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Aging Studies for the ATLAS MDTs

Aging Studies for the ATLAS MDTs. Dimos Sampsonidis for the ATLAS group of Thessaloniki. Outline. Background Environment at LHC Impact of the background on muon spectrometer Neutrons Aging Setup Results Collected charge calculation Summary. Background Environment @ LHC.

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Aging Studies for the ATLAS MDTs

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  1. Aging Studies for the ATLAS MDTs Dimos Sampsonidis for the ATLAS group of Thessaloniki

  2. Outline • Background Environment at LHC • Impact of the background on muon spectrometer • Neutrons • Aging Setup • Results • Collected charge calculation • Summary Athens, 17-04-2003

  3. Background Environment @ LHC • Background sources • Primary collision products • Prompt muons and meson decays in flight • Semileptonic decays of heavy flavours (c,b,t→μX) and Gauge Boson decays (W,Z,γ(*) →μX) • Hadronic debris • Decays in flight (h→μX) • Showers in Cal. decay into muons • Hadron punch-through • Radiation background • pp collision debris • Primary hadrons interact with forward Calorimeter, shielding, beam pipe and other materials • (nuetrons (Elow), photons, e, μ, hadrons) π / K→ μ dominate at low pT b, c →μ dominate at high pT Athens, 17-04-2003

  4. Background fluence Neutron and photon fluence have been computed taking into account the material distribution as well as the magnetic field in the ATLAS detector and exp. hall. (Bat94, Fer95, Fer96) 2.3<|η|<2.7, 1.4<|η|<2.3 |η|<1.4 The expected photon flux as a function of photon energy in different regions The expected neutron flux as a function of neutron energy in different regions To obtain detection efficiencies for the muon detectors Small prototypes were exposed to neutrons and photon sources Monte Carlo simulations Athens, 17-04-2003

  5. Rate at Inner μ-stations Background Rates Pseudorapidity dependence of the counting rate in the inner most MDT station at nominal luminocity MDT counting rate can reach 100 Hz/cm2 • MDT Rate Capability • Drift tube performance adequate at occupancy levels of 30% • Counting rates should remain below 300 Hz/cm2 • Accumulated charge 1 Cb/cm, for rate 500 Hz/cm2, gain 2x104, integrated luminocity 1042 cm-2 photon fluence (kHz/cm2) at nominal luminosity neutron fluence Athens, 17-04-2003

  6. Impact of the background on the Muon Spectrometer performance • Momentum Resolution • Resolution degradation by space-charge effects. • Electric field changes → e drift velocity changes → r-t relation shifted → single wire resolution is deteriorated • Reconstruction Efficiency • High background levels resulting in large chamber occupancies. • Radiation Damage (Aging effects) • At background rates ~ kHz/cm, with gas gain 2x104 a charge deposit of 0.6 Cb/cm wire for 10 years of high luminosity is expected Athens, 17-04-2003

  7. BIS MDT aging (Neutrons) • What can Neutrons (En>0.1 Mev) do? • Ionization charge deposition can be hundred times larger than that of a muon. • aging : can increase charge per unit length of anode by factor of several to more than an order of magnitude. • Front End Electronics Overload NO measurements of the ionization are done so far for neutrons. Evaluation of the ionization produced by fast neutrons in ATLAS muon detectors (Brookhaven group) α particles Have equivalent ionization to neutron recoil atoms and may imitate the single charge recoil nuclei very good. Athens, 17-04-2003

  8. MDT aging tests @ Thessaloniki • Goals • Measure the ionization that an α produces in MDTs • Study the aging effects on the MDTs due to the • collected charge to the wire • Aging depends on total collected charge Q • Q=G R T ne (Gain x Rate x Time x Primaries) Cb/cm • How • Use α-particles to irradiate the MDTs. • Use of a radioactive gas (Radon) in order to enrich the tube gas and irradiate the MDTs internally. Athens, 17-04-2003

  9. α 16,37 μs α 3.8 d α 3.05m α 1620 y β - 26.8 m β - 19.7m 226Ra 222Rn218Po 214Pb 214Bi 214Po 210Pb 6.0 MeV 5.5 MeV 7.7 MeV 222Rn 4 h later (radioactive equilibrium) 222Rn +dts :3α + 2β- Irradiation with α (222Rn) Radon gas emits alpha particles • Advantages • Uniform internal irradiation • No deterioration of the electric field in the tube • Known 222Rn activity Athens, 17-04-2003

  10. outlet Reference tubes Gas Radon source pump Flow meter Ar 93% CO2 7 % Lucas Cell Aging tests Set Up • 222Radon Source • Gas Flow through 226Ra source 20.6 KBq • Flow duration and initial 222Rn concentration in the source specify the concentration in the tubes • Source is removed • Gas circulates 20 times at atm. pressure for homogeneity (1h) • Lucas Cell (α-scintillation detector) monitor the 222Rn activity. Gas gain monitoring by pulse-height spectra and comparing to the reference tubes Athens, 17-04-2003

  11. Rb 13.4 KeV Mo 17.4 KeV Ag 22.1 KeV HV 2850 V Gate 130 ns Thres. 70 mV ADC Spectra Aging tests Set Up Preamplifier γSource Fun IN/OUT Disc. HV Shaper Amplifier Gate Pulser (calibration) ADC VME Crate MXI2 Athens, 17-04-2003

  12. HV : 3.04 KV Thres : 80 mV ADC spectra from MDT with Radon and the Mo source, with time difference 17.5 h. The calibration source cannot be distinguished. April 2002 MDT Aging tests 6x4 Drift tubes (4 tubes in operation) BNL electronics Gas Ar+N2+CH4 (96:3.9:0.1) Parallel distribution 222Rn : 4.91 KBq/tube Athens, 17-04-2003

  13. Wire before irradiation Wire after irradiation April 2002 MDT aging tests Very high activity, Radon concentration was high After the 4 days of operation at ~3 KV the tubes was flushed with the nominal gas. The tubes were dead (!!!) (Q=0.003 Cb/cm) • (Surface) analysis of wire: • Check for deposits on the wire. • Elemental analysis of any deposits by X-ray analysis (CERN EST-SM group) (has not been done) Athens, 17-04-2003

  14. MDT Aging tests HV 2.4 KV HV 2.6 KV After radon irradiation July 2002: with less radon 60 Bq/tube Sept. 2002:37 Bq/tube Improvements of the gas distribution system Use the nominal gas Ar:CO2 (93:7) Reference tubes were contaminated with Radon (Sept.) Reference tube Pressure effect Athens, 17-04-2003

  15. V Ccalw G= Eγ f e Absolute Gain Calibration γ - Mo 17.4 keV 2750 V <HV < 2950 V Pulses from Generator In Test Input of the Preamplifiers (V) Eγ/w : number of ion pairs released in the gas by each γ conversion 670 e for the 17.4 KeV Athens, 17-04-2003

  16. March 2003 72 ± 1 Bq/tube Looking for the alphas, HV scan 2.8 kV 2.7 kV 2.5 kV 2.3 kV 2.0 kV 1.8 kV Athens, 17-04-2003

  17. Radon α Activity in Lucas Cell and MDTubes 222Rn (theor.) Lucas Cell MDTube Radon Monitoring for 8 days HV: 2 kV Gas Gain: 120 Difference in λ between the theoretical value for Radon and Lucas Cell and tubes is due to gas leakage. Athens, 17-04-2003

  18. ΔΕ (MeV) ΔE of α • The 6.5 MeV αgive a • peak at ADC channel 550 • Gas Gain at 2kV ~ 120 • Calibration An α (6.5 MeV) produces 73 times more primary electrons than γ (17.4 keV) ~ 40900 e Energy scale has been calibrated by comparison with the charge detected on soft γ Athens, 17-04-2003

  19. Calculation of the collected charge • Monte Carlo simulation in order to estimate the energy deposition in the tubes for the alphas and betas • Stopping powers of e- and e+, ICRU 37. • Energy of the α scaled according to our exp. measurement. • Detector cylindrical geometry. Athens, 17-04-2003

  20. Calculation of the collected charge 16.7 μCb/tube March 2003 72 ± 1 Bq/tube Athens, 17-04-2003

  21. Summary • We have a setup for α-particles irradiation. • We control radon concentration. • Ionization produced by the alphas of <E> 6.4 MeV measured to be ~1.3 MeV. • The ionization of the α is 73 times larger than soft γ • With 222Rn of 5 KBq and collected charge 0.003Cb/cm using a gas Ar:N2:CH4 the tubes ‘died’. • We continue irradiation of the tubes in a well controlled way in order to reach the value 0.6 Cb/cm of the collected charge Athens, 17-04-2003

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