A GEM Detector System for an Upgrade of the CMS Muon Endcaps

7 Cooling, gas and cabling services
7.1 Gas
The GEM detector system for the CMS experiment has a total volume of about 3.4m 3 . The chambers are operated
with a three component gas mixture made of Ar/CO2/CF4 (45:15:40). The basic function of the gas system is
to mix the three components in the appropriate proportions and to distribute the gas mixture into the individual
chambers. The expected flow regime is 5 volume changes per day equivalent to about 700 nl/h. The gas system
for the GEM detector will re-use part of the infrastructure as they were foreseen for the innermost RPC end-cap
station, in particularly the distribution racks located in the experimental cavern. The detector volume, the required
gas flow and the use of a relatively expensive gas mixture make a closed-loop circulation system highly favorable.
The system proposed consists of several modules, which are designed using the standard adopted in all the gas
system for the LHC experiments. The gas system will be running on PLC and it will be controlled/monitored
using the standard PVSS 4) interface.
Figure 70: Layout of the mixer unit.
Primary supply The gas system will be connected to the existing primary supply network for Ar, CO2, CF4 and
N2 (the latter it is mainly used to control pneumatic valves located inside the experimental cavern).
Mixer The mixer unit will be located in the SGX5 building. The flows of each gas component will be metered
by mass flow controllers (MFC). The mixing ratio will be adjusted and monitored by the software control running
on the PLC. The mixer unit will contain two sets of MFCs: the first called run will be used for normal operation,
while for a fast filling of the detector a second set (fill) with higher flow capacity will be employed. Fig. ?? shows
the drawing of a typical mixer unit, while an approximate price quotation is given in Table 16.
Closed-loop circulation In order to reduce the operational cost, the gas is circulated in a closed loop circuit. The
circulation loop is distributer over three different areas:
1. Purifier (optional), gas input and exhausted gas connections are situated in the SGX building;
4) Process visualization and control system
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