Liquid Argon Time Projection Chamber: Purity and Purity MonitoringDAVID GERSTLE – LArTPC – YALE UNIVERSITY/FNAL

31 May 2006 – FNAL Users’ Meeting

Materials lock

LAr boil-off condenser

LN2

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) O2 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): Qanode/Qcathode = e-tdrift/

•O2 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.

Qanode

anode signal

cathode signal

photodiode

tdrift= 150 s, Qanode/Qcathode= ~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).

Qcathode

Ch3 100mVCh1 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 600 - 800 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 μs; 12.5 MS/s

Cathode Plane

Wire Plane

Carlo Rubia O

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G. Carugno et. al., NIM A292 (1990)

Argon gas