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the muon spectrometer of the atlas detector at the large hadron collider ( lhc ) is designed for the measurement of muon momenta with an accuracy of @xmath2 over a wide energy range reaching @xmath3 resolution at 1  tev .
the muon trajectories in the 0.4  t field of a superconducting air - core toroid magnet are measured in three stations of precision drift chambers chambers , the monitored drift - tube ( mdt ) chambers .
the mdt chambers consist of two triple or quadruple layers of aluminium drift tubes of 0.4  mm wall thickness and 29.170  mm inner diameter with a @xmath4 m diameter gold - plated tungsten - rhenium wire at the center .
the tubes are filled with ar : co@xmath5  ( 93:7 ) gas mixture at a pressure of 3  bar .
operated at a gas gain of 2@xmath610@xmath7 , the drift tubes must provide a spatial resolution of better than 100  @xmath1 m in order to reach the required chamber position resolution of better than 40  @xmath1 m  @xcite with a sense wire positioning accuracy of 20  @xmath1 m which has been achieved in the chamber serial production  @xcite .
the operating conditions of the atlas muon chambers at the lhc are characterized by unprecedentedly high neutron and @xmath8 backgrounds ( see figure  [ fig1 ] ) .
the chambers will experience background count rates ranging from 40  hz/@xmath0 to 500  hz/@xmath0 including a safety factor of 5  @xcite corresponding to count rates per tube between 45 and 300  khz for tube lengths of @xmath9  m.    in summer 2003 , one of the largest mdt chambers constructed for the atlas muon spectrometer containing 432 drift tubes of 3.8  m length  @xcite has been tested at the gamma irradiation facility  @xcite at cern with a 740  gbq @xmath10cs source in a 100  gev muon beam . the chamber was equipped with the final read - out electronics for atlas which measures both the drift time and the pulse height of ionizing particle tracks .
the chamber was operated at photon count rates of up to 990  hz/@xmath0 .
a silicon strip detector telescope ( see figure  [ fig2 ] ) was used as external reference to determine the space - to - drift time relationship and the spatial resolution and efficiency of the drift tubes .
expected background count rates in hz/@xmath0 ( including a safety factor of 5 ) in the mdt chambers in different regions of the atlas muon spectrometer .
the cross section of one quadrant of the atlas detector containing the lhc proton beam line is shown . ]
top view of the experimental set - up at the gamma irradiation facility at cern with a mdt chamber consisting of two triple layers of drift tubes and a silicon strip detector telescope in a 100  gev muon beam .
]    a ) the spatial resolution of the drift tubes as a function of the impact radius @xmath11 of the muon determined by the silicon detector telescope at different photon count rates after time - slewing corrections .
b ) improvement of the average drift - tube resolution due to time - slewing corrections using the pulse - height measurement of the read - out electronics .
the improvement increases at smaller impact radii and is independent of the irradiation rate . ]
a ) the average spatial resolution of the drift tubes with and without time - slewing corrections and b ) the drift - tube efficiency for 790  ns dead time as a function of the photon count rate . ]
the discriminator threshold of the read - out electronics was optimized to a value of 4.6 times the thermal noise fluctuations corresponding to the 25@xmath12 primary ionization electron . by extrapolating the muon trajectory measured by the silicon strip detector telescope with @xmath13
m accuracy to the nearest triple layer of the chamber at a distance of 58  cm , the muon impact radius @xmath11 in the drift tubes of this layer is determined with a precision of @xmath14 m . with this information , the space - to - drift time relationship and the spatial resolution of the drift tubes as a function of the impact radius
is determined depending on the @xmath8 irradition rate ( see figure  [ fig3]a ) .
the measured drift radii show a systematic deviation from the impact radii determined with the silicon detector telescope which increases with decreasing pulse height measured by the read - out electronics .
the deviation is interpreted as a time - slewing effect of the read - out electronics . by parametrizing the observed correlation as a function of pulse - height and impact radius ,
an average time - slewing correction function is determined .
the time - slewing corrections lead to a significant improvement of the drift - tube resolution of up to @xmath15 m at small radii independent of the irradiation rate ( see figure  [ fig3]b ) .
the ar : co@xmath5 gas mixture shows a strong dependence of the drift velocity on the electric field .
therefore , fluctuations in the space charge density created in the tubes by the @xmath8 irradiation cause an uncertainty in the space - to - drift time relationship .
this effect  @xcite,@xcite leads to a degradation of the spatial resolution with increasing irradiation rates ( see figure  [ fig3]a ) .
the degradation increases rapidly for large impact radii . the average drift tube resolution as a function of the photon count rate
is shown in figure  [ fig4]a with and without time - slewing corrections .
the resolution degrades linearly with increasing count rate .    without irradiation ,
the average drift - tube resolution is @xmath16 m without and @xmath17 m with time - slewing corrections . for the maximum expected background count rate in atlas , 500  hz/@xmath0 , the time - slewing corrections improve the resolution from @xmath18 m to @xmath19 m
because of the fixed dead time ( 790  ns in the test beam measurements ) built into the read - out electronics , muon hits in the drift tubes can be masked by earlier hits of @xmath20-rays and from the background radiation .
hence , a reduction of the drift tube efficiency by @xmath21 due to @xmath20-rays is expected even without irradiation and a further decrease of efficiency with increasing irradiation rate .
the probability of finding a hit with drift radius compatible with the muon impact radius determined by the silicon detector telescope within three times the spatial resolution is shown in figure  [ fig4]b .
up to the highest anticipated count rate per tube in atlas of 300  khz , the measured efficiency follows the expectation based on a simple model of the read - out electronics with 790  ns dead time without taking into account details of the pulse shape .
the muon hit efficiency drops from @xmath22 without photon irradiation to @xmath23 at 300  khz background count rate per tube . due to the redundant track - point measurements in the 6 to 8 tube layers of a mdt chamber
, the measured drift - tube efficiency allows for an efficient reconstruction of muon trajectories in the atlas muon spectrometer up to the highest expected background rates .
a full - scale monitored drift - tube chamber from the serial production for the atlas muon spectrometer equipped with final read - out electronics has been tested in a muon beam at @xmath8 irradiation rates of up to twice the maximum background rate expected during operation at the lhc . even at the highest anticipated background rate of 500  hz/@xmath0 which leads to a deterioration of the drift tube resolution by @xmath24 m , a spatial resolution of close to @xmath25 m
is achieved as required by applying time - slewing corrections . the measured drift tube efficiency as a function of the photon count rate follows the expectation from the built - in dead time of the read - out electronics .
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the resolution and efficiency of a precision drift - tube chamber for the atlas muon spectrometer with final read - out electronics was tested at the gamma irradiation facility at cern in a 100  gev muon beam and at photon irradiation rates of up to 990  hz/@xmath0 which corresponds to twice the highest background rate expected in atlas . 
 a silicon strip detector telescope was used as external reference in the beam . 
 the pulse - height measurement of the read - out electronics was used to perform time - slewing corrections which lead to an improvement of the average drift - tube resolution from 104  @xmath1 m to 82  @xmath1 m without irradiation and from 128  @xmath1 m to 108  @xmath1 m at the maximum expected rate . 
 the measured drift - tube efficiency agrees with the expectation from the dead time of the read - out electronics up to the maximum expected rate .    ,    ,    ,    ,    ,    ,    ,    ,    ,