The ATLAS Muon Spectrometer1 has ~355,000 drift tubes installed into 1200 precision Monitored Drift Tube (MDT) tracking chambers arrayed over a 22 m high, 45 m long barrel-shaped detector.

The 2%-3% (100 GeV) momentum resolution and high efficiency of the spectrometer relies on maintaining 80 m single drift tube resolution. Significant performance degradation occurs for mis-calibration of the maximum drift time of a few ns .

The drift tube is the fundamental sensitive element in the spectrometer. It consists of a 3 cm diameter extruded aluminum tube, pressurized to 3 bar with 93%Ar, 7% CO2 gas mixture. A 50 m diameter gold-plated tungsten wire stretched coaxially to 350 grams. Operated at 3080 V, the gain is 20000.

The spectrometer’s precision coordinate is the radial distance of closest approach of an ionizing particle passing through a drift tube. The spatial resolution depends critically on RT transfer function relating drift time to track impact parameter.

We have constructed a mini-MDT chamber employing 96 closed-packed drift tubes arrayed in two multi-layers of 3 closed-packed layers each. Dedicated to online gas monitoring and calibrations, the chamber is staged at the ATLAS gas-mixing facility at CERN. It is configured with two independent gas distribution manifolds and enables simultaneous direct comparison from different input sources. It routinely samples the gas supply and return lines servicing the underground spectrometer.

The chamber uses standard ATLAS muon spectrometer readout electronics and has an automated data acquisition and analysis program. Plastic scintillators trigger on cosmic ray muons at 20 Hz, two-fold coincidence.

Drift times are measured and collected into drift spectra. Which are analyzed to extract the maximum drift time. The maximum drift time is sensitive to the gas composition as well as gas temperature and pressure. The gas temperature and pressure are measured by embedded sensors and collected drift times are corrected to standard conditions of 3000 mbar and 20 C. After these corrections any changes to the drift spectra are attributable to changes in the gas composition. expressed as drift radii. Also from these drift times track are reconstructed and used to iteratively in autocalibration1 process to extract an RT function.

The primary task of the Monitor Chamber is to provide continuous recording of gas properties and to flag deviations sufficient to signal a re-determination of the RT calibration constants.

A sensitive parameter to gas properties is the maximum drift time, Tmax. A 5 ns change in the maximum drift corresponds to ~75 m error in track drift radius- about equal to a drift tube’s intrinsic resolution. Left un-calibrated this change in Tmax degrades efficiency and momentum resolution by several percent2.

Drift times are affected not only by the gas composition, but also by the temperature and pressure in the monitor chamber. Corrections are computed & validated with data.

ns ns

mbarC

% ppm

ns ns

CONFIGURATION

Drift Time Calibration & Gas Monitoring of the ATLAS Muon Spectrometer Precision Chambers

Daniel S. Levin,1 Nir Amram2, Meny ben Moshe2, Erez Etzion2, Tiesheng Dai1, Edward Diehl1,

Claudio Ferretti1, Jeffery Gregory1, Mike Kiesel1, Rudi Thun1,Curtis Weaverdyck1, Alan Wilson1, Bing Zhou1

for the ATLAS Collaboration

1University of Michigan, Department of Physics 2Tel Aviv University, School of Physics and Astronomy

Presented at IEEE-NSS, Honolulu, 2007

Daniel S. Levin,1 Nir Amram2, Meny ben Moshe2, Erez Etzion2, Tiesheng Dai1, Edward Diehl1,

Claudio Ferretti1, Jeffery Gregory1, Mike Kiesel1, Rudi Thun1,Curtis Weaverdyck1, Alan Wilson1, Bing Zhou1

for the ATLAS Collaboration

1University of Michigan, Department of Physics 2Tel Aviv University, School of Physics and Astronomy

Presented at IEEE-NSS, Honolulu, 2007



drift spectrum (combined tube hits of a chamber partition)

The rising edge and trailing edge are fit with modified

Fermi-Dirac functions :

The maximum drift time is defined :

Tmax = P2(trailing edge ) - P2(leading edge)

In normal operation:

MDT Supply/Return gas lines ===Right /Left partitions.

gas flows at ~40 l/hr through both chamber partitions

Monitor -chamber

Electron drift

to anode

muon

Drift tube with high voltage anode wire at center (no B field)

32 /)(41

1)( PtPe

tPPtf

Above: drawing of chamber showing the two multi-layers. Readout electronics are hidden from view in faraday cages on the near side. Eight temperature sensors are embedded. (Not shown).

The results of one week’s output is shown above. In this plot:

1) Each point represents one hour of data. Measurement error is < 1 ns.

2) The Tmax is computed as the difference in the tail and leading edge fit parameters

3) The difference in the gas drift time is consistent with ~50 ppm H2O. (Diffusion of small amounts of water through the ceramic endplugs of the drift tubes is possible)

REFERENCES

PERFORMANCE

1. ATLAS MUON TDR, CERN/LHCC/97-22

2. R. Veenhof, “GARFIELD”, CERN Program Library W5050.

3. A. Wilson et al , “Z ”ATLAS Physics Workshop, Rome, (2005)

After corrections to temperature and pressure the Tmax from a reference gas source is very stable. In plot below the dispersion over 115 hours is only 0.6 ns

CONCEPT

Below: Monitor-chamber placed in the MDT gas system.

Left/Right sides can receive inputs from 4 sources:

1. Fresh mix MDT gas

2. MDT Supply gas from main trunk line

3. MDT Return gas from trunk line

4. Reference calibration gas

Above: The variation of Tmax to pressure, temperature, CO2 fraction and water are computed from the Garfield program3

Top: Temperature and pressure diurnal variations in the monitor chamber for a 115 hour data run with a reference gas source. Bottom: the Tmax is extracted and plotted vs elapsed hours. All deviations can be attributed to the small diurnal pressure/temperature fluctuation as shown in the top plot. The bottom plot shows Tmax before and after the corrections are applied.

MONITORING RESULTS

Day/Date

Monitor-chamber

Left: Muon track reconstructed by pattern recognition algorithm which finds the line tangent to the drift radii.

Drift radii obtained with an RT function from autocalibration2.

Right: Hit distribution for each tube layer of the chamber:

This shows:

all channels functioning

high efficiency

no spurious noisy channels

tube hits by chamber layer

The results of one month (Sept 07) output In this plot: Each point = one hour of data. Measurement error < 1 ns. The Tmax computed as the difference in the tail and leading edge fit parameters Tmax from both gas lines is stable < 1 ns

A small, but easily measurable difference in the input/output Tmax

On Aug 22: Ar flow interruption immediate effect on drift time

Day/Date

mbar

P =3.2 barBuffer (optional)

P =3.3 bar

Purifier

(optional)

Turbo pump

P =2.8 bar

Ar CO2

Fresh gas flow

5-10 Nm/hr

Freq

control

Circulation flow

~ 100 Nm/hr

retu

rn

fres

h m

ix

cali

brat

ion

supp

ly

surface building

underground building

parallel feed

15 MDT racks

P = 3.0 bar

ATLAS

Return line

Total length of circuit ~ 800 m

P = 3.0 bar

exhaust

Pressure sensors

Drift distance = Distance of

closest approach

+

nshours

tube #

layer 1 layer 4

layer 2 layer 5

layer 3 layer 6