  After the high-luminosity upgrade of the Large Hadron Collider (LHC) at CERN,
the ATLAS muon spectrometer is expected to work at 10 times increased
background rates of gammas and neutrons. This is challenging as the momentum
resolution of the spectrometer is expected to be 10 %. This requires a single
tube resolution of the muon drift tubes of 80 mum. At background rates around
1000 Hz/cm2 space charge effects will lead in the slow and non-linear AR:CO2 =
93:7 gas mixture to a degradation of the drift-tube spatial resolution. This
was studied before experimentally for gammas and low energetic neutrons. Almost
no information exists for fast neutrons. Therefore, we organized our studies
under the following aspects: - We investigated the influence of 11 MeV neutrons
on the position resolution of ATLAS MDT chambers. At flux densities between 4
and 16 kHz/cm2, almost no influence on the position resolution was found, it
degrades by only 10 mum at a detection efficiency of only 4*10-4. - We
investigated inert gas mixtures on fastness and linearity of their
position-drifttime (r-t) relation. At a reduction of the maximum drift time by
a factor of 2, the use of the present hardware and electronics might be
possible. For our experimental studies we used our Munich cosmic ray facility.
Two gas mixtures show almost identical position resolution as the standard gas.
- For spectrometer regions of highest background rates we contributed to the
investigation of newly developed 15 mm drift tubes. Position resolutions have
been measured as a function of gamma background rates between 0 and 1400
Hz/cm2. - Garfield simulations have been performed to simulate space charge
effects due to gamma irradiation. Results will be presented for the standard
geometry as well as for the new 15 mm drift tubes.
