A model for the Global Quantum Efficiency for a TPB-based wavelength-shifting system used with photomultiplier tubes in liquid argon in MicroBooNE

Abstract

A: 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 PE ) given the known systematic errors on the simulation parameters. We compare results from four measurements of the N 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 ± 0.0009 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.

Convolutional neural network for multiple particle identification in the MicroBooNE liquid argon time projection chamber

“…MicroBooNE's photon detection system. A photon detection system [18] is installed behind the TPC anode plane to detect scintillation light emitted by the argon atoms that are excited by charged particles passing through the argon. Liquid argon is a highperformance prompt scintillator with a yield of about 30,000 photons/MeV at MicroBooNE's nominal electric field of 273 V/cm [19,20] with ∼23% of the total light emitted within a few ns [21].…”

“…Liquid argon is a highperformance prompt scintillator with a yield of about 30,000 photons/MeV at MicroBooNE's nominal electric field of 273 V/cm [19,20] with ∼23% of the total light emitted within a few ns [21]. The MicroBooNE photon detection system consists of 32 8-inch cryogenic Hamamatsu photomultiplier tubes (PMTs) equipped with wavelength-shifting tetraphenyl butadiene (TPB) coated acrylic front plates [18]. MicroBooNE's readout electronics [22] record 23.4 µs long waveforms starting at the beam trigger.…”

First Measurements of Inclusive Muon Neutrino Charged Current Differential Cross Sections on Argon at 0.8 GeV Average Neutrino Energy with the MicroBooNE Detector

“…The GQE is the convolution of the efficiency of the acrylic+TPB 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 PMT. This has been estimated for MicroBooNE using a similar procedure as described in [113], and amounts to qe = 0.0093. The solid angle covered by each PMT with respect to each track…”

A 4 tonne demonstrator for large-scale dual-phase liquid argon time projection chambers

“…A thin layer of TPB is used to convert the argon scintillation light centred at 128 nm [31] to the wavelengths in the range of the PMT photo-cathode sensitivity. TPB is particularly well-suited for the detection of VUV light in cryogenic environment and commonly used in experiments using liquid argon [32,33]. Three of the five PMTs are coated with TPB by evaporating it directly on the glass window while for the other two, the coating is applied on a transparent 4 mm thick acrylic 18 plate mounted in front of the photo-detectors.…”