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

Chapter 3: Luminosity Measurement at the LHC and in ATLAS 86
Figure 3.11: Concept of a Roman Pot. The detector is attached to a movable frame.
In the working position during the special runs (right), the detector approaches the
beam very closely, within 10 σx/y or about 1 mm. During normal running, it is
retracted away from the beam (left).
The ALFA detector consists of plastic scintillator fibers housed in so-called Roman
Pots, whose working principle is illustrated in Figure 3.11. Each Roman Pot unit
contains two pots, which approach the beam from top and bottom in the special
runs. There are two Roman Pot units separated by 4 m on each side of the IP about
240 m away. The pseudorapidity coverage of ALFA is 11 < |η| < 13.5.
The sensitive area in each Roman Pot consists of ten ceramic plates, each sup-
240 m
porting two layers of 64 fibers. The two layers are rotated by 90 o with respect to each
other. The achievable spatial resolution from a single plane is 14.4 µm [16]. The scin-
tillation light from the fibers is routed to multi-anode photomultipliers. Figure 3.12
shows a cut-away view of a Roman Pot unit with the various parts indicated.
The precision of the luminosity measurement from ALFA is expected to be 2-
3% [13]. The Roman Pots have already been installed in the LHC tunnel. Fiber
tracker construction is complete, and testbeam measurements were taken during 2010.
The complete installation will take place during the LHC shutdown in winter 2010-11.