Measurement of the Z boson cross-section in - Harvard University ...

Chapter 3: Luminosity Measurement at the LHC and in ATLAS 90
3.3.3 BCM
The main purpose of the BCM (Beam Conditions Monitor) is to protect ATLAS
and vital beamline components in the event of beam instabilities. For example, several
proton bunches hitting a TAS (T arget Absorber Secondaries) collimator can lead to
very high secondary particle densities that will damage the experiment. The BCM can
detect the initial stage of such an incident and abort the beam. The particle showers
in these anomalous cases originate well away from the ATLAS IP, so that if two BCM
detectors are placed symmetrically about the IP, the shower particles will reach them
with a time difference ∆t (Figure 3.14). By contrast, particles from collisions at the
IP will reach both detectors simultaneously. A coincidence condition can therefore
distinguish between the two types of events. This configuration also enables the
BCM to provide bunch-by-bunch collision rate and instantaneous luminosity, making
it potentially useful as a luminosity monitor.
Each BCM station is suspended from the beampipe support structure at a radius
of ≈55 mm from the nominal beam axis. They are located at z = +183.8 cm,
corresponding to a pseudorapidity of ≈4.2. For a particle coming from one side of
the IP, such as from a TAS event, the time difference between the signals at the two
stations will be 12.5 ns.
A BCM station consists of four diamond sensor modules each of area 10×10 mm 2 ,
placed symmetrically about the beampipe at an angle of 45 o (Figure 3.15). Diamond
sensors have a large drift carrier velocity, enabling a fast signal rise-time (≈1 ns) and
a narrow pulse (≈3 ns), so that operation at the nominal collision frequency of 40
MHz will be possible. In addition, leakage current due to radiation damage should