Measurement of space charge effects in the MicroBooNE LArTPC using cosmic muons

Abstract

A: Large liquid argon time projection chambers (LArTPCs), especially those operating near the surface, are susceptible to space charge effects. In the context of LArTPCs, the space charge effect is the build-up of slow-moving positive ions in the detector primarily due to ionization from cosmic rays, leading to a distortion of the electric field within the detector. This effect leads to a displacement in the reconstructed position of signal ionization electrons in LArTPC detectors ("spatial distortions"), as well as to variations in the amount of electron-ion recombination experienced by ionization throughout the volume of the TPC. We present techniques that can be used to measure and correct for space charge effects in large LArTPCs by making use of cosmic muons, including the use of track pairs to unambiguously pin down spatial distortions in three dimensions. The performance of these calibration techniques are studied using both Monte Carlo simulation and MicroBooNE data, utilizing a UV laser system as a means to estimate the systematic bias associated with the calibration methodology.

Space Charge Effects in Noble-Liquid Calorimeters and Time Projection Chambers

“…It has been operated at a nominal electric field of 274 V/cm. Under these conditions, and using µ + = 1.6 × 10 −7 m 2 s −1 V −1 , the α parameter is approximately equal to 0.81, and space charge effects were clearly observed and extensively reported by the MicroBooNE Collaboration [14]: uncorrected crossing tracks appear bent in the xy and xz projections more than in ICARUS, with sagittas on the centimeter scale. Besides, the most apparent effect of space charge occurs on the side faces: the transverse coordinates of entry or exit points of ionizing particles crossing the field-cage are shifted towards the center of the detector.…”

“…Besides the laser system, MicroBooNE has extended the method of crossing tracks, replacing laser beams with crossing cosmic muons [14]. The method relies on the position of the end-points of tracks crossing the detector boundaries.…”

“…The MicroBooNE Collaboration has provided an extensive report on transverse and longitudinal distortions caused by space charge [14]. As mentioned above in Section 2.2.2, the vertical extension of the detector (along y) is similar to the drift gap (along x), while the extension along the third coordinate (z) is much larger.…”

“…The data points accumulate on the cathode plane and on the profile of the top and bottom side faces, which appear distorted towards the center of the detector because of a transverse component in the electric field due to space-charge. Reproduced from reference[14].…”

Cosmic Ray Background Rejection with Wire-Cell LArTPC Event Reconstruction in the MicroBooNE Detector

“…While the active TPC volume is a rectangular cuboid defined by the rectangular wire-planes at the anode and the corresponding cathode at the opposite end, the reconstructed TPC boundary from trajectories of charged particles deviate from the physical boundary because of the space charge effect. The space charge effect [50,51] is caused by the drift of positively charged argon ions toward the cathode plane. Since the mass of the argon ion is much larger than the mass of the electron, the drift velocity of the ion is about five orders of magnitude slower.…”