Optical calibration of the SNO+ detector in the water phase with deployed sources

TL;DR

This paper details the optical calibration of the SNO+ detector during its water phase, using deployed sources to measure optical properties and validate the detector model, thereby reducing systematic uncertainties in energy response.

Contribution

It introduces an in situ calibration method for the SNO+ detector using deployed sources, improving the accuracy of the detector model and energy response systematic uncertainties.

Findings

Measured water attenuation coefficients across wavelengths

Validated detector model with a gamma source showing 0.6% energy scale variation

Characterized optical properties of acrylic vessel and PMTs

Abstract

SNO+ is a large-scale liquid scintillator experiment with the primary goal of searching for neutrinoless double beta decay, and is located approximately 2 km underground in SNOLAB, Sudbury, Canada. The detector acquired data for two years as a pure water Cherenkov detector, starting in May 2017. During this period, the optical properties of the detector were measured in situ using a deployed light diffusing sphere, with the goal of improving the detector model and the energy response systematic uncertainties. The measured parameters included the water attenuation coefficients, effective attenuation coefficients for the acrylic vessel, and the angular response of the photomultiplier tubes and their surrounding light concentrators, all across different wavelengths. The calibrated detector model was validated using a deployed tagged gamma source, which showed a 0.6% variation in energy scale across the primary target volume.