October 8, 2005
Last weekend we stopped taking data on MINOS because the horns (magnets) that bend all the particles so they are pointing towards our detectors could no longer hold the 180 thousand amps that are supposed to pulse through them every two seconds. Since there are two horns that are connected together, when we first turned off we 
didn't know if it was the upstream or downstream horn, and of course no one knew why they stopped being able to hold all that current. I heard about all this from my dad (who also works at Fermilab) right before going to an October Festival with my mom and kids, so while the kids were riding ponies and picking out pumpkins I had this sinking feeling about the beam and wondering how long it would be before the experiment started running again.
Designing a magnet that can take this much current, and can survive this many radioactive particles flying through it at once is really an art, and building it is yet another art. In a way the horns are the heart of the beamline, since they pump the blood (particles) through the arteries (decay pipe), and you can't run without them for very long. If there is 
something is wrong with the horns, you had better be able to fix or replace it. But imagine fixing something that is so radioactive that you can't actually get next to it! You need a very fancy operating room to fix a horn once it's been used for as long as we've been running.
The operating room for this beamline is basically a garage made of steel blocks with a window of lead glass on one side. This way a crane can drive the horn into the room, and people can look in from the side to see what happened. The experts figured out that it was the downstream horn that couldn't hold the current, and moved it into this room. The horn (as seen through the glass) is in the photo at the left. The scary thing that 
happened this week is that once it was in the operating room, the horn held current again, as if just going to the doctor was enough to be cured. Also, from looking at the horn they saw it looks just fine, there is no obvious place where a huge amount of current burned its way through to the ground.
But the mystery of why the horn failed was solved while I was in the middle of the MINERvA collaboration meeting: they found that one of the feet the horn stood on was 
shaken loose, and that's what let the horn short to the ground--you can see the photo at the left that was entered in the electronic logbook of the beamline, which shows that the foot 10 inches away from where it started and the burn makes on the "floor" as the 180 thousand amps met the earth.
It almost feels like a cheat to feel down when I find out about the horn problem and elated when I find out that the solution won't take too long, given that I have been so wrapped up in MINERvA (an experiment to run in a few years) instead of MINOS (the 
experiment running now). But actually the funnier thing is that I seem to always have something else creeping into my mind that's not in front of my eyes: just like I was distracted last weekend because of work stuff while I was with my kids, this Saturday I am at a MINERvA collaboration meeting and I can't help but think "okay, now Sonia's about to finish her swimming lesson" or "I wonder what Rob is giving the kids for lunch now". Luckily, Sunday and Monday I don't have any work planned and no crises to stress out about (for the moment: knock wood). I wonder what will wander into my head this time...
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