Ultra-pure liquid argon for future neutrino experiments

Scientists on Fermilab’s Liquid-Argon Purity Demonstrator recently showed that liquid argon can achieve a high purity without first having to evacuate the tank of contaminants.

Argon, a noble gas, comprises 1 percent of the Earth’s atmosphere, making it its third most common gas. When very pure, argon can also aid scientists in detecting neutrinos.

Fermilab’s Liquid-Argon Purity Demonstrator, or LAPD, a 30-ton-capacity tank of liquid argon connected to a purifying filter system, contains what scientists call a time projection chamber. Once a neutrino interacts with an argon nucleus, the resulting particles will remove electrons from the surrounding electron atoms. The electrons will then drift toward readout wires strung throughout the chamber. The so-called ionization electrons’ arrival at the wires will signal to scientists that a neutrino has just deposited its energy in the argon-filled chamber.

Ultra-pure argon is important for being able to detect the ionization electrons. If the argon contains electronegative impurities — substances that attract electrons, such as water and oxygen — they can derail the electrons before the electrons have a chance to get to the readout wires.

The LAPD tests whether it’s possible to achieve a liquid sufficiently free of contaminants without having to evacuate the vessel before filling it with argon, as previous experiments have done. Researchers are building several liquid-argon time projection chamber experiments, including MicroBooNE, with the ultimate goal of building a multikiloton-capacity chamber. In the case of these future, larger vessels, it becomes prohibitively costly to first evacuate them.

The LAPD was designed to achieve the ultra-high purity required by liquid-argon time projection chambers in a vessel that cannot be evacuated. Prior to filling with liquid argon, the ambient atmosphere in the cryostat is removed by purging the tank with argon gas. After the initial purge, the argon gas is subsequently circulated through filters to further reduce these contaminants. Liquid argon is then introduced into the vessel. The liquid is then continuously circulated through the filters to achieve concentrations of water and oxygen on the order of 0.1 parts per billion.

The LAPD’s second run period was from December 2012 through September 2013. The cryostat was completely filled with liquid argon, and scientists measured the liquid-argon purity under various operating conditions.

The measurements from purity monitors installed in the cryostat establish for the first time that purities that allow for electron drift lifetimes greater than 6 milliseconds can be achieved in a large volume of liquid argon without first evacuating the vessel. This achieved lifetime largely exceeds the value (1.5 milliseconds) needed by the LBNE for a 2.3-meter-long drift distance.

The LAPD group also measured that the present filter material can remove 0.49 grams of oxygen per kilogram of filter material. This is is a number of great interest to the engineers designing filtration systems for very large liquid argon experiments.

Tingjun Yang