Gas distribution in the outer tracking system

 

1.  General layout of the Gas system

The gas system consists of a mixing module, a closed circulation loop including the distribution system, pressure regulation and purifiers. A detailed description of the OT gas system can be found here. In this section the distribution of the gas within the OT stations and the gas flow through the detector modules is described.

2.  Gas flow through the detector modules

Detector modules are flushed by two independent gas lines, the counting gas and the envelope gas, as shown in the sketch below.

Simplified view of the gas flow through the detector modules.

There will be a single gas line from the mixing and purifying system, located at the gas building at the surface, to the gas distribution behind the radiation wall. In the distribution system described below the incoming gas line is split in the counting gas and envelope gas line. The counting gas enters straw tubes through the lower endpiece of a detector module. It flows in parallel through the straw tubes, reaches the upper endpiece where it exits the module. The impedance seen by the gas is determined by the input and output tubes to the module endpieces. They are made of two stainless steel pipes with an inner diameter of 3mm and a total length of 30cm. These dimensions guarantee a pressure drop across the module well below 0.5mbar.

The envelope gas line flushes the residual volume of the module box. The gas enters this volume through the aluminium inserts at the lower endpiece, runs through the module box and exits the module at the upper module endpiece.

The realisation of the gas piping is shown in these technical drawings ( 1 and 2).

The pressure in the counting gas will be regulated to 1mbar overpressure with respect to the ambient pressure, while the envelope volume will be at 0.5mbar overpressure.

The advantages of this gas flow scheme are:

  • The separation of the two gas volumes blocks the counting gas from impurities entering the modules by leaks.
  • The gas flow through the entire gas volume is guaranteed, even in case of leaks in the gas barrier separating the two volumes.
  • The counting gas can be monitored independent from the envelope gas.

    • 3.  Distribution in the outer tracker stations

    Every OT station is divided in quarter stations, building in total 12 individual subsystems. One quarter station comprises 22 detector modules. Each quarter station is supplied with the gas by one input and one return line for the counting gas, as well as one input and one return line for the envelope gas.

    There will be a single gas pipe from the mixing and purifying system to the gas distribution system. Here, the gas line is split in 24 parallel lines, two lines for every quarter station. Mass flow controllers control the gas flow through every individual line. The gas runs through the detector modules and back the gas distribution system. A monitoring system allows to survey the quality of the counting gas. Behind the monitoring system all lines rejoin and the gas is returned to the surface.

    The sketch shows a schematic view of the gas distribution in the outer tracking system. More details can be found in a seperate note.

    Gas distribution to the outer tracker stations.