the cylin-drical barrel part, its use is not straightforward for theendcap, for which another setup had to be inventednicknamed Spanish Fan. The azimuthal symmetry iskept for the endcap and the accordion plates' wavesare parallel to the vertex-pointing line. In the result-ing design the zig-zag amplitude, the Lar gap and theabsorber thickness are variable with the distance fromthe beam line so as to keep the sampling fraction con-stant with  [5]. A sketch can be seen in g. 3. Thefull radius of the endcap in the actual experiment willbe constructed in two parts. The prototype realised fortests is one sixth in azimuth of the inner endcap wheeland its rapidity coverage is 2:16 < jj < 2:89. Its gran-ularity is  = 0:03 0:05 and is segmented in 3samplings 9,9 and 7 X0's deep. Due to the variable gapwidth along R the HV applied was also variable anddierent for every two  lines to keep the calorimeterresponse constant. The HV values were calculated byMonte-Carlo and the residual response variation was atthe level of only a few percent.Figure 3: The design of the accordion for the endcap.The energy resolution of the endcap prototypefor dierent rapidity points is shown in gure 4. Its00.511.522.533.540 50 100 150 200 250 300Energy (GeV)η = 2.66η = 2.57η = 2.43η = 2.22σ/E (%)Figure 4: Energy resolution obtained with the Spanishfan prototype.parametrisation for  = 2:66 is :EE=(10:7 0:3)%pE(0:300:04)%0:51 0:02E(4)The uniformity of response over 48 cells gives a globalconstant term of (0:79 0:04)%, shared almost equallyby electronics (calibration, capacitance) and mechanical(modulation, gap, absorber thickness) non-uniformities.The position resolution of the prototype is measured tohave a sampling term less than 5mm=pE and a con-stant term of around 0.25 mm. The performance iscomparable with that of the barrel em prototype.5 PreshowersThe calorimeter prototypes described above were de-veloped following the requirements of the LHC physicsguidelines, among which a light Higgs decaying to twogammas is a big challenge. To detect it one needs apartfrom very good energy resolution, good photon direc-tion measurement and high =0and /jet separation.To compensate the energy lost in the upstream mate-rial, measure accurately the shower position and reject0's the role of the em calorimeter has to be assisted bya preshower. The LARG community has adopted andtested two approaches for this purpose, a separate andan integrated preshower.The separate preshower [6] is located in frontof the calorimeter and it has two highly segmented lay-ers with slanted strips, i.e. () = 2:5  10 3, in and in  respectively. The rst layer is after 2 X0's ofabsorber material inside the em barrel cryostat and thesecond layer after one extra X0to increase photon con-version probability. The prototype covered the rapidityrange between 0.2 and 0.8 and 9oin azimuth. The 150GeV muon signal was measured with a signal/noise ra-tio of 5.6 over 2 strips/layer (4 channels).The energy resolution measured with the preshower-calorimeter system shows a sampling term of (12:3 0:2)%pE. The position resolution in the two preshowerlayers is()= 0:16(0:19)1:67(2:02)pEmm: (5)The angular resolution measured by the preshower andthe middle calorimeter layer is()= 1:(1:)22:(27:)pEmrad; (6)for electrons and for photons is shown in gure 5.The integrated preshower [7] is actually re-alized by segmenting nely the rst sampling of thecalorimeter. This was achieved by drawing the strips ofthe kapton electrodes in a special way. The ones cor-responding to the rst sampling were oriented in twoviews, i.e. u = + and v =  , and the ones of thetwo other samplings remained  oriented as they werein the em barrel module described in 2. Kaptons ofu- and v-type where interleaved at stacking. The stripwidth of the rst sampling was 5 mm and the eectivegranularity was  = 0:004410:0784. The longi-tudinal depths of the three samplings are 5.6, 14.6 and9.1 X0's respectively. The prototype covered rapiditiesfrom 0.4 to 0.9 and 9oin azimuth. The sampling term ofthe energy resolution was found to be (12:30:2)%pE.The position resolution of the rst sampling is()= 0:18(0:16)1:8(2:2)pEmm: (7)The resolution of the angular measurement using oneFigure 5: Angular resolution for the separate preshowerprototype, measured with photons.point in the 1st (u- and v-strips) and another one inthe 2nd sampling is shown in gure 6 for electrons.angular resolution at η=0.605101520253025 50 75 100 125 150 175 200 225EBeam (GeV)σ (mrad)σ (mrad)Figure 6: Angular resolution for the integrated UVpreshower prototype, measured with electrons.6 From Prototypes to ATLASThe advances described above proved the capability ofbuilding large calorimeter systems with the Accordionprinciple. The ATLAS experiment has used these ideasto a large extent for the design of the detector.The ATLAS calorimeter setup was dened basedon the prototype results and the detailed simulations ofthe overall detector. The performance observed in thetestbeam was preserved in the simulated calorimeter inATLAS. After testing a lot of options for the calorime-ter system [1] the collaboration adopted the solutionsdescribed below. For the barrel part of the em calorime-ter the accordion design will be used with an integratedpreshower, i.e. with a 1st sampling nely segmentedin the  direction. To have the additional feature ofcorrecting for the energy lost at the upstream mate-rial a single layer presampler will be used in front ofthe calorimeter. In the endcap region the spanish fandesign will be implemented for the em part.A rst step towards ATLAS was the rst test-ing with electron and pion beams of the system of thebarrel em (Pb/LAr accordion) and hadronic calorime-ter (Fe/Scintillator Tiles) prototypes. This combinedtest was successful in showing that the system of a liq-uid ionization em and a scintillator sandwich hadroniccalorimeter is capable of reconstructing the energy of ahadronic shower with good energy resolution. More re-sults on that are presented at the same conference (seetalk by V.Boldea [8]). A preliminary analysis of 300GeV pion data gave a resolution =E = 4:1%.ConclusionsThe R&D on liquid ionization calorimetry combinedwith the Accordion technique has been very fruitfulduring the last few years with the realisation and testsof large scale prototypes. The achieved readout speed,granularity, radiation hardness, the energy and positionresolution and the response uniformity are compatiblewith the LHC requirements. Those concepts have beenadopted to the nal design of the ATLAS detector.References[1] ATLAS Collaboration, Technical Proposal,CERN/LHCC/94-43.[2] B. Aubert et al., NIM A309(1991)438,B. Aubert et al., NIM A321(1992)467,B. Aubert et al., NIM A325(1993)118.[3] D.M. Gingrich et al., NIM A364(1995)290.[4] D.M. Gingrich et al., NIM A355(1995)295.[5] A. Chekhtman et al., ATLAS-CAL-NO-067 (1995).[6] J. Collot, 5th Int.Conf. of Calorimetry in HEP,BNL 1994,L. Drage, private communication.[7] I. Wingerter-Seez, 5th Int.Conf. of Calorimetry inHEP, BNL 1994,O. Linossier, private communication.[8] V. Boldea, Four Seas Conf., Trieste 1995.

The accordion calorimetry for LHC

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