The protection system used to shield LEP experiments against particle backgrounds has been upgraded in view of the much higher photon background rates expected at LEP2 energies. General experience with background rates in the LEP experiments at highest beam energies is reported and special features are highlighted

Lamont, M

Von Holtey, G

The protection system used to shield LEP experiments against particle backgrounds has been upgraded in view of the much higher photon background rates expected at LEP2 energies. General experience with background rates in the LEP experiments at highest beam energies is reported and special features are highlighted. (10 refs)

CERN Document Server

Protection of LEP experiments against particle background at highest beam energies

PROTECTION OF LEP EXPERIMENTS AGAINST PARTICLEBACKGROUND AT HIGHEST BEAM ENERGIESG. von Holtey, M. Lamont, CERN, Geneva, SwitzerlandAbstractThe protection system used to shield LEPexperiments against particle backgrounds has beenupgraded in view of the much higher photon backgroundrates expected at LEP2 energies. General experiencewith background rates in the LEP experiments at highestbeam energies is reported and special features arehighlighted.1  INTRODUCTIONThe main particle backgrounds at LEP are off-energyelectrons and positrons produced by beam-gasbremsstrahlung and synchrotron radiation (SR) photonsproduced in the closest bending magnets and inquadrupoles in the straight sections on either side of theexperimental detectors. While off-energy electron ratesunder normal vacuum conditions are acceptable and arenot expected to increase with energy, the low energy SRphoton background is many orders of magnitude largerthan can be tolerated and increases rapidly with beamenergy. The LEP collimator protection system, designedfor 45 GeV, therefore had to be extended in order tocope with the much increased photon rates to beexpected at twice the beam energy. The most importantlimit on photon background comes from aging of gastracking detectors. They are designed to be sensitive toenergy depositions of ≤1keV for minimum-ionizingcharged particles and are therefore sensitive to keVphotons, which interact in the gas or other local material[1].After a description of the LEP2 backgroundprotection system, experience with detector backgroundsat high beam energies are reported.1SCAL2 3 4-2 5 6 7 8 9 10405060703020-3 -1-410VDETmaskIPSCAL80vertical Collimatorhorizontalback scattered photons(2.7 mrad)(0.7 mrad)direct photonsCOLH.QS1[mm][m]QS0QS0beam apertures30 mradFigure 1: Schematic layout of SR-mask around a LEP IP, indicating the boundary conditions for a mask solution.2  LEP BACKGROUND PROTECTIONThe LEP detectors are protected against beaminduced background particles by a system of movabletwo-jaw collimators placed at strategical locations in theexperimental insertions. The LEP1 system is composedof 16 horizontal or vertical collimators for eachexperiment, plus six aperture limiting collimatorslocated in a non-experimental insertion [2]. Simulationsof SR-photon background at 90 GeV indicated a largeincrease of the photon rate, mainly due to back-scattering of photons from the first 50 m of vacuumchambers around the IP [3]. The solution proposed wasto enlarge where ever possible vacuum chambers, toinstall eight more collimators per experiment, and as themain defense to incorporate cylindrical tungsten0-7803-5573-3/99/$10.00@1999 IEEE. 3008Proceedings of the 1999 Particle Accelerator Conference, New York, 1999absorbers into the experimental vacuum chambers [4].The functionality of these SR-masks and the MCpredictions were verified with a test SR-mask installedin DELPHI during 1994 and 1995 using 45 GeV and65 GeV beams [5]. A schematic layout of a SR-mask isshown in Figure 1, a comparison of measuredbackground rates with different simulations is given inFigure 2.20 30 40 5010-210-1100COLH.QS1 half opening (mm)photon hits / BX-mAFigure 2: Monte Carlo predictions and measurements in theDELPHI TPC. Data points (photons per bunch crossing BX)are from the TPC, solid line MC [3], broken line MC [6].Ebeam= 65 GeV.Following the positive test results, the upgradedLEP2 protection system, including SR-masks, wasinstalled for the 1997 run in all four experiments and hasproven since to protect the detectors well at increasingbeam energies up to the 94.5 GeV run during 1998. Asan example SR-photon and off-energy background ratesfor different energies and LEP optics measured byOPAL are shown in Figure 3. Rates increase linearlywith current. The large fluctuation of the SR-photon ratereflects the strong dependence of this background onseveral machine parameters.3  LIMITATIONS OF THE MASKSOLUTIONOn the other hand, as the SR-masks are protrudinginto the vacuum chamber only 2.5 m away from the IP,they become very effective sources for low energyphotons to be scattered into the detectors. The absorberrings are therefore shaped in such a way as to suppressall forward scattering. Back-scattering however from thedownstream mask can not be avoided. The only solutionis therefore to shield the downstream mask against directphoton hits.This is achieved by a pair of additional horizontalcollimators (COLH.QS3 or 4), placed close to ahorizontal beam waist, some 50 m from the IP’s.However, as LEP beam optics are considerably differentin insertions IP4 and IP8 compared to the two otherinsertions, the horizontal beta-functions at the collimatorlocation in IP4 and IP8 are two times larger and themask protection is consequently not as good.Sufficient protection can, however, still be achievedfor both types of insertions as long as the densitydistribution of the horizontal beam tails stay close toGaussian. An example is shown in Figure 4. Closing theprotection collimators to 14 σx, the minimum allowedsetting before scraping into the beam, good protection isobtained in OPAL (IP6) and DELPHI (IP8). Themeasurement also demonstrates the limited margin leftfor the collimator opening in DELPHI, at which pointthe SR-mask is no longer protected and the photon ratestarts to increase exponentially.According to simulations for IP8 the downstream SR-masks are hit by about 3.103 photons per bunch crossing(BX) with Gaussian beams of 3 mA, against about 7.106photons/BX with exponential beam distributions. Thesephotons are radiated by electrons on •6 sigma orbits instraight section quadrupoles 50 to 150 m upstream of theIP. The scattering probability from the tungsten maskinto the detector for photons in the several 100 keVenergy range is of order 5.10-5, which amounts to several100 photon hits/BX in the case of exponential beams. Asthe trip level for the central detectors is in the region of•10 photons/BX, such high rates could not be tolerated.Figure 3: Hit rates per bunch crossing of synchrotron radiationphotons and off-energy electrons measured in the OPALVertex Drift Chamber (CV) are shown as function of the totalcurrent in the two crossing bunches. The different symbolsbelong to different beam energies and LEP optics. One unit of3009Proceedings of the 1999 Particle Accelerator Conference, New York, 1999‘figure-of-merit’ corresponds to a photon hit rate of about0.6/BX and an electron hit rate of about 0.9/BX [7].10 15 20 25 300.00.51.01.5collimator half opening (x/s x)BGD1 / mAOPALDELPHIsettingFigure 4: “Figure-of-merit” for SR-photons (BGD1) measuredin DELPHI and OPAL as function of the opening of horizontalcollimators COLH.QS3 and COLH.QS4, respectively.(92 GeV, εx= 30 nm).While off-energy and SR-photon background ratesstayed acceptably small under normal beam conditionsduring 1998 running at 94.5 GeV with currents up to3 mA/beam, occasionally large spikes and short periodsof high photon background were observed, which led inmany cases to detector trips, particularly in ALEPH(IP4) and DELPHI (IP8) [8].These “photon storms” are believed to be due toresonance build-up of non-Gaussian beam tails in thehorizontal plane. In the two insertions, where limitedmask protection exists, SR-photons radiated by electronsin the far tails can reach the inner tip of the downstreammask and back-scatter into the detectors. Photon stormscan occur several times during a fill, apparentlyuncorrelated to beam current. The slow exponential riseof the photon rate over several minutes can be stoppedand restored by a very small increase of the horizontaltune of order 0.005 (see Figure 5).In a MD experiment [9] photon storms could betriggered by reducing the horizontal tune to reach the3Qx+Qy=1 resonance. No reduction of the beamlifetimes was observed, however, tail scans unveiled astrong increase of the particle density in the horizontaland vertical tails of one of the beams.Further beam studies are needed to better understandthe underlying effects during data taking conditions andto isolate the resonance’s in question. Better collimatorprotection can be achieved if the horizontal aperturelimit can be reduced below the present 12σx. Increasingthe horizontal beta-value at the interaction points willreduce the beam size at the low-beta quadrupoles andallow for closer collimator settings.0 10 20 300.05.010.0time (min)  figure-of-meritD Qx=+0.005fill #5185, t ‡ 20:50Figure 5: Background rates in ALEPH (dotted line), DELPHI(full line), OPAL (dash-dotted line) and L3 (dashed line)during a “photon storm” with data taking at 94.5 GeV. Theexponential rise was stopped and the rates restored by a Qxtrim of +0.005.REFERENCES[1] J. Rothberg, Limitations due to background atLEP1, Proc. 5th Workshop on LEP Performance,Chamonix, 1995[2] G. von Holtey and K.M. Potter, Estimation ofsynchrotron radiation background in LEPexperiments, IEEE Particle Accelerator Conference,Washington. DC, 1987[3] G. von Holtey, Simulations of photon backgroundsto LEP detectors at 90 GeV beam energy, CERNSL/92-54, 1992[4] G. von Holtey et al., Nucl. Instr. and Meth. in Phys.Res. A 403 (1998) 205[5] The LEPC Working Group on LEP2 Backgrounds,CERN/LEPC93-10, 1993[6] P. Roudeau, unpublished.[7] P. Estabrookes, OPAL, private communication,1998[8] G. von Holtey, Particle backgrounds at the LEPdetectors at 94.5 GeV, Proc. 9th Workshop on LEPPerformance, Chamonix, 1999[9] K. Affholderbach et al., “Beam tails and backgroundstorms with colliding beams at 94.5 GeV”, SL-Note-98-073 MD (1998)3010Proceedings of the 1999 Particle Accelerator Conference, New York, 1999

Protection of LEP Experiments against Particle Background at Highest Beam Energies

Abstract

Similar works

Full text

Available Versions

CERN Document Server

CERN Document Server