Background studies with HERA beam

1. Background studies with HERA beam Torsten Limberg

2. Overview of studies with beam Synchrotron radiation background studies: • Beam Based Alignment • Dedicated e+ runs with special filling patterns Positron particle background studies: • Collimator studies upstream of experiments • Beam tail studies with scrapers and increase of positron tail density with HERMES target • Beam position/angle scans at IP Proton background studies: • Background rates vs. local IR pressure bumps • Ramp detector solenoid to check for trapped ions • Use e+ beam as vacuum measurement inside detectors

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

4. BBA Method Principle:

5. BBA Results Controlled Bridge movement successfully measured Found magnets sufficiently close to Design positions (here at ZEUS)

6. e+ only Runs • Separates synchrotron radiation background (linear in positron current) from positron particle background (quadratic in good approximation) • Detector drift chamber currents vs. time differentiates between directly hitting synchrotron photons and backscattered radiation (needs special fill patterns) • Determine proton contribution to background rates

7. H1 Drift Chamber Current vs. Ie+

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

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

10. Beam Tail Studies with Scrapers

11. Scraper Measurement

12. Result of Scraper Studies • Measured horizontal and vertical tails of positron density distribution with scrapers: • Similar to 2000 running • Increased pressure in HERMES gas target to reduce positron life time. Repeated scraper measurement and found tail population increased by a factor of ten: • No significant deterioration of background conditions (at 4 mA)

13. Beam Position and Angle Scans at Ip’s

14. Results of Beam Position Scans … nothing unexpected

15. Studies of Proton Background • Measure increase in background rates for local pressure increase at different positions in the IR (Pressure bumps by controlled out-gassing of Hydrogen while heating TSP pumps) • Exclude ion trapping in solenoid fields • Measure detector vacuum with e+ bremsstrahlung rates

16. Controlled out-gassing by heating TSP Pumps

17. Measuring the Detector Vacuum with the e+ Beam 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

18. The Solenoid ExperimentTest for ions trapped by the solenoid fieldsFriday, late shiftrun down H1 Solenoid at the end of a luminosity run with keeping collisions and with chambers on RadMon H1 Solenoid CJC2 eBeam Current

19. Summary • Beam Based Alignment excluded grossly misaligned magnets as the source of the problem • Dedicated e+ runs yielded: • direct synchrotron radiation photons hits can be reduced to small values (between noise level and 10% of e+ background) with orbit optimization • Backscattered synchrotron radiation from absorbers is the main photon source • Ratio between positron background (synchrotron radiation and particles) and proton background • Proton background from beam-gas collisions is at least as severe as the positron background Backscattered photon rate can reduced by absorber plating and shielding => leaves proton background as the major problem • Proton background studies: • Excluded high density ion trapping (Solenoid experiment) • Provide data for of vacuum profile and detector sensitivity • Controlled out-gassing at TSP pumps • Measuring average detector vacuum with e+ bremsstrahlung rates lead to self-consistent model for vacuum pressure profile and detector response.