Neutrinos meet liquid argon

Like neon, argon is used to make colorful lighted signs. Particle physicists are now putting argon to a far more exciting use: detecting neutrinos. Image: Pslawinski

Fermilab’s flagship effort is its neutrino program, which is ramping up to be the strongest in the world. This means creating the world’s best neutrino detectors. To that end, scientists at Fermilab are pursuing one hot technology that is lighting up neutrino physics, detection based on cryogenic liquid argon.

At first, argon seems to be a pretty boring element. As a noble gas, it does not react chemically. Making up one percent of our atmosphere, it is its third most common component, surpassed only by nitrogen and oxygen. But don’t let its mundane properties fool you. When we cool it down to extremely cold temperatures, it turns into a liquid with incredible properties for cutting-edge neutrino detectors.

For particle physics, perhaps liquid argon’s most important feature is that it acts as both a target and detector for neutrinos, although it isn’t the only material that can be used this way. The Super-Kamiokande experiment in Japan used water stored in a deep-underground tank as large as Wilson Hall to detect neutrinos. Here at Fermilab, the MiniBooNE experiment used a giant sphere of oil that operated much the same way as Super-Kamiokande’s tank.

But with 40 protons and neutrons, liquid argon is denser than water or oil, so liquid-argon detectors see more neutrino collisions per unit volume than their oil- or water-based predecessors. That means faster measurements and consequently faster discoveries.

Another advantage of liquid argon is that, when a neutrino interacts with it and subsequently generates charged particles, it produces two separate kinds of signals; oil- or water-based detectors produce only one. One type of signal, unique to liquid argon, results from its ability to record the charged particles’ trajectories.

Charged particles are created in the liquid argon after a neutrino flies in and collides with an argon nucleus. The charged debris travels through the argon and easily knocks off electrons from the neighboring atoms along its path. The electronic traces in the liquid argon are pushed by an applied electric field toward an array of wires (similar to a guitar’s) on the side of the detector. The wires collect data on the particle trajectories, producing a signal.

The second signal type is one shared with oil- and water-based detection: a flash of light. When a charged particle bumps into an argon atom’s electron, the electron transitions to a higher energy. As the electron transitions back to its original state, the excess energy is emitted as light.

It turns out that argon is also relatively cheap. Companies liquefy air and heat it slowly. Since each of air’s components has a unique boiling temperature, they can be separated. The boiled-off argon is moved to a separate chamber where it is again condensed. The commercially available liquid argon that we buy is still not pure enough for our experiments, so once the liquid argon arrives at the lab, we filter out the remaining impurities by a factor of 10,000.

Using a common and innocuous gas, Fermilab is establishing itself to be the world’s premier neutrino physics research center. Stay tuned to discover what secrets this technology will unlock!

Tia Miceli