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Seth Zenz | Imperial College London | UK

View Blog | Read Bio

Why are you still doing night shifts?

This is a question I’ve received recently from a couple of my friends in the theory community.  Theoretical particle physicists are pretty smart people, and they do know a little something about particle detectors — so if they’re wondering, then I’m sure some of you will be curious too!  This is also a chance to see a snapshot of my psychological state at the end of a night shift: I wrote all of this to explain what I was doing between 6:20 and 6:45 in the morning a couple weeks ago.  My only edits are two places where I wrote something incorrect and replaced it with a new explanation in brackets.

To summarize: I’m busy this week and getting an easy entry out of cutting and pasting from my gChat log.

Again, the question was (more or less), “Why are you still doing night shifts when the accelerator, and large parts of the ATLAS detector, are off?”  Here’s my answer:

06:22 calibrate the detector
the pixel detector has 80 million channels (i.e. pixels, 400 x 50 microns)
06:23 they actually live, physically, on about 1700 modules, which talk to various hierarchically-organized computers
06:24 [to transmit the data the 100 meters to the counting room without high voltage or repeaters] we have optical links for transmitting the data from inside the detector until it gets outside
thus we need lasers to turn digital signals into optical light, and then we also need to convert the light back
the lasers have to be timed and powered correctly, as does whatever reads the information
06:25 at the moment, the ATLAS pixel detector isn’t using some fraction like [3%] of its modules, because they aren’t set correctly. in some cases, they may be impossible to set correctly until we can open the detector and replace components — which may be many years
but in other cases, the automatic-setting didn’t work, and we have to take a closer look.
06:26 some experts were in here today to try to recover a few such modules by taking that closer look; now I’m running scans that tell us if they were succesful or not.
06:27 that’s only one example of the kind of thing we do. there are a lot of things you can set on every module, and we have to get them all set right.
06:38 [My friend asks why we run all night, and if we run all the time]
06:43 me: yes, we have finite time, and lots of work to do
and clearly more people than pixel detectors.
06:44 once the cooling goes off, in a few weeks, we have to turn the modules off. then there’s only a few kinds of calibration scans/studies we can do

It’s worth noting that now, two weeks later, all the optical links are working well, except for a very few that are hard-core unrecoverable — thanks to the work of the experts who looked at the tuning and the very small contribution I made by running scans for them overnight.  Our night shifts continue, with a few nights each from over a dozen people in this month alone.   Although the details of the work at the moment are different, but the overall plan is the same: to have our subdetector, the last one installed, be as ready as the rest of ATLAS when data finally arrives next year!


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  • Stowell P. Robertson

    Hear is some Conjecture on the Higgs to think about.

    The Higgs

    The theoretical Higgs particle (mass), Higgs boson (God particle), Higgs field, and the Higgs mechanism ? The true question, is the Higgs, matter or energy ? The mass loss is equal to the energy (photons Y) gained, plus (+) the cost of exchange for the conversion from mass to energy. The theoretical energy gained, photons Y, and the Higgs, that may be the loss of mass, equal to the Z bosons, with the W+ & W- bosons, also neutrino and tackions. Theoretically, the cost of energy in the Universe, is high energy loss (HEL), in the conversion of mass to energy in the Universe.


  • mark matthews

    If all the links were tested prior to install why are there a few that are “hard-core unrecoverable”? Is there a built in redundancy factor? ie: How many can you afford to lose before your testing is compromised. Also, if the LHC is down for this extended period of maintenance would now not be a better time to recover those items instead of “in many years”, or is this easier in principle rather than in practice? Again thank you all for this blog as it is a fascinating insight into the daily lives/routines of physcists at the LHC.

  • Hi Mark,

    Good questions, and thanks for your kind words. Here are some answers:

    The reason we have failures even though we tested before installing is simply that components sometimes fail. Different phases of installation happened over a long period of time, so some of the internal components of the pixel detector had over a year to fail since sign-off!

    As for the question of how many modules we can afford to lose… well, for every part of the detector that isn’t working, we get a “hole” that charged particles can pass through where we meant to see them but we can’t. Fortunately, the pixel detector has three layers, and the other two tracking subdetectors have even more layers: 8 for the SCT and over 30 for the TRT. The pixel measurements are the most precise, and closest to the interaction point, so obviously we don’t want to lose any — but we can still find tracks even if we have some “holes.”

    Finally, your idea of getting in and making some fixes is a good one, and I’ve asked the same question myself. Unfortunately, the pixel detector is so deep inside ATLAS, and so intricately constructed, that no component of it can be accessed without taking a lot of things apart! We don’t have time to do that between now and next year.


  • mark matthews

    Thank you for the prompt reply. Reading the BBC homepage today (http://news.bbc.co.uk/1/hi/sci/tech/7734251.stm) they are stating that the repairs to the magnets will now take until summer (June?) with the LHC being switched on sometime in summer. This seems to be a bit longer than was initially forecast. Does an extended period of LHC dormancy have a greater effect on the detectors/electronics etc, because as in my experience with aviation electronics, the more they are powered up and running the better they behave.

  • I don’t really know the answer, Mark, but I have two reasons to doubt there’s any problem. First, these electronics are all designed to run for a number of years, so a few months’ holdup shouldn’t make much difference. Second, our detector electronics often remain powered up and running, even if there’s nothing to detect, unless they’re off for maintenance reasons.