Image credit: Fermilab Media Services.
Scientists and engineers working on the design of the particle detector for the Long-Baseline Neutrino Experiment (LBNE) celebrated a major success in January. They showed that very large cryostats for liquid-argon-based neutrino detectors can be built using industry-standard technology normally employed for the storage of liquefied natural gas. The 35-tonne prototype system satisfies LBNE’s stringent purity requirement on oxygen contamination in argon of less than 200 parts per trillion (ppt) – a level that the team could maintain stably.
The purity of liquid argon is crucial for the proposed LBNE time-projection chamber (TPC), which will feature wire planes that collect electrons from an approximately 3.5 m drift region. Oxygen and other electronegative impurities in the liquid can absorb ionization electrons created by charged particles emerging from neutrino interactions and prevent them from reaching the TPC’s signal wires.
The test results were the outcome of the first phase of operating the LBNE prototype cryostat, which was built at Fermilab and features a membrane designed and supplied by the IHI Corporation of Japan. As part of the test, engineers cooled the system and filled the cryostat with liquid argon without prior evacuation. On 20 December, during a marathon 36 hour session, they cooled the membrane cryostat slowly and smoothly to 110 K, at which point they commenced the transfer of some 20,000 litres of liquid argon, maintained at about 89 K, from Fermilab’s Liquid-Argon Purity Demonstrator to the 35 tonne cryostat. By the end of the session, the team was able to verify that the systems for purifying, recirculating and recondensing the argon were working properly.
The LBNE team then topped off the tank with an additional 6000 litres of liquid argon and began to determine the argon’s purity by measuring the lifetime of ionization electrons travelling through the liquid, accelerated by an electric field of 60 V/cm. The measured electron lifetimes were between 2.5 and 3 ms – corresponding to an oxygen contamination approaching 100 ppt and nearly two times better than LBNE’s minimum requirement of 1.5 ms.
The Phase II testing programme, scheduled to begin at the end of 2014, will focus on the performance of active TPC detector elements submerged in liquid argon. Construction of the LBNE experiment, which will look for CP violation in neutrino oscillations by examining a neutrino beam travelling 1300 km from Fermilab to the Sanford Underground Research Facility, could begin in 2016. More than 450 scientists from 85 institutions collaborate on LBNE.
