The Neutrino Detector at the Far Site
A neutrino can travel through the Earth without interacting with a single atom, therefore most leave no trace of their passage. To observe even just a few of the extremely rare interactions of neutrinos with matter, physicists build detectors with massive amounts of target material and run the experiments for several years. The detectors record signals from the detectable particles that have emerged from these rare collisions of neutrinos with atoms of the target material. LBNE plans to construct a neutrino detector known as a Liquid Argon Time Projection Chamber (LArTPC), in which liquified argon is the target material. It will be placed in the path of the neutrino beam coming from Fermilab.
Argon, a gas at room temperature, condenses to a liquid when cooled to cryogenic temperatures. LArTPC technology uses liquid argon (LAr) as a target material. The detection method is based on the collection of ionization electrons, resulting from particle interactions in the liquid argon, onto wire planes immersed in the fluid. Under the influence of an electric field, the electrons drift to the wire planes, thereby creating a signal. A predecessor experiment at Fermilab, MicroBoone, will build a 100 (metric) ton LArTPC, whereas LBNE's conceptual design as of September 2012 describes a 10,000 ton detector installed in an excavated pit near the surface, but nestled into a hillside and covered with 3 meters of shielding to limit cosmic ray exposure.
In a later phase of the project or with extra funding from non-DOE sources, LBNE hopes to construct an even larger detector and place it nearly a mile underground, which would extend the experiment's physics capabilities. Such underground placement of the detector would shield it from cosmic rays, which are abundant and would generate signals in the detector that serve only to complicate the data analysis. Most cosmic rays would get absorbed by the matter above the detector.
The largest neutrino detector to date is a 50,000-ton water Cherenkov detector in Japan called Super-Kamiokande. In a water Cherenkov detector, charged particles are created in collisions of neutrinos with water molecules and light is emitted. Super-Kamiokande is placed 3300 feet underground in a cylindrical cavern about 130 feet in both height and width. The cavern is filled with water and on its walls is an array of light-sensitive devices called photomultiplier tubes (PMTs) that collect the emitted light.
The largest existing LArTPC neutrino experiment is ICARUS, located at the Gran Sasso National Laboratory in Italy. It uses a detector containing 600 tons of liquid argon target material to detect the arrival of neutrinos sent through 730 km of rock from the CERN laboratory in Switzerland. The ICARUS detector is composed of two symmetric, LAr-filled, instrumented modules seated inside of vacuum-tight cryostats roughly 3.5m by 4m by 20m in size. It is also located underground to reduce unwanted signals from cosmic rays.
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