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Cleaning

Cleaning. Chamber gases and impurities Tolerable impurity levels Thermal desorption Particle induced desorption M. Taborelli’s conclusions and suggestions Open questions. Chamber gases and impurities. Chamber gases - advantages: N 2 :

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Cleaning

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  1. Cleaning • Chamber gases and impurities • Tolerable impurity levels • Thermal desorption • Particle induced desorption • M. Taborelli’s conclusions and suggestions • Open questions

  2. Chamber gases and impurities • Chamber gases - advantages: • N2: • Same as SPS chambers (experience, 2004 tests for ageing). • In case of leak – small changes in ionization properties. • Noble gas (e.g. Ar): • Cleaning with glow discharge done with noble gases. • Cleaning of gas with getter pumps before filling. • Possible problems with ageing: • Electronegative gases (O2, CO, CO2, H2O, …) capture electrons - drift velocity reduced by a factor ~1000  reduction in signal height. • Polymerization (should not pose a problem when properly cleaned and organic materials are avoided in the production process.) • Changes in ionization properties through change of gas composition.

  3. Chamber gases and impurities • Impurities/additives (like e.g. H2, CO2, CH4) change the chamber operation characteristics: • Drift velocity, • Recombination losses, • Gain (ionization/cm of charged particle), • Onset of gas amplification, • Flatness of signal vs voltage in the ionization region … • These effects are often used intentionally: • NuMi chambers add 2% H2 to He to suppress gas amplification, • CO2 additives to increase drift velocity …

  4. Tolerable impurity levels • After some literature search we have not found data which tells us exactly, what kind of impurities we can tolerate in which chamber gas. • We came up with an estimate that electronegative impurities should stay below ppm level to avoid electron attachment (hep-ex/0212011 (2002), NIM B 187 (2002) 535 – 547, NIM B 179 (2001) 412 – 435). • But no estimate for other impurities.

  5. Thermal desorption (M. Taborelli) • Calculation for the SPS ionization chamber layout and 1 bar • Thermal desorption (after 20 years) baked Al: • Impurity level: 1.6 10-4, mainly H2, 1% CO (1.6 10-6), < 1% CO2, O2 and H2O

  6. Particle induced desorption • Estimation of number of charged particles at the chamber locations (from the showers induced by local proton losses running at quench level): • Cleaning efficiency: 104, • Required life time of operation: 20 years, 4000 hours / year, • 10 % of time at 450 GeV and 90 % of the time at 7 TeV. • The drift velocity of electrons and ions created by ionization in the chamber stay around/below the thermal velocity  do not induce desorption.

  7. Particle induced desorption (M. Taborelli) • Calculation for the SPS ionization chamber layout and 1 bar • Particle induced desorption chemically cleaned Al (data not available for baked Al): • Maximum impurity level (all releasable gas): 7 10-3, mainly CO and CO2;H2 levels are higher (?) • Assume all “MIPs” are electrons (1 - 10 MeV)  impurity level • BLMA/BLMS: 10-8 (negligible) • BLMC: 10-3 (almost all desorbed) • assume all “MIPs” are photons (10 MeV)  impurity level • BLMA/BLMS: 4 10-11 • BLMC: 4 10-6

  8. M. Taborelli’s conclusions and suggestions • Thermal desorption acceptable if cleaned according to CERN standard for UHV application and backed in vacuum before filling. • Particle induced desorption: for BLMC possibly further cleaning necessary, for instance glow discharge during filling process. • Systematic He leak test of all chambers before baking. • Glow discharge: • needs to be done in a noble gas (use Ar as chamber gas?) • Insulators needs to be shaped especially to avoid metal coating by sputtering. • Cleaning of working gas before filling: advantage of nobel gas because NEG pumps can be used. • Rest gas (and outgasing) is dominated by H2. • Avoid closed volumes in design (from welding/brazing, from tubes, …) • Avoid threaded rod (use rod with treaded ends). • Feed through in one piece (no brazing).

  9. M. Taborelli’s conclusions and suggestions • Foresee baking of filling hoses and tubes and getter cleaning during filling. • Clean before weld. • Rather weld than braze (e.g. feed through). Define procedures for welding (brazing). • Avoid organic material in production process. • Completely penetrating welding to avoid pockets.

  10. M. Taborelli’s conclusions and suggestionsCleaning and Filling Procedure • CERN standard cleaning for UHV procedure • ultrasonic bath of the alkali detergent NGL 17.40 Alu from NGL Cleaning Technologies at 60 degree C • rinsing with cold demineralised water jet (conductivity < 5 uS cm-1) • immersion in hot ultrasonic demineralised water bath • drying with compressed dry nitrogen and afterwards in a hot (80 degree) air oven • Mount (several chambers at same time) • Pump (1-3 hours) • Leak detection (He from outside – for example in the oven) • Bake • Possibly: glow discharge (Ar or He at ~10-3 mbar) • Pump • Fill

  11. Open questions • Particle species and energy spectrum  GEANT simulations – not the same for BLMA/BLMS and BLMC. • Suggest to measure the signal characteristics with the LHC prototype chambers for Ar and N2 (possibly with additives) and defined levels of impurities (the ones from M. Taborelli’s estimate). • Cleaning procedure (and chamber gas) for the two chamber types. • Glow discharge necessary for BLMC?

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