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

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

3. D.SampsonidisAthens, 17-04-2003 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 (E low ), photons, e, μ, hadrons) π / K → μ dominate at low p T b, c → μ dominate at high p T

4. D.SampsonidisAthens, 17-04-2003 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) To obtain detection efficiencies for the muon detectors Small prototypes were exposed to neutrons and photon sources Monte Carlo simulations 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 2.3<|η|<2.7, 1.4<|η|<2.3 |η|<1.4

5. D.SampsonidisAthens, 17-04-2003 Background Rates MDT Rate Capability Drift tube performance adequate at occupancy levels of 30% Counting rates should remain below 300 Hz/cm 2 Accumulated charge 1 Cb/cm, for rate 500 Hz/cm 2, gain 2x10 4, integrated luminocity 10 42 cm -2 Rate at Inner μ-stations MDT counting rate can reach 100 Hz/cm 2 Pseudorapidity dependence of the counting rate in the inner most MDT station at nominal luminocity photon fluence (kHz/cm 2 ) at nominal luminosity neutron fluence

6. D.SampsonidisAthens, 17-04-2003 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 2x10 4 a charge deposit of 0.6 Cb/cm wire for 10 years of high luminosity is expected

7. D.SampsonidisAthens, 17-04-2003 BIS What can Neutrons (E n >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. α particles Have equivalent ionization to neutron recoil atoms and may imitate the single charge recoil nuclei very good. ion (MeV)0.410 (MeV)0.036 (MeV)0.024 (1/cm 2 sec)7.23 ∫ E n (MeV/cm 2 sec) 2.95 ∫ E γ (MeV/cm 2 sec) 2.46 ∫ E n / ∫ E γ 1.2 MDT aging (Neutrons) Evaluation of the ionization produced by fast neutrons in ATLAS muon detectors (Brookhaven group)

8. D.SampsonidisAthens, 17-04-2003 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.

9. D.SampsonidisAthens, 17-04-2003 Irradiation with α ( 222 Rn) Advantages Uniform internal irradiation No deterioration of the electric field in the tube Known 222 Rn activity Radon gas emits alpha particles 226 Ra 222 Rn 218 Po 214 Pb 214 Bi 214 Po 210 Pb α 1620 y 5.5 MeV 6.0 MeV 7.7 MeV α 3.8 d α 3.05 m β - 26.8 m β - 19.7m α 16,37 μs 222 Rn 4 h later (radioactive equilibrium) 222 Rn +dts : 3α + 2β -

10. D.SampsonidisAthens, 17-04-2003 Aging tests Set Up 222 Radon Source Gas Flow through 226 Ra source 20.6 KBq Flow duration and initial 222 Rn 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 222 Rn activity. Ar 93% CO 2 7 % outlet Gas Radon source pump Flow meter Lucas Cell Reference tubes Gas gain monitoring by pulse-height spectra and comparing to the reference tubes

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

12. D.SampsonidisAthens, 17-04-2003 MDT Aging tests 6x4 Drift tubes (4 tubes in operation) BNL electronics Gas Ar+N 2 +CH 4 (96:3.9:0.1) Parallel distribution April 2002 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. 222 Rn : 4.91 KBq/tube

13. D.SampsonidisAthens, 17-04-2003 (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) 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) April 2002 Wire before irradiation Wire after irradiation

14. D.SampsonidisAthens, 17-04-2003 MDT Aging tests July 2002: with less radon 60 Bq/tube Sept. 2002: 37 Bq/tube Improvements of the gas distribution system Use the nominal gas Ar:CO 2 (93:7) Reference tubes were contaminated with Radon (Sept.) After radon irradiation Reference tube HV 2.4 KV HV 2.6 KV Pressure effect

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

16. D.SampsonidisAthens, 17-04-2003 Looking for the alphas, HV scan 2.8 kV2.7 kV2.5 kV 2.3 kV2.0 kV1.8 kV March 2003 72 ± 1 Bq/tube

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

18. D.SampsonidisAthens, 17-04-2003 ΔE of α The 6.5 MeV α give 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 ΔΕ (MeV) Energy scale has been calibrated by comparison with the charge detected on soft γ

19. D.SampsonidisAthens, 17-04-2003 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.

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

21. D.SampsonidisAthens, 17-04-2003 Summary We have a setup for α-particles irradiation. We control radon concentration. Ionization produced by the alphas of 6.4 MeV measured to be ~1.3 MeV. The ionization of the α is 73 times larger than soft γ With 222 Rn of 5 KBq and collected charge 0.003 Cb/cm using a gas Ar:N 2 :CH 4 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