Liquid Argon Time Projection Chamber: Purity and Purity Monitoring DAVID GERSTLE – LArTPC – YALE UNIVERSITY/FNAL 31 May 2006 – FNAL Users’ Meeting Materials lock LAr boil-off condenser LN 2 Gas contaminant Purity Monitor Terry Tope Achieving Purity: FNAL PAB set-up We are building a filtering device (left) at the Proton Assembly Building (PAB). The LAr must be pure to 0.03 ppb (for a 3m drift) O 2 to prevent the absorption of ionization electrons as they drift toward the readout planes. To achieve this, we are developing LAr filtration system employing dually a Trigon™ oxygen filter and a molecular sieve. LARGE Issues to be Addressed Above: a rough schematic of our proposed detector, weighing in at 15kton. For massive detectors, we must additionally resolve the issues of the initial cleaning/purging of the tank (bottom) and of long wires (directly below). Air (ex-) Village Water Tank Project Such a large vessel cannot be evacuated, a considerable challenge when the removal of oxygen is paramount. We are exploring using argon gas as “piston” (right) to displace the air from a large tank, requiring fewer volume changes to achieve a low level of oxygen contamination. Data from tests on a small tank at PAB show that our approach has great promise. Why Build It? Need Precision Detectors How It Works Monitoring Purity by Drifting Electrons Do neutrinos and anti-neutrinos oscillate at the same rate? What is the rate of  - e oscillation? The liquid argon TPC is the device to answer these questions. It could also be used to detect proton decay (right) photocathode (-V) anode (+V) cathode grid (0V) anode grid (+V) Electron Flow due to E field Electrons are ejected from a photocathode by a Xe lamp and drifted to the anode Cathode and anode signals are compared to determine drift-time (see bottom left): Q anode /Q cathode = e -t drift /  O 2 concentration = 3E-13 /  (in seconds) We have achieved drift lifetimes of 12ms which meets the specifications for a 3m drift and a 20% loss. Next we plan on testing the effect of impurities (other than oxygen and water) with the apparatus directly below. Q anode anode signal cathode signal photodiode t drift = 150  s, Q anode /Q cathode = ~1 drifttime The LArTPC provides very clear tracks by producing bubble-chamber-like images from wire-plane readouts and provides for total absorption calorimetry. However, larger detectors (and consequently new technology) are needed Ionization electrons from passing charged particles are drifted over meters to wire chamber readout planes which detect the electrons in predictable ways (right). Each drift region is surrounded by a field cage to ensure a uniform E field. Each set of signal wires consists of 3 wire planes offset at known angles (left). Q cathode Ch3 100mV Ch1 100mV Ch2 50.0mV Long Wires Toru Goto and Takeshi Nihei sample ICARUS LAr events The wires have a capacitance of ~12 pF/m; this capacitance affects the signal, so there is a limit to their length (max. of pF for reasonable signal). The wires’ structural integrity is stressed during the process of cooling to the temperature of LAr Long wire test set-up M 40.0 μ s; 12.5 MS/s Cathode Plane Wire Plane Carlo Rubia Out of service for decades G. Carugno et. al., NIM A292 (1990) Argon gas