Bell, K W

Brown, R

Cockerill, D J A

Flower, P S

Hobson, P R

Imrie, D C

Kennedy, B W

Lintern, A L

Sharif, O

Sproston, M

Williams, J H

CERN Document Server

PrHEP hep2001International Europhysics Conference on HEPPROCEEDINGSThe Vacuum Phototriodes for the CMSElectromagnetic CalorimeterR M Brown∗†, K W Bell, D J A Cockerill, P S Flower, B W Kennedy, A L Lintern,M Sproston and J H WilliamsCLRC Rutherford Appleton Laboratory, Didcot, Oxfordshire, OX11 0QX, UKE-mail: R.M.Brown@rl.ac.ukP R Hobson, D C Imrie and O SharifDepartment of Electronic and Computer Engineering, Brunel University, Uxbridge,Middlesex, UB8 3PH, UKAbstract: The measurement of scintillation light from the lead tungstate crystals ofthe CMS ECAL poses a substantial technical challenge, particularly in the Endcap re-gions, where the radiation levels are highest. The photodetectors must be fast, sensitive,radiation-hard, and operate with significant internal gain in a magnetic field of 4 Tesla.The measured performance characteristics of 500 pre-production VPTs, developed to sat-isfy the needs of CMS, are presented.1. IntroductionThe electromagnetic calorimeter (ECAL) [1], for CMS [2], uses lead tungstate (PbWO4)scintillating crystal as the active medium. This is the first application of this material inparticle physics. In many respects, lead tungstate is an ideal crystal for use at the LHC.It has:- a very short radiation length (8.9 mm) and Molie`re radius (21.9 mm), allowing acompact design- fast scintillation emission (> 90% of light emitted within 100 ns) in the visible region(420 nm)- strong resistance to radiation induced darkening through the formation of stablecolour centres.∗Speaker.†We wish to thank H F Heath and colleagues at Bristol University, UK, and D Seliverstov and colleaguesat PNPI, Russia.PrHEP hep2001International Europhysics Conference on HEP R M BrownFigure 1: A vacuum phototriode with a nominal external diameter of 1 inch, supplied by ResearchInstitute Electron for the CMS Electromagnetic Calorimeter Endcaps.However, the light yield is rather low (50 photons/MeV), necessitating the use ofphotodetectors with internal gain. Furthermore, in CMS, the ECAL is located withinthe magnetic field volume of the superconducting solenoid. Thus the photodetectors arerequired to operate in a 4 T magnetic field.The ECAL comprises two subsystems: the ‘Barrel’ (61000 crystals) covering thepseudorapidity range jηj< 1.48, and a pair of ‘Endcaps’ (15000 crystals) covering therange 1.48 < jηj< 3.0. The photodetectors chosen for the Barrel are avalanche photodiodes(APDs) [3], described elsewhere in these proceedings. In the Endcaps, the radiation levelsare much higher than in the Barrel, precluding the use of silicon-based photodetectors.The solution chosen in this region is therefore to use vacuum phototriodes (VPTs). Themagnetic field orientation is favourable for the use of such detectors in the Endcaps, butnot in the Barrel.2. Application of VPTs in CMSA vacuum phototriode is essentially a photomultiplier tube with a single stage of gain(figure 1). Optical photons strike a semi-transparent photocathode deposited on the innersurface of the front window. The emitted photoelectrons are accelerated towards the anode,which consists of a very fine metal mesh. A fraction (50%) of the electrons pass throughthe anode and strike the solid metal dynode, located just behind the anode. Secondaryelectrons ejected from the dynode are accelerated back towards the anode, where most arecollected. The dynode surface is coated with the same bialkali material as that forming thephotocathode and under operating conditions (VA = 1000 V, VD = 800 V) has a secondaryemission coefficient of approximately 20. Thus, taking into account the transparency ofthe anode mesh, the effective gain of the VPT is of order 10. A more complete discussionof the operation is given elsewhere [4].– 2 –PrHEP hep2001International Europhysics Conference on HEP R M BrownFigure 2: Details of the construction of the ECAL Endcaps. On the left, elements of a ‘supercrystal’(a module of 25 crystals): mechanical components, a crystal/VPT assembly, monitoring systemfibres. On the right, the arrangement of supercrystals on a pair of ‘Dees’.The Endcap crystals have a square back face with dimensions 30x30 mm2. Theyare grouped in ‘supercrystals’ consisting of 5x5 crystals housed in a thin walled ‘alveola’structure made from carbon fibre composite material. An Endcap consists of two ‘Dees’,each of which contains 150 supercrystals. Figure 2 shows the components of a supercrystal,and the arrangement of supercrystals on an Endcap. The VPTs developed for CMS havean outer diameter of 26.5 mm – the largest dimension that could be accommodated withinthe mechanical constraints of the supercrystal. The effective photocathode diameter is19 mm, providing adequate coverage of the crystal face. The crystals are slightly taperedalong their length, to give a projective geometry. The angle of the crystal axis (and hencethe VPT axis) with respect to the direction of the magnetic field varies from 5.7◦ at theinner edge of the Endcap to 25.6◦ at the outer circumference.Following several years of intensive R&D [4], a tender exercise was conducted withpotential producers, resulting in the placement of an order for 500 ‘pre-production’ deviceswith Research Institute Electron (RIE) in St. Petersburg. Delivery was completed at theend of 2000.3. Performance CharacteristicsSamples of the pre-production tubes have been evaluated at Brunel University, both at zeromagnetic field, and at 4 T field with a fixed tube orientation of 15◦ with respect to the field.All the devices have been measured at Rutherford Appleton Laboratory at several magneticfield values up to 1.8 T and over a range of angles from −30◦ to +30◦. Figure 3 showshow the anode response changes as a function of the angular orientation in a magneticfield. The oscillatory structure at small angles is associated with the periodic pattern of– 3 –PrHEP hep2001International Europhysics Conference on HEP R M Brown0.00.20.40.60.81.01.2-90 -60 -30 0 30 60 90VPT angle (deg.)Rel.AnodeResponseArrows indicateangular regionsof end capsTypical magnetic response0.00.20.40.60.81.01.20 0.5 1 1.5 2Magnetic Field (Tesla)Rel.AnodeResponseFigure 3: The behaviour of a typical VPT in a magnetic field. On the left, the anode response asa function of the angle between the VPT axis and field direction for a field of 1.8 T. On the right,the anode response as a function of field strength for a VPT/field-angle of 15◦.the anode mesh. At angles beyond 45◦, the response falls rapidly as photoelectrons strikethe walls of the glass envelope before reaching the anode and dynode. The variation inanode response as a function of magnetic field strength is also shown in figure 3. Theoutput displays significant field dependence below 1 T, but at higher fields the responsechanges very slowly. The fixed field measurements at Brunel confirm that the performanceis maintained up to 4 T.The measured distribution of anode response (at 1.8 T and 15◦) is shown in figure 4(left). The entries have been separated into three classes corresponding to the start, middleand end of the production run. There is a clear indication that the average response tendedto improve through the course of the manufacturing run. The proof-of-principle that thePbWO4 – VPT combination could meet the performance requirements of the CMS ECALwas demonstrated using a 5x5 test array of crystals in a high energy electron beam atCERN in 1999 [5]. The anode responses of the 5 prototype RIE tubes from that test arrayhave also been measured on the test set-up at RAL. They appear as the open entries infigure 4. It is encouraging to note that the average performance of the pre-productiontubes is substantially better than that of the prototype devices.4. Radiation hardnessThe main susceptibility of vacuum photodetectors to damage by radiation arises fromdarkening of the window caused by the formation of colour centres. Tubes with a standardborosilicate window are quite readily degraded in this way. Radiation hard tubes can berealised by incorporating a fused silica window, but these would be prohibitively expensivefor application in CMS. A good compromise is to make the window from ultraviolet trans-mitting glass, which is much more radiation resistant than borosilicate glass, yet adds little– 4 –PrHEP hep2001International Europhysics Conference on HEP R M Brown051015202530354045501 11 21 31 41Anode pulse height (Arb. unit)Post-irradationPrototypesBatch 301-500Batch 101-300Batch 1-1000 10 20 30 40 50 60 70 80 90 100 012345350 375 400 425 450 475 500 525 550 575 600 625 650 675 700Wavelength (nm)TransmissionLoss(%)Sample 1Sample 2Figure 4: The histogram on the left shows the distribution of anode responses for 500 pre-production VPTs (shaded) and 5 earlier prototype devices (unshaded). The anode responses oftwo pre-production tubes, after 60Co irradiation to 25 kGy, are also shown (black). The curveson the right show the internal optical transmission as a function of wavelength for two faceplatesamples, after 60Co irradiation to 25 kGy.to the cost of the tube. Measurements on face plate samples using the 60Co irradiationfacility at Brunel show that at doses up to 25 kGy (the expected dose at jηj= 2.6, after10 years of LHC operation) the loss in transmission in the region of PbWO4 emission is lessthan 4% (figure 4 (right)). Complete tubes have also been irradiated while under bias. Theanode responses of two irradiated pre-production tubes are indicated by the black entriesin figure 4 (left). The post-irradiation performance is typical of the unirradiated devices.5. SummaryHigh-field, radiation-resistant VPTs have been developed to fulfil the requirements of theCMS ECAL Endcaps. Test measurements on 500 pre-production devices show that thedesign specifications can be met.References[1] The CMS Electromagnetic Calorimeter Project Technical Design Report,CERN/LHCC 97–33 (1997).[2] The Compact Muon Solenoid Technical Proposal, CERN/LHCC 94-38 (1994).[3] K. Deiters et al., Properties of the avalanche photodiodes for the CMS electromagneticcalorimeter, Nucl. Instrum. Meth. A453 (2000) 223.[4] K.W. Bell et al., The Development of Vacuum Phototriodes for the CMS ElectromagneticCalorimeter, Nucl. Instrum. Meth. A469 (2001) 29.[5] M. Appollonio et al., Test Results from a Prototype Lead Tungstate Crystal Calorimeter withVacuum Phototriode Readout for the Compact Muon Solenoid Experiment,Nucl. Instrum. Meth., to be published.– 5 –

The Vacuum Phototriodes for the CMS Electromagnetic Calorimeter

The Vacuum Phototriodes for the CMS Electromagnetic Calorimeter

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