The progress towards the realisation of the LHC-B Ring Imaging Cherenkov
detector is reported.Comment: 5 pages. 5 postscript figures + 1 postscript preprint logo + 1 LaTeX
  file + 1 style file. Also available at
  http://ppewww.ph.gla.ac.uk/preprints/97/14/ Invited talk given at Beauty'9

Anghinolfi

Heijne

N.H Brook

Weilhammer

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The progress towards the realisation of the LHCb Ring Imaging Cherenkov detector is reported.The progress towards the realisation of the LHC-B Ring Imaging Cherenkov detector is reported

Brook, N.H.

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Department of Physics & AstronomyExperimental Particle Physics GroupKelvin Building, University of Glasgow,Glasgow, G12 8QQ, ScotlandTelephone: +44 (0)141 339 8855 Fax: +44 (0)141 330 5881GLAS-PPE/97{14December 1997LHC-B Ring Imaging Cherenkov DetectorN. H. Brookon behalf of the LHC-B collaborationAbstractThe progress towards the realisation of the LHC-B Ring Imaging Cherenkov detector is reported.invited talk given at the5th international workshop on B-Physics at hadron machines (Beauty’97),Los Angeles, California, USA.Oct 13-17, 19971 IntroductionThis paper reports on the development of the Ring Imaging Cherenkov (RICH) detector for LHC-B. The LHC-Bexperiment is a proposed single arm spectrometer for the LHC optimised for B physics. Particle identication iscrucial for the study of B-physics and CP violation at the LHC. The many B-meson decay modes to be studiedmakes it necessary to have =K separation over a momentum range of 1 < p < 150 GeV=c: This reduces thebackground of the selected nal state and, in addition, provides an ecient flavour tag of the signal B-meson.The overall concept and performance of LHC-B is discussed in more detail elsewhere in these proceedings [1].LHC-BCoilRICH-2E-calH-calMuon systemTracking chambersMirror plateRICH-1MagnetSi vertexDetector2 m2 mFigure 1: The LHC-B spectrometerRICH detectors meet the experimental criteria for particle identication in the required momentum range.It is proposed to use two RICH detectors in the LHC-B experiment, see g 1. The upstream detector (RICH1),g. 2, has a combined gas and aerogel radiator and is situated in front of the dipole magnet. The aerogel radiatoris placed against the entrance window of the second gaseous (C4F10) radiator. A spherical mirror with a radiusof curvature of 190 cm is tilted by  250 mrad to reflect the Cherenkov light onto an array of photodetectorssituated outside the experimental acceptance. The downstream RICH (RICH2), g. 2, uses CF4 as its radiatorwith a spherical mirror with a radius of curvature of 820 cm tilted by 370 mrad. An additional flat mirror istilted by 240 mrad to bring the Cherenkov photons out of the acceptance of the experiment.Interaction pointPhotodetectors(HPD)Gas  (C4F10)Aerogel(n = 1.03)Window(Mylar)TrackingchamberMirror(R = 200 cm)Centre-of-curvature(200 mrad tilt)0 1 2400 mrad [m]Top View5 10120 mrad300 mradmirrorsPhoton detectors (HPD's)beam pipeCF4RICH-1 RICH-2LHC-BFigure 2: Layout of the LHC-B RICH detectors(note dierent scales).Each RICH detector has two photodetector planes giving a total area of 2:9 m2: The chosen detectortechnology must have a high quantum eciency, a spatial resolution of at least 2:5  2:5 mm2 and to have afast readout consistent with 25 ns bunch crossing of the LHC. Two candidate technologies exists [3]: hybridphotodiodes (HPDs) and multianode photomultipliers. These detectors are commercially available but notin designs that meet the experimental needs of LHC-B. A program of R&D is currently underway on thedevelopment of a HPD with a large active area.12 HPD developmentTwo complimentary approaches to the development of the HPDs are being investigated: the ‘pixel HPD’, whichuses the bump bonding of a silicon pixel detector to a readout chip, and the ‘pad HPD’ where a silicon paddetector is readout using routing lines wire-bonded to front-end chips around its circumference.2.1 Pixel HPDThe pixel HPD is being developed in close collaboration with DEP [4]. It is based on standard image intensiertechnology that strongly focuses photoelectrons onto a segmented silicon pixel array. The feasibility of thisapproach has already been demonstrated with the ‘IPSA-tube’ [5]. Work is ongoing on a scale model prototypewith an active photocathode diameter of 40 mm and an anode diameter of 11 mm. The anode is assembled withthe LHC1 chip developed at CERN [6]. This chip contains a detector array of 2048 pixels, 50 500m2 in sizeand it has a lowest achievable comparator threshold of 4000e− with a spread of 1000e−: Unfortunately thesedetector properties are not compatible with the needs of LHC-B. In particular, the pixel dimension (50m) is toosmall compared to the RICH granularity ( 500500m2): Nor are the the threshold properties of the LHC1 chipcompatible with LHC-B requirements, though recent development in pixel electronics have achieved comparatorthresholds down to 1400e− with an RMS of 90e−: The specic needs of LHC-B are being investigated.2.2 Pad HPDThe pad HPD will be housed in a cylindrical glass envelope capped with a UV-glass entrance window witha K2CsSb photocathode. A visible light photocathode deposition facility that allows a high vacuum seal ofthe HPD baseplate onto the metal flange of the glass envelope has been designed. The nal assembly of thisapparatus was performed at CERN at the end of 1997. The signal from the photoelectrons in the silicon sensorwill be detected by front end chips placed around the edge of the sensor via wires bonded to the routing linesfrom the pad. Focussing electrodes, xed in the glass envelope, will demagnify the image by a factor of 2:3:The 2048 pad silicon detector contains pads of dimensions 1mm2. Successful tests have been performed onthe sensor with photoelectrons up to 20 keV with (the non LHC speed) VA3 chip [7]. A signal/noise ratio of 10 has been achieved. Earlier tests with a 256 pad sensor had achieved a signal/noise ratio of  18: Thisdierence is under investigation, but one possible reason for this degradation in performance is dierences inthe manufacturing of the wafer.The SCT-128A [8] analogue chip which was developed for the ATLAS silicon tracker is being modied toachieve a noise level of  600e−: Additional modications will be needed to meet the requirements of the padHPD for LHC-B, in particular the multiplexing properties of the chip.LHCB RICH:  1/4 scale prototypeAerogelBeamC4F10 GasMirror240mm focal length400 mmHPD Read-out Electronics Figure 3: Conguration 1: the 1=4 scale prototype RICH vessel3 RICH PrototypeA prototype of the downstream RICH detector was tested in the T9 test beam at the CERN SPS duringthe Spring and Summer of 1997. A planar array of seven 61-pixel HPD’s from DEP were used to detect the2Cherenkov photons produced in aerogel, air and C4F10 radiators. In conguration-1, g. 3, of the prototypethe light is focussed by a 240 mm focal length mirror which corresponds to a 1=4 scale of the RICH1 detector.A full scale prototype (conguration 2) was also used which has a 1143 mm mirror to focus rings from C4F10onto an array of six 61-pixel HPD’s. This was achieved by adding extension arms to conguration 1.The 61 pixel HPD has a silicon diode detector segmented as a hexagonal array with pad dimensions of 2 mmface-to-face. The HPD was operated at a high voltage of 12kV. Using a pulsed light emitting diode the completereadout and data acquisition chain was tested. The pedestal, the single, double and triple photoelectrons peakswere clearly visible with a signal/noise ratio of  5:7: Most of this noise is associated with the input capacitanceof the feedthrough and printed circuit boards.The test beam provides charged particles of either polarity and the momentum can be tuned in the range2 − 15:5 GeV=c: The particle type is identied by measuring the signal pulse height from a CO2 thresholdCherenkov counter installed 30 m upstream from the prototype. The prototype vessel was aligned with thebeam axis. Charged particles which provide the trigger are selected using using scintillation counters, twoupstream and two downstream of the vessel. A photoelectron hit is dened to be a HPD pixel with a signalpulse height 4 above the pedestal mean, where  is the rms width of the pedestal peak.Using RICH conguration 1, data were taken with a 10 GeV/c negatively charged beam with 18 mm thicknessof aerogel. Fig. 4 shows an arc of a ring on the central HPD, whose radius is compatible with that expectedfrom C4F10: The outer HPD’s clearly exhibit a ring which originates from the aerogel radiator.Event Display of Run 487  (6225 triggers)-80-60-40-200204060-60 -40 -20 0 20 40 60 80mmmmHPD 138605 photonsHPD 21038 photonsHPD 31131 photonsHPD 41320 photonsHPD 51204 photonsHPD 6 HPD 71147 photons 1137 photonsFigure 4: An event display from aerogel and C4F10 radiators in RICH conguration 1, integrated over run 487.The number of photoelectrons per triggered event was measured for all three radiators in the vessel. For thisanalysis a threshold of 3 was set for individual pixels and multiple photoelectrons were taken into account. Themean number of photoelectrons are shown in table 1. The partial geometrical coverage of the aerogel and gasrings was calculated from simulation with  5% uncertainity. The expected photoelectron yields was calculatedfrom simulation which included the properties of the aerogel, mirror and photocathode eciencies. The overallprecision in the expected yield is estimated to be 15%. The comparison between observed and expected yieldsare given in table 1. The numbers from this preliminary analysis are compatible within 30%.Radiator Raw Bkg. E. Ratiohits corr. corr.Air 4.92 4.56 4.80 0.99C4H10 7.85 7.49 33.55 1.07Aerogel 1.79 1.31 10.71 0.72Table 1: Observed number of photoelectrons per event for air, aerogel and C4F10 radiators. The columns givethe number of raw hits, the numbers after correction for background and for geometrical eciency, and thecomparison with the expected yield.The full scale RICH1 prototype was studied using conguration-2. The longer focal length of the mirrormeans the C4F10 ring now spans the outer 6 HPDs. The event display shown in g. 5 is obtained from negativelycharged 15.5 GeV/c momentum beam. The K :  ratio of the triggering particles has been enhanced to 1 : 2using the threshold Cherenkov counter. Fig. 5 shows segments of two rings; an inner ring from the incidentkaons and an outer ring from the pions. It can be seen that the number of hits observed in HPD 3 is lower than3in HPD 4. (Similarly HPD 5 has fewer hits than HPD 2.) This is because HPD’s 3 and 5 have mylar windowsin front of their photocathodes which absorb the UV photons.Event Display of Run 587  (24575 triggers)-80-60-40-200204060-60 -40 -20 0 20 40 60 80mmmmHPD 1HPD 231943 photonsHPD 38579 photonsHPD 415201 photonsHPD 510613 photonsHPD 626014 photonsHPD 729944 photonsFigure 5: An event display showing =K separation, using C4F10 radiator in RICH conguration 2, integratedover run 587.4 SummaryThe major outstanding issue for the LHC-B RICH detector is the demonstration of a photodetector that matchesLHC-B requirements. For the pixel HPD it is envisaged to produce and test a 80 mm diameter tube whilsta pixel chip is developed in parallel that meets the experimental needs. Test in 1998 are planned for the padHPD using the 2048 pad detector under vacuum with the designed focussing and then eventually with a bialkaliphotocathode.The RICH prototype tests have been successful. Clear Cherenkov rings from gas and aerogel radiators havebeen observed for the rst time using HPD’s as photon detectors. The preliminary measured photon yields arecompatible within 30% of expectations based on simulations. Further analysis of the data is investigating thereconstruction resolution of the Cherenkov angle for each recorded hit from both the gas and aerogel radiators.Further prototype testbeam runs are being planned to study RICH 2. It is also planned to use the RICHprototype to test the various photodetectors as they become available.References[1] N. Harnew, LHC-B B-Physics Performance, to appear in these proceedings.[2] LHC-B Collaboration, LHC-B Letter of Intent, CERN/LHCC 95-5.[3] R. Rusack, Advances in Photon Detectors, to appear in these proceedings.[4] Delft Electronic Products BV, 9300AB Roden, Netherlands.[5] T. Gys et al, Nucl. Instr. Meth. A355 (1995) 386.[6] E. Heijne et al, Nucl. Instr. Meth. A383 (1996) 55.[7] P. Weilhammer et al, Nucl. Instr. Meth. A383 (1996) 89.[8] S. Anghinol et al, IEEE Trans. Nucl. Sci. 44 (1997) 298.[9] E. Albrecht et al, First Observation of Cherenkov Ring Images using Hybrid Photon Detectors, submittedto Nucl. Instr. Meth.4LHCB RICH:  1/4 scale prototypeAerogelBeamC4F10 GasMirror240mm focal length400 mmHPD Read-out Electronics 

LHCb Ring Imaging Cherenkov Detector

Brook, N H

LHCb ring imaging Cerenkov detector

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Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment

LHC-B ring imaging Cherenkov detector

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LHC-B Ring Imaging Cherenkov Detector

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