Tuesday, March 30, 2010

LHC Has Collided Protons at 7 TeV!!!

Read the posting here.

If you're wondering how colliders work, ArsTechnica put up a great article this week on how they work.

As a freshman at Michigan State University, my family and I had the pleasure of being able to take one of their periodic public tours of the Cyclotron facilities. One of my favorite parts was how they curved the acceleration path at one point to separate particles by mass, so they could then sort of collect particles in Faraday Cups at the end of the path. That a machine so complex and powerful replies on the fundamental principle of inertia to sort the subjects of its studies by mass is somehow poetic.

Turns out we use that exact principle for isotope detection, except that it's on a far smaller scale. My research group makes frequent use of a multicollector induced-coupled plasma mass spectrometer (MC-ICP-MS), which allows us to measure with a high degree of precision (when the machine's behaving; I swear it's sentient sometimes) isotopic ratios. In particular, I'm looking at stable (or non-radioactive) iron isotopes. In the future, I'll be looking at stable magnesium isotopes, (obviously unstable) uranium isotopes, and possibly stable silicon isotopes. Silicon's difficult because of its low solubility in most solutions and its tendency to fractionate.

Fractionation occurs when some mechanism allows the preferential movement of one isotope over another. I'm being vague on that definition for a reason; there are multiple ways in which this can happen. One example is heating the sample to a level where silicon melts and could possibly vaporize. Heavier isotopes require more energy to lift, just as lifting a car requires more energy than lifting a bicycle. If there is only enough energy in a system to vaporize a few silicon atoms at a time, it's much more probable that the lighter silicon atoms will vaporize.

It doesn't exclude the possibility of vaporizing the heavy silicon atoms at this time - it's just less probable. However, if more energy is introduced to the system, the probability of heavier silicon isotopes being vaporized increases greatly. Were I to analyze collected silicon vapor collected from each of the energy levels, I'd find that the lower energy vapor has a "lighter" signature, whereas the higher energy vapor has a "heavier" signature.

Recall, though, that this is only one way to fractionate stable isotopes. It's by far the most common process, but there is also chemical fractionation. I won't go into as much detail about this process as I'm less familiar with it, but it's definitely a fascinating study and I'm hoping we go into great detail about it in my isotopes class next fall.

Why silicon is being problematic isn't clear yet. Part of our chemical procedure to prepare samples for analysis hasn't been perfected yet, and we know that for sure. One particular chemical added in extremely small amounts is intended to "anchor" the silicon in solution so it doesn't form a colloid or precipitate out, trapped in telltale wispy flakes that settle at the bottom of the sample tube. Too much of this chemical and it will occupy all available site on the silicon atoms in solution and turn into a gas, which means it will fractionate out an eventually escape. Dilution of the sample beyond the theoretical minimum volume required to dissolve the amount of silicon present hasn't entirely helped, either.

But the best part by far is that mass spectrometry and photospectrometry of the samples have provided results exactly the opposite of each other (there should be at least a rough direct correlation between the two). Hard to say. We're still working on the method.

And on that note, I should head into work fairly soon. My first class of the day was delayed by half an hour, but I have an array of small tasks I should plow through in some capacity before then.

No comments: