News from the pit

3 March 2008

The muon barrel during installation

It has been an eventful two weeks at Point 1, with the final two large pieces of the detector being put in place. But elsewhere in the cavern, commissioning work on the muon barrel has been underway for quite some time.

“There are two levels of commissioning,” explains Ludovico Pontecorvo, Barrel Commissioning Coordinator, and newly appointed Muon System Project Leader. “We start with the individual chambers, basically checking that all the functionality is there. Then, once every two months, we do a common run of the whole muon system.”

The muon barrel is arranged in three layers of twelve to sixteen chambers, made up of Monitored drift tubes (MDT) for precision tracking, and resistive plate chambers (RPC) for triggering. Service testing started back in April 2007, and this week, checks were being carried out for the first time on the MDTs in sectors 11 and 12.

Ludovico and colleagues are busy testing whether the Ar/CO2 gas mix is maintained at 3bar absolute pressure throughout each of the 3cm diameter aluminium tubes. At only 400 microns thick, these can be easily damaged, so testing is crucial to ascertain whether any loses have been suffered during the installation work. Concurrently to this, the team are also monitoring whether high voltage is preserved, without discharge, in the wires which run down the centre of each aluminium tube. If a wire is broken, a shortage will occur, and high voltage will not be maintained.

Due to the tightly-packed nature of the aluminium tubes, problem wires or tubes cannot be replaced; instead they’re simply disconnected from the high voltage supply, meaning reduced detection and monitoring capacity in the local area. Thankfully the track record so far is excellent: “There are maybe 100 wires not working, out of 400,000,” says Ludovico.

Following these basic checks, the chamber being scrutinised is connected up to the detector control system (DCS). This takes precise measurements of the magnetic field acting on the chamber, in order to allow accurate calculation of the momentum of particles passing through the detector; and also records temperature as part of the system calibration. Finally, the front-end electronics are connected up to the low voltage supply, so that readings can start to be taken from the chambers.

“This initialisation is not a trivial thing,” says Ludovico. “Many times we’ve had problems, and we’ve had to locate where these are in the chain. Maybe it’s a fibre that’s dirty, or a connector that isn’t well plugged in. Normally the problems are not very important, but sometimes there will be a fault in one of the electronic pieces, and we have to find and replace it.”

In addition to the commissioning work on sectors 11 and 12, a combined run is also taking place this week, meaning that the trigger technology is tested and assessed at the same time as the precision equipment. RPCs in sectors 7 and 8 are the focus of scrutiny this time around. “11 and 12, 7 and 8: This is the new stuff that we’re adding on. Of course we’re still monitoring the rest at the same time as we’re commissioning these latest sectors,” explains Ludovico, “because once you’ve done it, you keep on exercising it to make sure it’s stable.”


Colin Barras


Ceri Perkins