Friday, March 12, 2010

The Elephant and the Electron Beam

At Jefferson Lab, where I and a lot of other nuclear physicists from around the world conduct experiments, we have a high quality, 6 GeV continuous beam electron accelerator. Such a machine is ideal for nuclear physics, and it is only going to get better--we are in the process of upgrading to twice the energy, or 12 GeV.

There are currently three experimental halls cleverly named Halls A, B, and C. Each has a specialized package of massive particle detectors. (With the upgrade a fourth hall, with the surprising name Hall D will be added.)

It is sometimes hard to explain to the layman the differences among these halls.

Here is what they have in common: the beam enters the hall an strikes a nuclear target. The reaction products emerge from the collision. The idea is to measure these outgoing particles and then reconstruct what the the nuclear forces and nuclear structure must have been that produced them.

The trade-off (there is always a trade-off) is this: you can collect all the particles at all angles but at lower resolution, or you can detect fewer particles and a smaller set of angles but at higher resolution. Or you can fall in between.

Where I conduct research is in Hall B. There we take the first approach: our detector is called a 4π detector, because 4π is the complete solid angle. (And why the surface area of a sphere is 4πR2.) We detect everything, but at lower resolution. Naturally we do the type of experiments that take advantage of our detector's 4π acceptance, and the other halls do the same with their acceptance--and the bottom line is that we complement each other.

That gets me to the point. The best way I ever heard this explained to a lay audience was using an adaptation of the story of the blind men and the elephant.

It goes like this. Suppose at Jefferson Lab our experiment's target, instead of things like helium or hydrogen, was an elephant. After the first round of experiments, the conclusions published would be:

Hall C: It is gray and rough
Hall A: It's thin and rope-like
Hall B: It's an elephant!

No, suppose a second round of experiments is performed, using the information learned in the first. The new results would be:

Hall C: It's an Indian elephant
Hall A: Subspecies Ghandis Grande
Hall B: It's an elephant!

That paints a picture of the differences--Yes?

(Stolen from Mac Mestayer)

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