High-rate irradiation of 15mm muon drift tubes and development of an ATLAS compatible readout driver for micromegas detectors

The upcoming luminosity upgrades of the LHC accelerator at CERN
demand several upgrades to the detectors of the ATLAS muon
spectrometer, mainly due to the proportionally increasing rate of
uncorrelated background irradiation. This concerns also the "Small
Wheel" tracking stations of the ATLAS muon spectrometer, where
precise muon track reconstruction will no longer be assured when
around 2020 the LHC luminosity is expected to reach values 2 to 5
times the design luminosity of $1 \times 10^{34}
\text{cm}^{-2}\text{s}^{-1}$, and when background hit rates will
exceed 10 kHz/cm$^2$. This, together with the need of an additional
triggering station in this area with an angular resolution of 1
mrad, requires the construction of "New Small Wheel" detectors for
a complete replacement during the long maintenance period in 2018
and 2019. As possible technology for these New Small Wheels,
high-rate capable sMDT drift tubes have been investigated, based on
the ATLAS 30 mm Monitored Drift Tube technology, but with a smaller
diameter of 15 mm. In this work, a prototype sMDT chamber has been
tested under the influence of high-rate irradiation with protons,
neutrons and photons at the Munich tandem accelerator, simulating
the conditions within a high luminosity LHC experiment. Tracking
resolution and detection efficiency for minimum ionizing muons are
presented as a function of irradiation rate. The experimental muon
trigger geometry allows to distinguish between efficiency
degradation due to deadtime effects and space charge in the
detectors. Using modified readout electronics the analog pulse
shape of the detector has been investigated for gain reduction and
potential irregularities due to the high irradiation rates and
ionization doses. This study shows that the sMDT detectors would
fulfill all requirements for successful use in the ATLAS New Small
Wheel endcap detector array, with an average spatial resolution of
140 $\mu$m and a track reconstruction efficiency of around 72\% for
a single tube layer at 10 kHz/cm$^2$ irradiation rate. A second
proposal for a New Small Wheel detector technology are Micromegas
detectors. These highly segmented planar gaseous detectors are
capable of very high rate particle tracking with single plane
angular resolution or track reconstruction. The ATLAS community has
decided in 2013 in favor of this technology for precision tracking
in the New Small Wheels. A prototype Micromegas detector will be
installed in summer 2014 on the present ATLAS Small Wheel to serve
as test case of the technology and as template for the necessary
changes to the ATLAS hardware and software infrastructure. To fully
profit from this installation, an ATLAS compatible Read Out Driver
(ROD) had to be developed, that allows to completely integrate the
prototype chamber into the ATLAS data acquisition chain. This
device contains state-of-the-art FPGAs and is based on the Scalable
Readout System (SRS) of the RD51 collaboration. The system design,
its necessary functionalities and its interfaces to other systems
are presented at use of APV25 frontend chips. Several initial
issues with the system have been solved during the development. The
new ROD was integrated into the ATLAS Monitored Drift Tube Readout
and into a VME based readout system of the LMU Cosmic Ray Facility.
Additional successful operation has been proven meanwhile in
several test cases within the ATLAS infrastructure. The whole data
acquisition chain is ready for productive use in the ATLAS
environment.