Ars photo essay: standing in the beam line of a neutrino detector

Although I was lucky enough to tour Brookhaven’s RHIC accelerator during a period of scheduled downtime, my trips to the LHC and Fermilab both took place while the particle accelerators were in operation. Given the tremendous energies involved, it meant that it was simply not safe to go anywhere near the active hardware, since that’s a sure way to pick up a healthy dose of ionizing radiation. But Fermilab had an exception to that, a place where it wasn’t just acceptable to look at working hardware, but it was actually possible to walk right through a particle beamline. The secret? The particles were neutrinos.

Neutrinos are uncharged particles and are so light that, for decades, most physicists assumed they were actually massless. As if that weren’t enough, they only interact with other matter via the weak force, which is only significant at short distances. Thus, for the most part, they generally pass through matter without incident—trillions go through your body every minute, but most of us will only have them hit anything a total of about three times in our entire lives.

They are so disinterested in interacting with matter that Fermilab is able to create a beam of neutrinos and direct them to a mine in Minnesota without losing enough of them on the way to interfere with the experiment.

Since neutrinos aren’t interested in doing much other than shooting through the Universe at nearly the speed of light (given their extremely low mass, it doesn’t take much energy at all to get them there), how do physicists actually work with them? That’s what we’ve got the photos for.

Accelerating the neutrinos isn’t an issue, but creating a beam of them is—since they’re uncharged and not prone to much in the way of interactions, there’s no way to focus them. So the people at Fermi don’t. Instead, they focus the particles that decay into neutrinos. This starts by taking some of the protons out of the chain of accelerators that normally boosts them to high energy before their injection into the Tevatron. Instead, these protons are directed at a solid target, where they create a shower of unstable particles, many of which are charged.

That spray is focused into a beam using a combination of a metal horn, shown here, and a precisely timed electrical pulse.

Deborah Harris, the other physicist who gave us a tour of Fermi’s on-site neutrino experiments, said that the electric pulses, timed to coincide with the arrival of the particles from the solid target, are powerful enough to make the horns hum. To demonstrate, she sent along an audio file.

This isn’t a perfect process—by the time the beam gets to Minnesota, it’s about a kilometer in diameter—but it’s good enough to send a high concentration of neutrinos in a fairly specific direction, something nature is generally not inclined to do. The showers of charged particles involved, however, makes getting close to that part of the experiment very dangerous when it’s active. So we took a short drive out to the building put in place to house Fermi’s Minos experiment.

All the action goes on underground; the building is there to provide access to the site, which is a few dozen meters beneath the surface. When new hardware is put in place, it’s sent down this drop shaft. The Fermi staff was kind enough to let me ride the elevator.

The elevator wasn’t the only way back to the surface, however. In case of an emergency, we were warned we might be asked to walk back out the line the boring equipment had created when it carved out the underground facility.

Once at the bottom of the shaft, Harris led us past some areas where water was dripping from the ceiling. This isn’t a problem for the hardware; in fact, the water gets used to cool equipment before getting piped back to the surface.

Once past the bend, however, the tunnel opened up and dried out, revealing one last bit of shielding before the actual detector hardware. Right now, there are three detectors—two active and one inactive—on the beam line. That’s another advantage of working with neutrinos: the average neutrino typically won’t end up hitting the detector, so you can simply stack detectors one in front of the other, with each of them picking off a few stray neutrinos as the majority of the beam continues on through the Earth.