LArTPC in Brief: This is intended to be a quick summary of our efforts. Please note that [bracketed] items provide additional technical information. Green links provide background to students who are unfamiliar with the concepts discussed. Red links link to the area in our site where that facet of our project is discussed.
Purpose: Liquid argon time projection chambers (LArTPCs) are the future of long-baseline neutrino oscillation physics. These detectors, with their fine-grained tracking and total absorption calorimetry, are ideal for investigations of theta_13, CP violation, and the mass hierarchy. As well they serve as multipurpose detectors for physics beyond accelerator neutrino physics, such as for nucleon decay searches.
How it works: When a high energy particle passes through a medium, the particle leaves a path of ionization electrons which can be detected, tagging the path of the incoming particle. In a LArTPC, the medium is LAr and the paths of ionization electrons are detected by drifting the paths over meters to wire planes. These wire planes are oriented in such a way that the time, magnitude and position of each path can be reconstructed. Thus a data acquisition system records many snapshots of the relative appearance of ionization electrons each second [at ~40 MHz]. Put in sequence, we can reconstruct each particle's path, which results in gorgeous bubble-chamber-like images. From the topology and energy deposited along each track, specific interactions can be reconstructed.
Where are we & Where are we going?: LArTPC technology has been pioneered by the ICARUS collaboration and successfully constructed and operated on "small" scales, as demonstrated by the ICARUS T600 program [ICARUS T600 http://www.aquila.infn.it/icarus/]. An R&D path towards scaling these detectors up to the size needed for the next generation of long baseline neutrino oscillation physics -- ~10-50 ktons is described below.
R&D Effort: Two challenges in scaling from small to large detectors include: achieving and monitoring the purity levels necessary and the challenges associated with the long wires of the wire planes.
Purity is an issue because a track of electrons is analogous to Hansel and Gretel's trail of bread. The culprits in liquid argon are not hungry rodents, but rather polar molecules and atoms without full outer electron shells (which every element has except for noble gases, which argon is). These particles-predominantly water and oxygen-will absorb the ionization electrons to make themselves happy, but at the expense of our evidence of a particle having passed through. Fortunately, filtration devices are available which achieve acceptable purity levels for drift regions at least 3m in length [20% loss over a 3m drift requires 20 ppt oxygen contamination and a 12 ms drift time] The filtration device tested here, used in conjunction with a purity monitor similar to those used by ICARUS [designed by C. Carugno et al.], consists of a molecular sieve to remove water and a Trigon™ filter to remove the oxygen [it is a copper filter which reacts with the oxygen to form copper oxide]. The filter can get 'full' [when all of the copper has reacted] and can then be regenerated.
The purity monitor measures purity by firing a light pulse from a xenon lamp at a photocathode and then drifting the ejected electrons to the anode with an electric field. The number of electrons surviving the transit from the cathode to the anode gives a measure of the argon purity from those numbers. Beyond these first pass purity tests, a large-scale argon purification system will be designed and tested.
Another challenge in achieving purity is how to clean and purge the contents of the vessel before an initial liquid fill. As the air around us is ~21% oxygen, as much air as possible must be removed before putting in LAr. In the past, taking a vessel down to vacuum has solved this problem; however, evacuation is impossible for the massive tank sizes -- 40m diameter and height for a 50kton tank-being considered here. A solution is to use a gaseous argon 'piston' to push the air out of the tank. Preliminary tests have shown this method is promising: the argon does not significantly mix with the air. These tests reached acceptable oxygen purity levels after only three volume changes of gaseous argon.
Another major challenge is that of long wires. Long wires introduce capacitance, and therefore noise, and are susceptible to breakage during cooling and in the event of convection currents. The close proximity of the wires [5mm] causes a significant amount of capacitance [~12 pF/m for stainless steel in cold conditions] which manifests itself as noise in the event signals. Fortunately this linear capacitance leaves us within the 600-800 pF range we have deemed to be acceptable to record a usable signal. We are still investigating the effects of cooling the wires: as the wires have a very low volume to surface area ratios, they will cool to the temperature of LAr almost immediately after LAr touches them. However the apparatus holding them will take significantly longer to cool. Consequently, the wires will be put under more tension than desirable while the apparatus cool. During this time, the wires are liable to break or be stretched. Naturally, neither of these is welcome.
Analysis Effort: Under Construction