aging studies for the atlas mdts

Aging Studies for the ATLAS MDTs

Dimos Sampsonidis

for the ATLAS group of Thessaloniki

D.Sampsonidis Athens, 17-04-2003

Outline

• Background Environment at LHC

• Impact of the background on muon spectrometer

• Neutrons

• Aging Setup

• Results

• Collected charge calculation

• Summary


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 muonsHadron 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


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 sourcesMonte 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


Background Rates

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

Rate at Inner μ-stations

MDT counting rate can reach 100 Hz/cm2

Pseudorapidity dependence of the counting rate in the inner most MDT station at nominal luminocity

photon fluence (kHz/cm2) at nominal luminosity

neutron fluence


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


BISWhat 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.

α particles

Have equivalent ionization to neutron recoil atoms and may imitate the single charge recoil nuclei very good.

<En>ion (MeV) 0.410

<Eγ> (MeV) 0.036

<Emuon> (MeV) 0.024

<Raten> (1/cm2sec) 7.23

∫En (MeV/cm2sec) 2.95

∫Eγ (MeV/cm2sec) 2.46

∫En / ∫Eγ 1.2

MDT aging (Neutrons)

Evaluation of the ionization produced by fast neutrons in ATLAS muon detectors (Brookhaven group)


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 QQ=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.


Irradiation with α (222Rn)

Advantages•Uniform internal irradiation

•No deterioration of the electric field in the tube

•Known 222Rn activity

Radon gas emits alpha particles

226Ra 222Rn 218Po 214Pb 214Bi 214Po 210Pbα

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

222Rn

4 h later (radioactive equilibrium)

222Rn +dts : 3α + 2β-


Aging tests Set Up222Radon 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.

Ar 93%CO2 7 %

outlet

Gas

Radon source pumpFlow

meter

Lucas Cell

Reference tubes

Gas gain monitoring by pulse-height spectra and

comparing to the reference tubes


Aging tests Set UpPreamplifier

Pulser(calibration)

Shaper Amplifier

Fun IN/OUT

Disc.

Gate

HV

ADC

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


MDT Aging tests

6x4 Drift tubes (4 tubes in operation)

BNL electronics

Gas Ar+N2+CH4 (96:3.9:0.1)

Parallel distribution

April 2002

HV : 3.04 KVThres : 80 mV

ADC spectra from MDT with Radon and the Mo source, with time difference 17.5 h.

The calibration source cannot be distinguished. 222Rn : 4.91 KBq/tube


(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


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:CO2 (93:7)

Reference tubes were contaminated with Radon (Sept.)

After radon irradiation

Reference tube

HV 2.4 KVHV 2.6 KV

Pressure effect


Absolute Gain Calibration

γ - Mo 17.4 keV

2750 V <HV < 2950 V

Pulses from GeneratorIn Test Input of the

Preamplifiers (V)

G=V Ccalw

Eγ f e

Eγ/w : number of ion pairs released in the gas by each γ conversion

670 e for the 17.4 KeV


Looking for the alphas, HV scan

2.8 kV 2.7 kV 2.5 kV

2.3 kV 2.0 kV 1.8 kV

March 200372 ± 1 Bq/tube


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

222Rn (theor.)

Lucas Cell

MDTube


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 eΔΕ (MeV)

Energy scale has been calibrated by comparison with the charge detected on soft γ


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.


Calculation of the collected chargeAr CO2

Wi (exp.) [eV] 26 33 [F.Sauli 1977] Gas Composition 0.93 0.07

Rn-222 Po-218 At-218 Bi-214 Po-214Energy / disintegration [keV] 5489 6001 6686 1.14 7687Npairs / disintegration 207981 227386 253336 43 291254Initial Radon Concentration 72.25 Bq in tube

Pb-214 Bi-214 Tl-210 Pb-210Energy / disintegration in Ar 36.22 31.82 61.37 34.66Energy / disintegration in CO2 45.96 41.69 77.26 35.63Energy / disintegration in mixture 36.90 32.51 62.48 34.73Npairs / disintegration 1398 1232 2367 1316

High Voltage Gain Npairs Q [Cb] Npairs Q [Cb] Total Q [Cb]2000 120 1.81E+13 2.90E-06 6.55E+10 1.05E-08 3.493E-04

High Voltage Gain Npairs Q [Cb] Npairs Q [Cb] Total Q [Cb]2000 120 5.03E+12 8.05E-07 1.80E+10 2.89E-09 1.67E-05

electrons

α particles

electrons

α particlesWithout leakage of the tube

With gas leakage of the tubeα particles electrons

16.7 μCb/tube

March 200372 ± 1 Bq/tube


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 0.003 Cb/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

aging studies for the atlas mdts

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