Aging and Long Term Operation

1. Aging and Long Term Operation

2. Aging Effect in Wire Chambers The degradation of operating conditions of wire chambers under sustained irradiation are the main limitation to the use of gas detector in high-energy physics. ‘Classical aging effects’ are deposits formed on electrode surfaces by chemical reactions in avalanche plasma near the anode. During gas avalanches many molecules break up and form free radicals (unionized atomic or molecular particles with one or more unsatisfied valence bonds). Free-radical polymerization is regarded at the dominating mechanism of wire chamber aging

3. Aging Effects (cont) Free radicals either recombine to form the original molecules or cross-linked molecular structures of increasing weight Leads to the formation of deposits (conducting or insulating) on electrode surfaces. • Decrease of the gas gain (due to modification of electric field) • Excessive currents • Sparking and self-sustained discharges • Radiation-induced degradation depends on • the nature and purity of the gas mixture • different additives and trace contaminants • materials in contact with the gas • materials of the electrodes • electric field configuration

4. Aging Effects in Wire Chambers

5. Premature aging in Ar/CH4 Free radicals are hydrogen deficient and are therefore able to make bonds with hydrocarbon molecules. Therefore CH4 polymerizes in the avalanche plasma, which causes premature aging. Aging rate of Ar/hydrocarbon gases can be reduced by adding oxygen-containing molecules, which allows large systems to operate at low intensity with only a small performance loss. Not trustworthy for long-term, high-rate experiments.

6. Silicon Deposits Si-deposits on anode wires Silicon has been detected in the analysis of many wire deposits, although the source has not been clearly identified in all cases. Si-compounds can be found in many components including lubricants, adhesives, rubber, silicon-based grease, oils, O-rings, fine dust, gas impurities, diffusion pumps, molecular sieves, and many more. Most dangerous are Si-lubricant traces used for the production of gas system components. Cleaned by flushing the system with DME.

7. The Malter Effect Microscopic insulating layer deposited on a cathode from quencher dissociation products and/or pollutant molecules. Some metal oxide coatings, absorbed layers or even the cathode material itself may not be initially conducting enough inhibit neutralization of positive ions from the avalanche. These ions generate a strong electric field across the dielectric film and cause electron field-emission at the cathode. Positive feedback between electron emission at the cathode and anode amplification leads to the appearance of dark current, increased rate of noise pulses and finally exponential current growth (classical Malter breakdown) Adding water prevents Malter discharges, because water increases the conductivity of partially damaged electrodes

8. Malter Effect Malter Effect first imaged in the CRID RICH detector. Wire deposits in CH4+TMAE after a charge dose of 6x10-3 C/cm

9. Ionization Density Detector lifetime depends on ionization density and in turn on the irradiation rate, particle type and energy. Space-charge effects (at large current densities) reduce the electron energy in the avalanche and thuse the ion and radical density in the avalanche plasma. This can be related to the charge density and total energy dissipated in the detector from incoming particles. • Counting rate capabilities are limited by space-charge effects in the avalanches • gain reduction • formation of self-quenching streamers

10. Ionization Density Transition between proportional and streamer mode depends on both the HV but also the primary ionization density. α-particles may reach the streamer mode for moderate multiplication factors. Streamers produces a densely ionized and low resistivity plasma between the anode and cathode which may lead to spark events. Sparks may ‘prime’ the electrode surface and begin the build-up of deposits. Tips left from sparks enhance the electric field locally. The Malter current and electron jets from these may be as significant as heavily ionizing particles.

11. Aging from α-particles Energy loss per α track can be 103 times larger than primary ionization with X-rays or MIPS. This ages shows up as hair-like deposits within the irradiated area. Aging rate in Ar/CF4/CH4 ~100x higher in a 100 MeVα-beam than with a Fe55 sources with similar current densities.

12. High Voltage Dependence of aging properties on HV may relate to the electric field strength on the anode surface. This enhances inelastic processes and determines the electron temperature in avalanches. Detector lifetime decreases nearly exponentially with increase of anode voltage. Aging rate can also be accelerated by avalanche formation at the cathode (if field there > 10-20 kV/cm) Lifetime of ATLAS muon drift tubes as a function of HV for different particle fluxes

13. High Voltage Triple GEM detectors are more reliable and radiation hard than double GEMs, with the same total gain. This gain is due to the reduction in the electric strength across a single GEM. Triple GEM Detector of the COMPASS experiment GEMs age less than other wire-detectors because multiplication and readout are separate (gas amplification only within GEM holes) and the rate of impurity polymerization caused by the lower electric field.

14. Size of irradiated Area Detector lifetime depends on the size of the irradiated area. Aging rate increases in the direction of serial gas flow. Progressive deterioration of MIP efficiency in the direction of serial gas flow Some aggressive free radicals may diffuse within the irradiated area and react with other avalanche generated dissociative products, construction and electrode materials and enhancing aging effects with increasing gas usage.

15. Useful Guidelines for Wire Detectors • Carefully choose construction materials that are radiation hard and have appropriate outgassing properties • Limited set of aging resistant gases can be used in high intensities experiments: CF4, C02,O2, H20. • Validate assembly procedures and ensure maximal cleanliness • Carefully control any anomalous activity in the detectors: dark currents, changes of anode current and remnant activity in the chamber when the beam goes away • If aging effects are observed add oxygen-based molecules; operations with CF4 decreases risk of Si polymerization • Surface conductivity of electrodes is important because it relates closely to operational capability at high ionization densities

16. RPCs and Aging Efficiency Drop with increasing current The two B-factories, Belle and BaBar use RPCs in streamer mode. High currents appeared in Belle’s RPCs almost immediately upon installation. This was due to the formation of hydrochloric acid formed due to the presence of high levels of water. Cured by replacing plastic rubes with copper cones

17. STM Imaging of Anodes Virgin Glass Surface “Good” anode “Bad” anode Surfaces etched by HF acid formed from water and Freon in the gas

18. Aging of BaBarRPCs The BaBarRPCs were made of Bakelite coated with Linseed oil. A permanent reduction in efficiency was caused by the lack of polymerization in the linseed oil and the formation of oil droplets under the high temperatures and currents.

19. Images of RPC Aging Damaged Cathode Raw Glass Damaged Anode AFM scans Raw Glass Damaged Electrode SEM scans

20. Aging in Gaseous PhotoDetectors Gaseous photodetectors need to ensure efficient conversion of UV photons into electrons and to detect single photoelectrons. Systematic aging studies of TMAE and TEA vapors. TMAE has best quantum efficiency, but high gas gain loss due to deposits on anode wires. TMAE has a larger aging rate because the molecule is more fragile. The aging rate in both is inversely proportional to the anode wire diameter.

21. Damage to MSGCs Discharges measured in the CMD MSGC prototypes Strip Damage due to discharges and sparks