Channel-Level Calibration Methods of Silicon Photomultiplier for JUNO-TAO Central Detector

TL;DR

This paper develops channel- and tile-level calibration methods for SiPMs in the TAO central detector to achieve sub-percent energy precision. It combines hit-time and charge-based analyses to calibrate DCR, time offset, PDE, gain, IOCT, and EOCT, introducing a novel LED-switching approach for EOCT rate and emission-angle calibration. Using one million simulated events, the study quantifies calibration biases: relative PDE ~2.1%, IOCT ~1.4%, DCR ~0.4%, EOCT rate <0.1%, gain <0.1%, and time offset ~0.027 ns, with EOCT angle bias <4% in the main angular range. The results guide calibration strategies in TAO and demonstrate the feasibility of achieving the detector’s requested energy resolution, while noting limitations such as AP calibration and surface-reflection effects to be addressed with real data. The work has practical impact for precision reactor antineutrino measurements and provides methods adaptable to similar SiPM-based detectors.

Abstract

The Taishan Antineutrino Observatory (TAO or JUNO-TAO) is a satellite observatory for the Jiangmen Underground Neutrino Observatory (JUNO), located 44 meters away from the No.1 reactor of the Taishan Nuclear Power Plant. TAO can measure the reactor antineutrino energy spectrum with excellent energy resolution (better than 2\% at 1 MeV) using state-of-the-art Silicon Photomultipliers (SiPMs) operated at low temperature. To achieve this goal, the SiPMs (together with their readout electronics) must be well calibrated. This paper presents the channel-level calibration methods for the dark count rate (DCR), relative photon detection efficiency (PDE), time offset, gain, and internal optical crosstalk (IOCT) of the SiPMs based on charge and time information of the collected events. For the calibration of the external optical crosstalk (EOCT), in terms of its rate and emission angle distribution, a novel method is proposed by switching on and off different groups of SiPMs with an LED placed in the detector. Using one million simulated events, the expected calibration biases are evaluated for all the aforementioned parameters: relative PDE (2.1\%), IOCT (1.4\%), DCR (0.4\%), EOCT Rate ($<0.1\%$), gain ($<0.1\%$), time offset (0.027 ns). The emission angle distribution of the EOCT photons could be measured with a bias of less than 4\% in main angular range.

Figures (19)

• Figure 1: Design of the TAO detector
• Figure 2: The Simulated Hit Time Distribution for a Channel. The trigger algorithm slides a 300 ns time window along the time axis. An event is triggered when the number of channels with hit within this trigger time window exceeds a preset threshold (set to 1000 in the TAO electronics simulation). The trigger time corresponds to the start of the trigger time window and is aligned to t = 0 ns. The scintillation light from the physics event is emitted within a short time after the trigger time, so hits occurring before the trigger time primarily originate from DN.
• Figure 3: Bias Between DCR Calibration Result and Mock Truth. The left and right figures show the results before and after the EOCT effect is turned off. The true DCR in simulation is set to 20 Hz/.
• Figure 4: Double-Sided Crystal Ball Function Fit to the Time Spectrum.
• Figure 5: Time Offset Calibration Result. The left figure shows the calibration result for each channel. In this simulation, half of the channels are set to the time offset of 0 ns, while the other half are set to the time offset of 20 ns. The middle and right figures show the bias between the time offset calibration result and the mock truth.