Background studies with Background studies with HERA beam HERA beam

Torsten LimbergTorsten Limberg

Overview of studies with beamOverview of studies with beamSynchrotron radiation background studies:Synchrotron radiation background studies:

Beam Based AlignmentBeam Based AlignmentDedicated e+ runs with special filling patternsDedicated e+ runs with special filling patterns

Positron particle background studies:Positron particle background studies:Collimator studies upstream of experiments Collimator studies upstream of experiments Beam tail studies with scrapers and increase of positron tail density Beam tail studies with scrapers and increase of positron tail density with HERMES targetwith HERMES targetBeam position/angle scans at IPBeam position/angle scans at IP

Proton background studies:Proton background studies:Background rates vs. local IR pressure bumpsBackground rates vs. local IR pressure bumpsRamp detector solenoid to check for trapped ionsRamp detector solenoid to check for trapped ionsUse e+ beam as vacuum measurement inside detectors Use e+ beam as vacuum measurement inside detectors

Beam Based AlignmentBeam Based Alignment

Question: Question: Are magnets so far Are magnets so far off their design position that off their design position that steering through the steering through the Interaction Regions without Interaction Regions without radiating into the experiment radiating into the experiment is impossible?is impossible?

BBA MethodBBA Method

Principle:Principle:

in itia l o rb it

d ire c tion IP d ire c tio n a rc

M isa lig n ed q u ad is v a rie d in s tre ng th

R esu lting o rb it d iffe re n ce is ta k en o u t w ith tw o co rre c tio n co ilsQ u ad ru p o le o ffse t is ca lcu la te d fro m the n ec essary co rre c to r c h an g e s.

BBA ResultsBBA ResultsControlled Bridge movement successfully measured

Found magnets sufficiently close to Design positions (here at ZEUS)

e+ only Runs e+ only Runs

Separates synchrotron radiation background (linear in Separates synchrotron radiation background (linear in positron current) from positron particle background positron current) from positron particle background (quadratic in good approximation)(quadratic in good approximation)

Detector drift chamber currents vs. time differentiates Detector drift chamber currents vs. time differentiates between directly hitting synchrotron photons and between directly hitting synchrotron photons and backscattered radiation (needs special fill patterns)backscattered radiation (needs special fill patterns)

Determine proton contribution to background ratesDetermine proton contribution to background rates

H1 Drift Chamber Current vs. IH1 Drift Chamber Current vs. Ie+e+

ZEUS Drift Chamber Hits vs. TimeZEUS Drift Chamber Hits vs. Time(for the passing of a ‘single’ bunch)(for the passing of a ‘single’ bunch)

Positron Particle Collimator StudiesPositron Particle Collimator Studies

Positron particle collimators at 15 and 40 Positron particle collimators at 15 and 40 meters do not improve e+ particle meters do not improve e+ particle backgroundbackgroundSimulations show that they will not be Simulations show that they will not be effective against e+ particles scattering in effective against e+ particles scattering in an 80 meter range upstream of the an 80 meter range upstream of the detector detector

Beam Tail Studies with ScrapersBeam Tail Studies with Scrapers

Beam Loss Monitors

Scraper Jaws

Scraper MeasurementScraper Measurement

Result of Scraper StudiesResult of Scraper Studies

Measured horizontal and vertical tails of Measured horizontal and vertical tails of positron density distribution with scrapers:positron density distribution with scrapers:– Similar to 2000 runningSimilar to 2000 running

Increased pressure in HERMES gas target Increased pressure in HERMES gas target to reduce positron life time. Repeated to reduce positron life time. Repeated scraper measurement and found tail scraper measurement and found tail population increased by a factor of ten:population increased by a factor of ten:– No significant deterioration of background No significant deterioration of background

conditions (at 4 mA) conditions (at 4 mA)

Beam Position and Angle Scans at Ip’sBeam Position and Angle Scans at Ip’s

Results of Beam Position ScansResults of Beam Position Scans

… nothing unexpected

Studies of Proton BackgroundStudies of Proton Background

Measure increase in background rates for Measure increase in background rates for local pressure increase at different local pressure increase at different positions in the IRpositions in the IR(Pressure bumps by controlled out-gassing of Hydrogen while heating TSP (Pressure bumps by controlled out-gassing of Hydrogen while heating TSP pumps)pumps)

Exclude ion trapping in solenoid fieldsExclude ion trapping in solenoid fieldsMeasure detector vacuum with e+ Measure detector vacuum with e+ bremsstrahlung ratesbremsstrahlung rates

Controlled out-gassing by heating TSP PumpsControlled out-gassing by heating TSP Pumps

Comparison of e+ bremsstrahlung rates from non-colliding bunches (measured with the ‘Luminosity Monitor’) yields a pressure increase between factor 5 and 10 between the years 2000 and 2002

Measuring the Detector Vacuum with the e+ Beam

The Solenoid ExperimentThe Solenoid ExperimentTest for ions trapped by the solenoid fieldsTest for ions trapped by the solenoid fields Friday, late shiftFriday, late shiftrun down H1 Solenoid at the end of a luminosity run with keeping run down H1 Solenoid at the end of a luminosity run with keeping collisions and with chambers oncollisions and with chambers on

H1 SolenoidRadMon CJC2

eBeam Current

SummarySummaryBeam Based Alignment Beam Based Alignment excluded grossly misaligned magnets as the source of the excluded grossly misaligned magnets as the source of the problemproblem

Dedicated e+ runs yielded:Dedicated e+ runs yielded:– direct synchrotron radiation photons hits can be reduced to small values (between direct synchrotron radiation photons hits can be reduced to small values (between

noise level and 10% of e+ background) with orbit optimizationnoise level and 10% of e+ background) with orbit optimization– Backscattered synchrotron radiation from absorbers is the main photon sourceBackscattered synchrotron radiation from absorbers is the main photon source– Ratio between positron background (synchrotron radiation and particles) and proton Ratio between positron background (synchrotron radiation and particles) and proton

backgroundbackground– Proton background from beam-gas collisions is at least as severe as the positron Proton background from beam-gas collisions is at least as severe as the positron

backgroundbackground

Backscattered photon rate can reduced by absorber plating andBackscattered photon rate can reduced by absorber plating andshielding => leaves proton background as the major problemshielding => leaves proton background as the major problem

Proton background studies:Proton background studies:– Excluded high density ion trapping (Solenoid experiment)Excluded high density ion trapping (Solenoid experiment)– Provide data for of vacuum profile and detector sensitivityProvide data for of vacuum profile and detector sensitivity

Controlled out-gassing at TSP pumpsControlled out-gassing at TSP pumpsMeasuring average detector vacuum with e+ bremsstrahlung ratesMeasuring average detector vacuum with e+ bremsstrahlung rates

lead to self-consistent model for vacuum pressure profile and detector response.lead to self-consistent model for vacuum pressure profile and detector response.