As discussed earlier, the Roman pots are the vessels that house the detectors. There are nine independent spectrometers each consisting of two Roman pots for a total of 18 pots. There are four stations with four pots each that comprise the quadrupole spectrometers and two single pots for the dipole spectrometer. This configuration results in the optimal acceptance as discussed in the previous section.
Figure 23 shows a front view and a side view of the beam pipe section including a pair of Roman pots. Each pot is a small steel box that completely encases the scintillation fiber detector (described in the next section) and keeps it isolated from the machine vacuum, although the pot itself remains inside the machine vacuum. The dimensions are labelled on the figure and show that the pot is very compact, with a length of only 3.8 cm along the beamline, a height of 13 cm, and a width of 7 cm. The width and height are determined by the bending radius of the fibers. The pot will be fully retracted in the bay area for beam injection, and can be moved into the beam pipe at a position close to the beam for normal running. A small diameter bellows surrounds the cylindrical chimney and supports the structure. The chimney is used to route the fibers to the phototubes.
Figure 23: A front view and side view of
the conceptual design for a pair of Roman pots.
The pots are located in a bay area inside
the beam pipe. A thin window covers the 2 cm active area of the detector.
The length of the detector in the beam direction is only about 3.8 cm.
The Roman pot is
composed of 2 mm thick steel except for a thin window which brackets
the active area of the detector traversed by the scattered protons.
The window is composed of a 50 
m stainless steel foil in order
to reduce multiple scattering.
Once the detector is placed inside the box,
a steel lid with a cylindrical chimney is welded to the top of the box.
A low viscosity epoxy will be injected through the chimney
in order to fill the remaining space on either side of the detectors,
thus creating a solid one-piece detector.
The box design produces the smallest possible
pot, reducing the space needed in the beam pipe region. This allow us to
have pots in both the x and y planes in order to maximize
the acceptance as discussed in Sec. 3.3.2.
Another advantage of this
design is a much lower cost relative to standard Roman pot designs
which are at atmospheric pressure on one side, and require
a pressure compensation system to
combat the forces
caused by the imbalance in pressure between the inside and outside of
the pot. Our design also only requires a small diameter bellows and
a small range of motion.
A step motor drives a cam system that moves the pot along the
direction of the chimney axis. A system of bearings keeps the box movement
from deviating from the direction perpendicular to the beam
line. The position sensing system is based on two high precision linear
potentiometers (LVDT's), one performing the primary
position measurement and
the second providing redundancy. The position sensor
signal will be sent to the Main Control Room, where the Roman
pot movement will be monitored.
The whole positioning system will be
capable of a displacement precision of better than 25 m.