Characterization of the Spontaneous Light Emission of the PMTs used in the Double Chooz Experiment

TL;DR

This study investigates the spontaneous light emission from PMTs in the Double Chooz experiment, identifying its causes, effects, and mitigation strategies to reduce background noise in neutrino detection.

Contribution

It provides a detailed characterization of light noise in PMTs and introduces mitigation techniques, including hardware modifications, to improve detector performance.

Findings

Light noise originates mainly from the PMT base due to heat and high voltage effects.

Correlation between temperature and light noise rate was observed.

Mitigation achieved through signal analysis and hardware modifications.

Abstract

During the commissioning of the first of the two detectors of the Double Chooz experiment, an unexpected and dominant background caused by the emission of light inside the optical volume has been observed. A specific study of the ensemble of phenomena called "Light Noise" has been carried out in-situ, and in an external laboratory, in order to characterize the signals and to identify the possible processes underlying the effect. Some mechanisms of instrumental noise originating from the PMTs were identified and it has been found that the leading one arises from the light emission localized on the photomultiplier base and produced by the combined effect of heat and high voltage across the transparent epoxy resin covering the electric components. The correlation of the rate and the amplitude of the signal with the temperature has been observed. For the first detector in operation the induced background has been mitigated using online and offline analysis selections based on timing and light pattern of the signals, while a modification of the photomultiplier assembly has been implemented for the second detector in order to blacken the PMT bases.