Speaker
Dr
Andrej Gorisek
(Jozef Stefan Institute (SI))
Description
Beam conditions and the potential detector damage resulting from
their anomalies have pushed the LHC experiments to build their own
beam monitoring devices. The ATLAS Experiment decided to build two
independent systems based on Chemical Vapor Deposition (CVD)
diamond material.
The ATLAS Beam Conditions Monitor (BCM) consists of two stations
(forward and backward) of detectors each with four modules. The
sensors are required to tolerate doses up to 500 kGy and in excess
of 10^15 charged particles per cm^2 over the lifetime of the experiment.
Each module includes two diamond sensors read out in parallel. The
stations are located symmetrically around the interaction point,
positioning the diamond sensors at z = ±184 cm and r = 55 mm (a
pseudorapidity of about 4.2). Equipped with fast electronics (2 ns
rise time) these stations measure time-of-flight and pulse height
to distinguish events resulting from lost beam particles from those
normally occurring in proton-proton interactions. The BCM also provides
a measurement of bunch-by-bunch luminosities in ATLAS by counting
in-time and out-of-time events. Eleven detector modules have been fully
assembled and tested and the best eight installed in ATLAS. Testbeam
results from the CERN SPS show a module median-signal to noise of 11:1
for minimum ionising particles incident at a 45-degree angle. The BCM
has operated reliably in ATLAS for the last 24 months, has provided
feedback on every beam dump during that time and is required to show a
clean abort before ATLAS returns the LHC injection permit. In addition
the BCM provides collision rate and background measurements that have
been instrumental in ATLAS achieving a luminosity precision of better
than 4%. As luminosity reached unprecedented levels in 2011 and other
luminosity monitors in ATLAS reached saturation, ATLAS reported
luminosity comes from BCM measurements.
For the ATLAS Beam Loss Monitor (BLM) a simpler detector was installed and
connected to LHC beam abort system. The readout electronics is based on
LHC BLM systems, with a few modifications to adopt to CVD diamond sensors.
There are 12 sensors of 8x8 mm^2 area and 0.5mm thickness attached to
the ATLAS Inner Detector end plate at z = ±3450mm, r= 65mm, six on each
side of interaction point. ATLAS BLM justifiably aborted the LHC beams
twice in Summer 2011 and thus prevented possible damage to the Inner
Detector.
The performance of the BCM and BLM detectors and their contributions
to ATLAS physics will be presented.
Authors
Dr
Andrej Gorisek
(Jozef Stefan Institute (SI))
Prof.
William Trischuk
(University of Toronto)
