Liquid Argon Time Projection Chamber: Purity and Purity Monitoring

1
Liquid Argon Time Projection Chamber Purity and
Purity Monitoring
Why Build It?
LARGE Issues to be Addressed
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.

Toru Goto and Takeshi Nihei

• 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)

• 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).

Need Precision Detectors
Long Wires
Monitoring Purity by Drifting Electrons

• 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

anode grid (V)
cathode grid (0V)

• 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.

• 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

Long wire test set-up
Electron Flow due to E field
(ex-) Village Water Tank Project
How It Works
G. Carugno et. al., NIM A292 (1990)
anode (V)
photocathode (-V)
Air
anode signal
Out of service for decades
Argon gas
photodiode
Carlo Rubia
drifttime

• 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).

• 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.

Qcathode
cathode signal
M 40.0 µs 12.5 MS/s
tdrift 150 ?s, Qanode/Qcathode 1