The effect of the machine induced background is studied for the low luminosity insertions of the LHC. Estimations for the fluxes of the secondary particles, induced by the proton losses in the LHC, are presented for several running conditions of the collider. The formation of the background in the machine structure is discussed

Azhgirey, L S

Baishev, I S

Potter, K M

Talanov, V V

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsMACHINE INDUCED BACKGROUNDIN THE LOW LUMINOSITY INSERTIONS OF THE LHCI. Azhgirey1, I. Baishev1, K.M. Potter2 and V. Talanov1The effect of the machine induced background is studied for the low luminosity insertions of the LHC.Estimations for the fluxes of the secondary particles, induced by the proton losses in the LHC, arepresented for several running conditions of the collider. The formation of the background in the machinestructure is discussed.1  IHEP, Protvino, Russia2  CERN, EST Division, Geneva, SwitzerlandPresented at the Eighth European Particle Accelerator Conference (EPAC)3-7 June 2002 - La Villette, Paris, FranceGeneva, Large Hadron Collider ProjectAdministrative SecretariatLHC DivisionCERNCH - 1211 Geneva 23SwitzerlandLHC Project Report 567Abstract2 July 2002MACHINE INDUCED BACKGROUND IN THE LOW LUMINOSITYINSERTIONS OF THE LHCI.Azhgirey, I.Baishev, V.Talanov∗, IHEP, Protvino, RussiaK.M.Potter, CERN, Geneva, SwitzerlandAbstractThe effect of the machine induced background is studiedfor the low luminosity insertions of the LHC. Estimationsfor the fluxes of the secondary particles, induced by theproton losses in the LHC, are presented for several runningconditions of the collider. The formation of the backgroundin the machine structure is discussed.1 INTRODUCTIONFluxes of the secondary particles, induced by protonlosses upstream and downstream of the interaction points,affect the radiation environment in the corresponding in-sertion regions and can contribute to the detector back-grounds [1]. The characteristic feature of these secondaryfluxes, or machine induced background, is that its value isproportional to the machine beam current, while the parti-cle fluxes, produced in the proton-proton collisions, scalewith the luminosity at a particular interaction point. Thismeans that the relative effect of the machine induced back-ground depends on the machine operating conditions andis different for different run scenarios.The LHC project [2] has two insertion regions (IRs) withrelatively low luminosity of proton-proton interactions, IR2and IR8. Each IR consists of the straight section around theinteraction point (IP) and two dispersion suppressors, up-stream and downstream of IP. One half of the straight sec-tion, both in the IR2 and IR8, houses the injection equip-ment together with the appropriate injection shielding, andwe present below the results of the machine backgroundanalysis for this particular part, based on the IR2 opticsstructure.2 ORIGINS OF MACHINEBACKGROUNDSecondary particle cascades can be initiated in the inser-tion region due to different physical processes. The genericproton losses are the interactions of the beam protons withresidual gas nuclei, resulting in multiple production of sec-ondary particles. Their distribution along the machine thusreflects the distribution of the residual gas pressure. Theseinteractions can be divided into two types, inelastic andelastic. The secondaries from inelastic interactions havein general a resulting momentum significantly smaller thanthe beam particles, and can not be transported through the∗ talanov@mx.ihep.sumagnetic structure of the machine. In the arcs and disper-sion suppressors, being enclosed by the machine tunnel,these particles are stopped due to the curvature of the ac-celerator ring and affect the radiation field mainly in the re-gions close to the point of the original collision. In contrastto this, secondaries produced in elastic interactions havea different behaviour. Elastic collisions of beam particleswith the residual gas nuclei give in the final state energeticprotons with a momentum much closer to the initial one.These quasi–beam particles can successfully travel with thebeam long distances through the lattice to interact with themachine equipment far from the point of the primary colli-sion [3].3 THE CHOICE OF THE STUDIEDMACHINE LENGTHThe choice of the machine length which is necessary tointroduce in the simulation of the background, induced byproton losses, as well as how to account for different ori-gins of this background, depends on the position of thestudied region in the machine structure. The values of theresidual gas densities determine the rate of the beam pro-tons losses in a particular section of the machine and thusthe absolute values of the machine induced background.Recently numbers became available for the H2 equivalentaverage gas density in the vacuum chamber of the LHCIRs, together with the residual gas composition in the cryo-genic and room temperature vacuum chambers [4]. Thegiven numbers can be compared with the previously pub-lished data on the residual gas composition [5], taking intoaccount the already estimated values for the machine back-ground, induced by the proton losses in the arc cell, dis-persion suppressor and straight section of IR8 [6]. This al-lows one to conclude that limiting the studied region to thestraight section part of the insertion region will be sufficientto estimate the values of the secondary fluxes, induced bythe beam–gas interaction, for the low luminosity insertioncase.4 PARTICLE CASCADE SIMULATIONInteractions of the beam protons with the nucleus of theresidual gas in the straight section and the transport of thesecondary cascades were simulated by the IHEP MARSprogram [7]. An overall threshold of 20 MeV on the par-ticle kinetic energy was applied to the simulation of thetransport for both hadrons and muons. Secondary particleswere transported along the straight section up to the scoringFigure 1: Number of hadrons as a function of primaryproton-nucleus interaction distance to IP2.plane, located close to the IP, where the trajectory of eachparticle was cut and particle characteristics were recorded.These characteristics included particle kinetic energy, di-rection cosines and coordinates of particle track crossingthe scoring plane. In order to be able to provide the sub-sequent reconstruction of secondary particle fields in thepresence of the residual gas density distribution, each par-ticle track was tagged with the distance to the interactionfrom the point of the primary proton–nucleus collision, andthe type of the residual gas nucleus, with which the interac-tion occurred. In addition, for each particle which fell intothe recorded source, the coordinates of the hadron–nucleusinteraction point were kept, where this particular hadronwas born, or, for muons, the parent pion. This made itpossible to trace back the history of the background parti-cles and not only to study the contribution to the machineinduced background from different parts of the insertionregion, but also to spot the sections where most of the sec-ondary particles that reach the experimental zone escapefrom the structure of the machine.5 SECONDARY PARTICLE FLUXESThe results of the simulations, folded with the gas den-sity profiles, are presented in Figures 1–4, for the three sce-narios of the LHC operation [8] — a basic one, “3rd year+90 days”, labeled Year 3, and for two other cases, “1styear after 70 days” and “2nd year +10 days”, labeled Year 1and Year 2 respectively. At the top of each figure a sketchof the corresponding machine structure is given, which in-cludes all the major elements of the straight section.The first two figures give the number of hadrons andmuons in the insertion region, as a function of the primaryproton–nucleus interaction distance to the interaction point.The values of secondary flux eg. for muons can be obtainedFigure 2: Number of muons as a function of primaryproton-nucleus interaction distance to IP2.by integrating the given distributions along the S axis. Thisgives the value of 1.03× 106 muons/s for the muon flux un-der the “3rd year +90 days” LHC running conditions, andthe values of 0.17× 106 and 0.75× 106 muons/s, for the“1st year after 70 days” and “2nd year +10 days” cases,respectively. The relatively smaller values for the first twoyears of the accelerator run reflect the decreased value ofthe nominal current at that period, which is assumed to be20 % in the first year, and 30 % in the second year of LHCoperation. As for the values of the secondary hadron flux,they are about an order of magnitude higher than the corre-sponding muon flux values for all three cases.In the basic case, after 90 days of LHC operation in thethird year, about 80 % of the background appears to be in-duced by the beam–gas interactions in the region of the in-jection absorber TDI. Among the other auxiliary peaks inthe gas density profiles, the most significant increases canbe seen in the injection line magnets region for the case ofthe second year operation, but even there it does not exceedthe effect of the peak in the TDI. Thus the region of the in-jection shielding is currently the most critical region of thestraight section, because of the residual gas density.6 BACKGROUND FORMATIONThe results, presented in Figures 3 and 4, show the sec-ondary fluxes in the insertion region from a different pointof view. These two figures give the number of hadronsand muons as a function of the distance to the IP from thepoint of the hadron–nucleus interaction, where a particu-lar hadron was born, or, for muons, from the origin of theparent pion. Since these distributions are obtained usingthe same normalization, as those in Figures 1 and 2, theypresent the same integral values for the secondary fluxes,but distributed in a different way along the S axis of theFigure 3: Number of hadrons as a function of last proton-nucleus interaction distance to IP2.Figure 4: Number of muons as a function of last proton-nucleus interaction distance to IP2.straight section. So by their definition Figures 3 and 4show the sections of the machine structure, where the ac-tual background particles, rather than their ancestors, areproduced.The distributions of hadrons in Fig. 3 demonstrate thatthe particles that interact with the nuclei of the residual gas,for example, around the TDI, do not necessarily producea secondary cascade, which will contribute to the back-ground, in the region close to the point of the primary in-teraction. The distributions still have distinct peaks in theposition of the TDI, formed by the particles, escaping di-rectly from the TDI–D1 drift space. But in the new rep-resentation this peak is now followed by a further increasealong the inner triplet, illustrating the previously discussed“bottleneck” effect [3]. Generally for all three profiles ofthe gas density, the closer to the scoring plane the regionof the last interaction, the more secondary hadrons are col-lected from it. In the muon distributions, the contributionof the D2–Q1 section is almost constant, with peaks in be-tween the elements, where parent pions are not stopped bythe material of the magnets.7 CONCLUSIONThe introduction of realistic profiles for the averageresidual gas density in the machine induced backgroundsimulations in the low luminosity insertion regions enablesthe estimation of contributions of secondary particle fluxesfrom the different parts of the straight section. The new es-timates can be compared with the ones that were obtainedfor IR8 [6] and which were based on the previously pub-lished data on the residual gas composition. This compar-ison shows an increase by a factor of 5 in the secondarymuon and hadron flux at the entrance to the experimentalcavern. The increase for the scenarios of the long–termLHC operation is almost entirely due to the assumed veryhigh residual gas density in the region of the TDI absorber.8 REFERENCES[1] I. Azhgirey et al. “Background muons generated in the CMSdetector area by the beam losses in the LHC”. In: One DayWorkshop on LHC Backgrounds, CERN, Geneva, March 22,1996.[2] The LHC Study Group. “The Large Hadron Collider Concep-tual Design”. CERN-AC-95-05, Geneva, 1995.[3] I. Baishev et al. “Proton losses upstream of IP8 of LHC”.CERN LHC Project Report 500, Geneva, 2001.[4] I.R. Collins and O.B. Malyshev. “Dynamic Gas Density in theLHC Interaction Regions 1&5 and 2&8 For Optics Version6.3”. CERN LHC Project Note 274, Geneva, 2001.[5] A. Mathewson. “First Estimates of the Gas Density in theLHC Vacuum System”. In: One Day Workshop on LHCBackgrounds, CERN, Geneva, March 22, 1996.[6] I. Azhgirey et al. “Methodical study of the machine inducedbackground in the IR8 of LHC”. CERN LHC Project Note258, Geneva, 2001.[7] I. Azhgirey and V. Talanov. “The status of MARS programcomplex”. In: Proc. of XVIII Workshop on the charged par-ticles accelerators, Protvino, 2000, vol. 2, p. 184–187.[8] I. Azhgirey et al. “Calculation of the machine induced back-ground in the IR2 of LHC using new residual gas density dis-tributions”. CERN LHC Project Note 273, Geneva, 2001.

Machine Induced Background in the Low Luminosity Insertions of the LHC

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