A Model for the Global Quantum Efficiency for a TPB-based Wavelength-Shifting System used with Photomultiplier Tubes in Liquid Argon in MicroBooNE

TL;DR

This paper develops a GEANT4-based model to estimate the global quantum efficiency of TPB-coated optical units in MicroBooNE, validated with measurements, providing a broadly applicable approach for liquid argon detectors.

Contribution

The paper introduces a first-principles simulation model for the GQE of TPB-based optical units, validated with experimental data, applicable to liquid argon light detection systems.

Findings

Estimated GQE of 0.0055±0.0009 for MicroBooNE units

Model accurately predicts radial quantum efficiency dependence

Validated model with experimental measurements

Abstract

We present a model for the Global Quantum Efficiency (GQE) of the MicroBooNE optical units. An optical unit consists of a flat, circular acrylic plate, coated with tetraphenyl butadiene (TPB), positioned near the photocathode of a 20.2-cm diameter photomultiplier tube. The plate converts the ultra-violet scintillation photons from liquid argon into visible-spectrum photons to which the cryogenic phototubes are sensitive. The GQE is the convolution of the efficiency of the plates that convert the 128 nm scintillation light from liquid argon to visible light, the efficiency of the shifted light to reach the photocathode, and the efficiency of the cryogenic photomultiplier tube. We develop a GEANT4-based model of the optical unit, based on first principles, and obtain the range of probable values for the expected number of detected photoelectrons ($N_{\rm PE}$) given the known systematic errors on the simulation parameters. We compare results from four measurements of the $N_{\rm PE}$ determined using alpha-particle sources placed at two distances from a TPB-coated plate in a liquid argon cryostat test stand. We also directly measured the radial dependence of the quantum efficiency, and find that this has the same shape as predicted by our model. Our model results in a GQE of $0.0055\pm0.0009$ for the MicroBooNE optical units. While the information shown here is MicroBooNE specific, the approach to the model and the collection of simulation parameters will be widely applicable to many liquid-argon-based light collection systems.