Next, the 0.2-millimeter proton beam passes through 10 dilution magnets, which cause the protons to fan out until the beam has thickened to a lower-intensity diameter of 1.5 mm.
Now fattened to the width of a human hair, the beam continues down the tunnel to the beam-dump cavern. Inside waits a cylindrical block of a dense, absorptive graphite composite that is 8 meters long and 0.7 meters in diameter.
The 10-ton graphite cylinder is encased in 1000 metric tons of steel and concrete. Why not just make the whole thing out of lead or another heavy metal? It turns out that graphite is the only material whose low density and high melting point can resist the ravages of the proton beam. In experiments, researchers found that an 86-microsecond exposure of the beam would bore a hole 40 meters into a block of copper.
Even though the beam’s damage potential has now been reduced by its increased girth, the beam would still handily eat through the graphite composite cylinder. So instead of letting it burn a single 1.5-mm-wide hole into the cylinder, CERN engineers designed the system to ”scan” the beam onto the face of the cylinder, much as the electron beam is scanned in a cathode-ray-tube television screen. To ensure that the intense beam never lingers too long in one place, it is scanned as a pattern—which vaguely resembles the letter e —onto the cylinder.
Though the graphite beam dump becomes very hot (about 750 °C), it does not melt. In fact, after it cools down it can be reused a few hours later.
