The innovative effort to fix the cooling problem of ALICE’s Silicon Pixel Detector (SPD) has been successful. Filters blocked by dust and pollutants have been reducing the efficiency of the detector’s cooling system since its installation in 2008. The successful intervention, carried out on February 15th, returned the performance of the worst affected filter to high levels. This resulted in a 10% increase in detector performance.
Rosario Turrisi Rosario Turrisi (left) with team members Yannick Lesenechal, Claudio Bortolin and Andrea Francescon
The solution involved drilling a small hole in the filter’s mesh by feeding a cable, with a sharp milling tool on one end, down the cooling line. This allowed the cooling liquid, perfluorobutane, to flow to the heat producing SPD electronics once more. The challenge was made particularly difficult as the SPD lies buried deep inside ALICE and the closest access point is 5 metres from the offending filters. ALICE Matters reviewed the proposed solution in January with SPD cooling coordinator Rosario Turrisi.
Rosario explains how the intervention progressed, “Surprisingly everything went as we thought it should. We tried this procedure many times in the lab and the actual drilling operation was really fast. It took us about half an hour.” The whole team was involve in the intervention but preparation, precision, and a steady hand dictated the success of feeding the cable without being able to see where it was going. Technician Yannick Lesenechal carried out the actual drilling, supported by Andrea Francescon and Samuel Rambaut, while Rosario, helped by Claudio Bortolin, judged the operation using a manometer; Gas was passed through the cooling system and a drop in pressure signalled the filter was unblocked.
“After that we cleaned the cooling system because you have to be sure you don’t leave anything inside,” Rosario continues. Once cleaning was completed samples of dust were sent for analysis. “And indeed the filter was pretty clean. The result was perfectly consistent with what we did in the lab. Once we had the green light from the analysis lab we started the cooling immediately and saw that the flow inside the line was back to nominal value.” After this the team began to turn sections of the detector back on one by one. “We saw that everything was working perfectly and we were able to turn on 100% of the detector. We really couldn’t believe our eyes at first. If you think of all the things that can go wrong you really don’t count on such a good result. So, we were really happy and there was a bottle of champagne of course.”
Rosario stresses why the success of this intervention is so important. “There are a few items of physics you just cannot do without the SPD so people were worried about its performance. This is a big, big change for us. We had 62.5% efficiency of the detector before the winter shutdown. But with one line cleaned we immediately gained 10% and with the second line done it will be another 10%.”