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Exciting tasks in the underground CMS cavern February 28, 2008

Posted by dorigo in personal, physics, science.

I am currently at CERN, where I am on shift for the CMS tracker (right). The CMS detector is in its final assembly phase, and the tracker – a cylindrical volume made by a dozen layers of silicon microstrip sensors- is being wired to its power, cooling, and readout systems. While this is being done, an important feedback can be given to the technicians by powering up the parts already wired and checking that everything is in good shape.

The microchips installed on silicon detectors in operation produce heat which, if not removed promptly and efficiently, causes overheating and eventually damage to the device. Because of that, one cannot power up the system without proper cooling in place. While the final cooling circuit is not operational yet, a temporary circuit of coolant has been installed on a part of a layer inside the tracker, such that we have a chance to spot problems connected to the assembling procedures early on.

Any temporary system requires a close babysitting. That is presently my job: as a SLIMOS (Shift Leader In Matters Of Safety) I am sitting in front of a desk in a control room inside the P5 cavern, 100 meters underground. I have to watch the temperature of the coolant, its pressure,  and take a walk to the detector hall to check periodically the dry air flowing in the system, the cooling pump and the flow circuits, plus the power supplies that run the modules we are testing.

The walk is the fun part of my job: even though I have not contributed in any way to the mechanical construction of the titanic project, I feel it as my own as I walk with steel-tip shoes through the maze of stairs and passages surrounding the detector, a hard hat on my head, a notebook and pen in my hands, and a professional look printed on my face.

Above you can see a part of the temporary cooling circuit that has been assembled to allow the powering up of layer 4 of the TIB (the tracker inner barrel). Pressure gauges, valves, and tubing – cool stuff! Every couple of hours they get checked up by the SLIMOS on duty.

Below instead you can see the graphs I have to keep an eye on during most of my shift. The temperature oscillates between 10 and 12 degrees during cooling cycles. Of course, an automated system is capable of alerting and shutting down the power to the sensors by itself, but in a temporary setup one has to be careful and oversee things: we do not want to burn our detector before it has a chance of seeing proton collisions!


So, if you watch closely, you can see that indeed, human intervention is required now and then. The jump up of the curves in the top graphs signals abnormal growths of temperature in the detectors being powered at about 12:17 hours today. Following the event, sensors were powered down with no trouble, but I then had to run and enable the power interlock and check the status of the cooling pump: the latter was showing an overpressure situation. I restarted it by following a 13-step idiot-proof procedure (in course of being updated!), and the system is now up and running smoothly again (as shown by the normal cycles on the right of the spike).


1. Fred - February 28, 2008

Loving it, Tommaso.

Is this the kind of job you could teach any one of us to do or is there some special skill and knowledge involved. For instance, would you have to be a physicist to be a SLIMOS? How are you commissioned or sponsored to work at the CMS tracker? Is it an agreement with your employer and CERN and are all in your capacity temporarily employed the same way? Who foots the bill and is this extra pay for you? Funny, I like to tear apart and help rebuild performance cars at my buddy’s shop. The similarities and considerations are many but obviously at a ridiculously smaller scale. There are always sensors, cooling pumps, pressure gauges, flow circuits, wires, tubing, tape, sharpies, paper, pens, etc. involved but we don’t wear steel-toed boots for safety reasons. What’s the purpose of steel-tip shoes? Do you ever get to crank the engine up?! Thanks again for the shots.

2. dorigo - February 28, 2008

Hi Fred,

glad you liked my post.
Answers to your questions:
1) no particular skills involved – anybody capable of walking through the labyrinth of ladders, bridges, and underpasses can do it. Not even a undergrad degree needed, really – but you need to use a lot of common sense. For instance, if the cooling fails, and the detector is powered off, you should first of all interlock the system, such that nobody can decide to turn the power on again before you restart the pump. Also, when you start the pump, it is common sense to tell you you should have a bypass system on, rather than a specific item in the checklist.
2) My sponsor is my employer, INFN. It pays for my trips here, and I decide what I am doing here – of course, money for travel is in a budget allocated for the commissioning, but when at CERN one can do other things too, like following meetings. And in fact, this is a CMS week, and there are lots of meetings to follow. That is no chance: I chose this week to come and do some shifts and follow meetings as well.
3) extra pay, yes. I get a per-diem which pays the hotel bills plus about 80 euros a day. And travel expenses are paid too, of course.
4) Steel-tip shoes are really needed. This morning I was walking on the floor of the CMS cavern, on the side of a 20-meters-tall muon station wheel, and a few meters above my head was a set of muon chambers hanging from a crane. Well, if the crane had failed, they would have at least been able to bury my feet. Seriously, though, with so much busy work going on around, steel tips are a good idea. Anything can fall on your feet, from a screwdriver to a steel pipe. And the helmet is VITAL! I banged my head TWICE on the same I-beam this morning, and even with the helmet on I did not like it… Without helmet, I’d be in the hospital.
5) I did restart the pump by myself today… with the cooling expert on a side!


3. DB - February 28, 2008

“I banged my head TWICE on the same I-beam this morning, and even with the helmet on I did not like it… Without helmet, I’d be in the hospital.”

I am reminded of the scene in Star Trek – The Undiscovered Country (I think) when Scotty meets a similar fate while walking around an unfamiliar ship. Only he wasn’t wearing a helmet.

4. Kea - February 29, 2008

WOW! They let you be a SLIMY? No wonder those technicians are sweating all the time… Seriously though, Tommaso, it is fantastic that you are willing to share this experience with everybody… although it makes me very, very nervous to see the schedule moving along this well when I know there are no decent post SM predictions for the new E regime.

5. Louise - February 29, 2008

Great photos, looks like the interior of the Death Star or another Sci Fi spaceship.

6. dorigo - February 29, 2008

And I am not from Klingon, DB🙂

Kea, yes, they allow me to fiddle with the expensive apparatus. But techs do not worry if things go awry, it is physicists who sweat more. As for post-SM predictions or lack thereof, you are right – but today Joe Incandela gave a nice talk in the auditorium, and he said he liked the idea that it is experimentalists who have an open field now. Who needs predictions after all… We’ll just smash those protons and see what the hell happens.

Hi Louise, the CMS tracker is soooo cool… It would be a pity to burn it out before due time!


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