The LHC luminosity will reach 1034 cm-2s-1 but special runs at 1028 cm-2 s-1 are foreseen. Thus a luminosity monitor must have a dynamic range of six orders of magnitude. A good tolerance to radiation is also required. A detector using both ionisation and secondary emission techniques has been studied in this context. Its design is based on monitors used previously at the CERN PS and SPS machines. Special attention was devoted to minimize leakage currents. Linearity in both Secondary Emission Counter (SEC) and Ionisation Chamber (IC) modes has been tested from ~104 incident particles to ~108 incident particles. SEC is linear above ~3 x 106 incident particles while IC is linear over the full studied range. However, because of the radiation environment at the LHC, the SEC mode is much preferred at high intensity. A solution actually foreseen is to switch from IC to SEC mode when the incident current on the monitor is ~10-13 A corresponding to an LHC luminosity of ~1031 cm-2 s-1

Perrot, Anne Laure

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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN – ESTCERN   EST/2001-003 (LEA)A luminosity monitor for the Large Hadron ColliderAnne-Laure PerrotCERN/EST-LEA, Meyrin CH 1211Geneve 23IPHE Université de Lausanne CH 1015 DorignyAbstractThe LHC luminosity will reach 1034 cm-2s-1 but special runs at 1028 cm-2 s-1 are foreseen.Thus a luminosity monitor must have a dynamic range of six orders of magnitude. A goodtolerance to radiation is also required. A detector using both ionisation and secondaryemission techniques has been studied in this context. Its design is based on monitors usedpreviously at the CERN PS and SPS machines. Special attention was devoted to minimizeleakage currents. Linearity in both Secondary Emission Counter (SEC) and IonisationChamber (IC) modes has been tested from ~104 incident particles to ~108 incident particles.SEC is linear above ~3 x 106 incident particles while IC is linear over the full studied range.However, because of the radiation environment at the LHC, the SEC mode is muchpreferred at high intensity. A solution actually foreseen is to switch from IC to SEC modewhen the incident current on the monitor is ~10–13 A corresponding to an LHC luminosity of~1031 cm-2 s-1.Paper presented at the “2000 IEEE Nuclear Science Symposium and Medical ImagingConference”Geneva, SwitzerlandJune 2001A luminosity monitor for the Large Hadron Collider *Anne-Laure PerrotCERN/EST-LEA, Meyrin CH 1211Geneve 23IPHE Université de Lausanne CH 1015 DorignyAbstractThe LHC luminosity will reach 1034 cm-2s-1 but special runsat 1028 cm-2 s-1 are foreseen. Thus a luminosity monitor musthave a dynamic range of six orders of magnitude. A goodtolerance to radiation is also required. A detector using bothionisation and secondary emission techniques has been studiedin this context. Its design is based on monitors used previouslyat the CERN PS and SPS machines. Special attention wasdevoted to minimize leakage currents. Linearity in bothSecondary Emission Counter (SEC) and Ionisation Chamber(IC) modes has been tested from ~104 incident particles to~108 incident particles. SEC is linear above ~3 x 106 incidentparticles while IC is linear over the full studied range.However, because of the radiation environment at the LHC,the SEC mode is much preferred at high intensity. A solutionactually foreseen is to switch from IC to SEC mode when theincident current on the monitor is ~10–13 A corresponding to an LHCluminosity of ~1031 cm-2 s-1.I.  INTRODUCTIONCERN is building a new accelerator, the Large HadronCollider (LHC). It will give high luminosities to experimentsto study very rare processes. However, the precisemeasurement of the luminosity, L, is difficult on this p-p(proton-proton) collider. A direct evaluation with the beamparameters (intensity and emittance) [1] or with the “van derMeer method” [2] will lead to a great uncertainty. TOTEM(TOTal and Elastic Measurement) will measure the p-p cross-section with high precision using the “luminosity independentmethod ”[3], [4]. This will allow the calibration of theluminosity monitor at L ~1028 cm-2s-1.The challenge for a luminosity monitor at the LHC is thedynamic range of six orders of magnitude (L between1028 cm-2s-1 and 1034 cm-2s-1). Being close to the interactionpoint it also requires a good tolerance to radiation. Theserequirements are not fulfilled by detectors currently in use. Itis proposed to study if a detector based on the concepts ofSecondary Emission Chamber (SEC) and Ionisation Chamber(IC) developed years ago and still in use at CERN could be aluminosity monitor for the LHC. After a short introduction toIC and SEC principles, the design and performance of aprototype are reported.II.  IC AND SEC PRINCIPLESAn IC belongs to the family of ionisation detectors whichare based on the direct collection of the electrons and ionsproduced by an ionising particle in the gas [5], [6]. A basicconfiguration is a pair of metallic plates surrounded by gas(see figure 1). A voltage difference is applied between theFigure 1: Sketch of an ionisation detectorplates. An incident particle between the plates creates along itspath a number of ion-electron pairs proportional to its pathlength in the gas. The electrons and positive ions move towardthe anode and the cathode respectively, where they arecollected. As the voltage is increased, the recombination forcesare overcome and the current begins to rise as more and moreof the electrons are collected. At some point, all createdelectrons will be collected. This is the IC mode in which wewant to operate.For many years SEC have been used as standard intensitymonitors in proton external beam lines. It is a simple andradiation hard detector. The SEC operation is based on theSecondary Electron Emission (SEE) process [7]. This lattercan be divided into three steps [8]. First, the interaction ofincident particles with target foils and the correspondingexcitation of electrons. Some of these electrons receive enoughenergy to be knocked out from the atoms. Second, thediffusion of the electrons toward the foil surface with energyloss through inelastic collisions. The probability of reachingthe surface decreases with the depth at which the electrons areexcited. Finally, some of the electrons can pass the surfacepotential barrier. The SEE due to incident charged particles ofhigh energy is a surface phenomenon independent of the targetthickness. The principle of operation of a SEC is very simple(see figure 2). When a charged particle passes through a thinfoil of metal, SEE occurs (its accompanying electromagneticfield interacts with the peripheral electrons on the foil surfaceatoms, as well as with the free electrons of the metal,providing some of them with enough energy to be ejected fromthe atoms and to escape from the surface). The foil surface isthen an electron emitter. Another foil positively biased willcollect these electrons. Collection is optimised when invacuum.anodecathode*Paper presented at the ‘2000 IEEE Nuclear ScienceSymposium and Medical Imaging Conference’signal+HVincident particleFigure 2: Principle of Secondary Emission ChamberIII.  CONSTRUCTION OF THE NEW PROTOTYPEThe conception and design of the new monitor are basedon those of SECs used at the PS (Proton Synchrotron) andSPS (Super Proton Synchrotron) [9]. The overall mass of thefoils and windows has to be kept to a minimum. It determinesthe monitor dimension, the foil material and the windowmaterial. The SEC has to be bakeable, in order to reduceoutgassing to obtain high vacuum and clean surfaces. Finally,it has to present good resistance to radiation damage. Theconversion SEC to IC is simply done by filling the chamberwith argon. Before presenting the monitor construction andintroducing briefly the associated electronics, theimprovements of the new prototype are pointed out.A.  Improvement of basic configurationOur prototype differs from previous SECs used at CERNin two main characteristics. First, the addition of a groundinsulated stainless steel cylinder inside the vacuum tank   (seefigure 3). Second, the use of triaxial connections. The aim isto minimise leakage currents, mass loops and noise pick-ups.The external tank acts now as a Faraday cage, it isolates themonitor electrodes (aluminium foils) from the externalelectromagnetic noise while the internal cylinder acts as a“floating” shield.Figure 3: Triaxial configuration of the new prototypeB.  Construction of the new prototypeIt can be divided into four main steps. Step1: aluminiumfoils are mounted on circular stainless steel frames. They formthe monitor electrodes (see figure 4). Step 2: twenty oneelectrodes are piled up on six support bars (see figures 5 and6). In the final state, each collecting electrode is surroundedby two emitting ones. Step 3: introduction of the “electrodesblock” in a stainless steel cylinder which will act as a floatingshield (see figure 7). Step 4: the overall assembly is insertedinside an external stainless steel tank which will act as aFaraday cage (see figure 8). In SEC mode, the vacuum insidethe chamber is around 10-7 Torr. In IC mode, the chamber isfilled with argon at atmospheric pressure. Table 1 reportssome technical specifications of the monitor.Figure 4: Aluminium foil on a stainless steel frameFigure 5: Electrode pile-up: Figure 6: Electrode pile-up:intermediate step final stepFigure 7: Insertion inside the Figure 8: Insertion inside theinternal cylinder external tankThe chamber is presented in figure 9. It is a flangedstainless steel cylinder. Each of the two external flangesinclude a foil window (curved dotted line) made of stainlesssteel 25 µm thick. They make the vacuum sealing. Thechamber components are chosen to withstand a bake-outemitting polarisedaluminium foilbeamelectronicsstainlesssteel windowcollecting non-polarised aluminiumfoilbias voltagepower supply∼HVvacuum(∼10-7 Torr)HVsignalinternal cylinderor “floating shield”external tankor“Faraday cage”temperature of the order of 250°C. On the top of the externaltank a “four-ways” cross allows the signal output, thepolarisation input and the vacuum system connections. Twoprototypes have been built. They differ in the insulation modebetween two consecutive electrodes. In the first prototype,insulation is assured by ceramic while vacuum or gas is usedin the second prototype. The resistivity is increased in this lastversion, thus minimizing the leakage current. The choice ofaluminium as electrode material results from a compromisebetween radiation hardness and efficiency [10].Table 1Monitor technical specificationsfoils 21 (11 emitting electrodes      10 collecting electrodes)Al (99% purity)thickness: 10 µmused diameter: 120 mmspacing: 5 mminsulation ceramic (macor and alumineAl2O3), thickness: 3 mmother pieces stainless steel (304 L type)Figure 9: New prototypeC.  Associated electronicAs already mentioned, the monitor will work at luminositybetween 1028 cm-2s-1 and 1034 cm-2s-1. Estimations of the monitoroutput current (or electronic input current) in SEC mode givevalues between 10-15 A and 10-9 A respectively. In ionisationmode the current is three orders of magnitude higher. Theelectromagnetic noise becomes important below 10-12 A andlimits the signal detection. TheFigure 10: Sketch of the electronic chainelectronic components were chosen with the lowest intrinsicnoise. A sketch of the electronic chain is presented infigure 10. The two main components of the circuit are aprecision switched integrator transimpedance amplifierIVC102 by Burr-Brown [11] and a voltage to frequencyconverter (VFC) AD650 by Analog Devices [12]. Theacquisition gate is activated by a trigger. IVC102 outputvoltage increases linearly with the integrated charge. After afixed delay the gate is closed and the read-out is activated.The output voltage is converted into frequency by the AD650and sent to a counter unit. At the end of the reading the systemis reset. Then the IVC102 output voltage is set to zero. Thisrepresents a complete acquisition and reading cycle.IV.  PERFORMANCESThe performances of the monitor in both SEC and IC modewere determined comparing the response with a referencedetector. The tests were carried out at CERN on the PS-T11and the SPS-H6 extraction beam lines. The reference detectoris a pair of scintillator-photomultipliers in coincidence inPS-T11 and a Precision Ionisation Chamber in SPS-H6,placed upstream of the luminosity monitor. The beamintensity could be adjusted with collimators. Before eachextraction burst, the PS and SPS machines deliver a prepulseacting as trigger for the electronic readout.A first series of tests performed during summer and fall1998 allowed to specify the detection threshold of a previousSEC built around 1970. The background noise, electronicmass loops and pick-up limited the SEC detection to a currentof around 10-12 A. The simulations show that the monitor inthe LHC conditions must detect currents between 10-9 A and10-15 A, thus a new SEC prototype was built. Its linearity testswere performed during 1999 and 2000 with intensities rangingfrom 3 x 104 to 6 x 107 incident particles per burst (see figure11). This range corresponds to the intensities of the availablePS and SPS beam lines.Front view Side viewA,B: internal flanges Abis, Bbis: external flangesC: “4-ways” flange D: triaxial supportE: insulator cylinder F: stainles steel barG: internal cylinderH: external tankI: electrode assembly supportK: Al foil frame L: stainless steel bar positionM: Al foildetectorsignal AD 650(VFC)IVC102(integrator)pulses counton interval ∆tPC∫= dteIC1VFigure 11: SEC-IC response versus the number of incident particleAbove ~ 3 106 incident particles per burst the SEC response isa linear function of the number of incident particles within1%. We then deduce that at the LHC the SEC will be able tomeasure luminosity with a precision of ~3% for an integrationtime of 1 s, for luminosity above 1031 cm-2 s-1.      A conversionof the SEC into Ionisation Chamber (IC) is proposed to coverthe LHC low luminosity region. We have transformed theSEC into IC by filling the chamber with argon at atmosphericpressure. The IC response is linear at the 1% level over thefull experimental range of incident particles available. Itsuncertainty during our tests is less than 2%. The IC could beused to measure the LHC luminosity from 1028 cm-2s-1. At highintensity, the LHC induced radiation may activate the innergas of the IC which will result in the distortion of the ICresponse. The present idea is to use the monitor in IC mode atlow LHC luminosity and in SEC mode at high LHCluminosity with a switch for a luminosity of 1031 cm-2 s-1.V.  ACKNOWLEDGMENTSI wish to express my thanks to S. Weisz (CERN/EST-LEA) and Prof. A. Bay (IPHE, Lausanne university) for muchhelpful advice and discussions throughout the course of thiswork. I am very grateful to G. Molinari, J. Bosser and theCERN/PS-BD team for their assistance and help in theconstruction, setting-up and perfomance tests of the monitor. Iwould also like to thank K. Potter, CERN/EST-LEA groupleader and W. Kienzle of the TOTEM collaboration for theirencouragement and stimulation.VI.  REFERENCES[1] K. Piotrzkowski, Experimental aspects of the luminositymeasurement in the Zeus experiment, internal reportDESY F 35D–93-06, 1993.[2] S. van der Meer, Calibration of the effective beam heightin the ISR, CERN-ISR-PO/68-31, 1968.[3] The TOTEM collaboration, Letter of intent,CERN/LHCC 97-49, LHCC/I11, p1, p30-31, fig.1 p9.[4] M. Bozzo et al, Measurement of the proton-antiprotontotal and elastic cross-sections at the CERN SPS collider,Phys. Lett. 147B, p392, 1984.[5] W. R. Leo, Techniques for nuclear and particle physicsexperiments, Springer-Verlag, chap.6, p119-127.[6] K. Kleinknecht, Detectors for particle radiation,Cambridge University Press, chaps.1 and 2.[7] V.Agoritsas and R. L. Witkover, Tests of SEC stability inhigh flux proton beams, IEEE Transactions on NuclearScience, vol. NS-26 No 3, June 1979.[8] The encyclopedia of physics, 2nd ed., edited by RobertM. Besançon, Van Nostrand Reinhold Company, p825.[9] V. Agoritsas, Secondary Emission Chambers formonitoring the CPS ejected beams, Symposium on beamintensity measurement, Daresbury, proceedings edited byV. W. Hatton and S.A. Lowndes, DNPL/R1, 22-26 April1968.[10]G. Ferioli, R. Jung, Evolution of the Secondary EmissionEfficiencies of various materials measured in the CERNSPS secondary beam lines, CERN-SL-97-71 (BI), 1997.[11]Burr-Brown, email: http://www.burr-brown.com[12]Analog Devices, email: http://www.analogdevices.com1.E-041.E-031.E-021.E-011.E+001.E+011.E+04 1.E+05 1.E+06 1.E+07 1.E+08number of incident particles per burstmonitor signal [V] SEC IC IC SEC

A luminosity monitor for the Large Hadron Collider

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