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Cryogenic performance of Silicon Photo-Multipliers at LN $_2$ temperature and at 3 K in the RES-NOVA cryostat

This paper presents measurements of the breakdown voltage and gain performance of Hamamatsu S13360-1350CS Silicon Photo-Multipliers at cryogenic temperatures of 77 K and 3 K within the RES-NOVA cryostat, supporting their application in detecting scintillation light from radiopure PbWO $_4$ crystals for supernova neutrino studies.

Original author: Riccardo Rossini, Massimo Rossella, Massimiliano Clemenza, Simone Copello, Carlo De Vecchi, Stefano Dell'Oro, Luca Gironi, Claudio Gotti, Lisa Malliolio, Nicola Manenti, Alessandro Menegolli, Luca Pat

Published 2026-11-03

📖 4 min read🧠 Deep dive

CC BY 4.0

Original author: Riccardo Rossini, Massimo Rossella, Massimiliano Clemenza, Simone Copello, Carlo De Vecchi, Stefano Dell'Oro, Luca Gironi, Claudio Gotti, Lisa Malliolio, Nicola Manenti, Alessandro Menegolli, Luca Pattavina, Daniele Peracchi, Simone Quitadamo, Marco C. Prata, Gian Luca Raselli, Laura Trombetta

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the author. For technical accuracy, refer to the original paper. Read full disclaimer

Deep in the heart of stars, violent explosions known as supernovae release a flood of ghostly particles called neutrinos. These particles are so elusive that they pass through the Earth and our bodies without ever touching anything, making them incredibly difficult to catch. To study them, scientists build massive, ultra-sensitive detectors designed to listen for the faintest whisper of energy when a neutrino finally interacts with an atom. One promising approach involves using crystals made of lead tungstate, a material that glows when struck by these particles. However, this glow is a shy one; at the comfortable temperature of a human room, the crystals are nearly silent. They only begin to shine brightly when cooled to temperatures far colder than any winter on Earth, approaching the absolute zero of the universe. To hear this faint light, researchers need a special kind of eye, a sensor capable of seeing single photons in the freezing dark, and they must be sure this sensor works perfectly in such extreme cold.

A team of researchers from Italy set out to test a specific type of sensor, known as a silicon photo-multiplier, to see if it could serve as that eye for the RES-NOVA experiment. These sensors are small, rugged chips that can detect light with high precision, but their behavior changes drastically when the temperature drops. The team wanted to know exactly how these chips would perform when cooled to the temperature of liquid nitrogen and even further down to the near-absolute cold of three Kelvin. They built a custom setup to hold two of these sensors inside a specialized machine called a dilution refrigerator, which can reach temperatures as low as ten thousandths of a degree above absolute zero. The goal was to map out how the sensors' ability to detect light and their internal electrical properties shifted as the cold deepened, ensuring they would be reliable tools for catching the faint signals from supernovae.

The researchers began their investigation by testing the sensors in a controlled climate chamber, where they could slowly lower the temperature from room temperature down to minus fifty degrees Celsius. They measured how much voltage was needed to make the sensors work and counted how many false signals, or "noise," the sensors produced when no light was present. As the temperature dropped, the sensors became quieter, producing fewer false alarms, and the voltage required to activate them changed in a predictable way. This initial phase confirmed that the sensors were behaving as expected as they cooled, setting the stage for the more extreme tests to come.

Next, the team moved the sensors into a bath of liquid nitrogen, chilling them to seventy-seven Kelvin. Here, they measured the electrical current flowing through the sensors to find a critical point called the breakdown voltage, which is the specific electrical pressure needed to turn the sensor on. They found that this voltage dropped significantly compared to room temperature, a result that makes sense because the cold makes it easier for electrical charges to move and multiply within the silicon. The sensors remained stable, and the team could clearly see the distinct steps in their response to light, proving they were ready for even colder conditions.

The final and most challenging test took place inside the dilution refrigerator at the University of Milano-Bicocca, where the sensors were cooled to just three Kelvin. In this near-absolute cold, the researchers repeated their electrical measurements. Surprisingly, they found that the breakdown voltage did not drop any further; it stayed exactly the same as it had been at the temperature of liquid nitrogen. This stability is a crucial discovery. It suggests that once these sensors are cooled below seventy-seven Kelvin, their core electrical behavior settles into a steady state and does not continue to shift as the temperature plummets toward absolute zero. This finding gives the scientists confidence that the sensors will remain reliable and predictable when installed in the final detector, which will operate at temperatures even colder than three Kelvin.

The study concludes that these silicon sensors are well-suited for the task of detecting the faint glow of lead tungstate crystals in the deep cold. By mapping out their performance from room temperature down to three Kelvin, the researchers have provided a clear roadmap for how these devices behave in extreme environments. The fact that the sensors stabilize at such low temperatures means they can be trusted to distinguish real signals from background noise in the quest to detect neutrinos from distant supernovae. With this validation, the path is now open to use these sensors to characterize the crystals and, eventually, to listen for the cosmic whispers of exploding stars.

Technical Summary: Cryogenic Performance of Silicon Photo-Multipliers at LN2 Temperature and at 3 K in the RES-NOVA Cryostat

Problem Statement
The RES-NOVA experiment aims to detect astrophysical neutrinos, specifically those from core-collapse supernovae, using Coherent Elastic Neutrino-Nucleus Scattering (CEνNS). The proposed detector utilizes an array of cryogenic calorimeters based on lead tungstate (PbWO4) crystals. A critical challenge in this design is the detection of scintillation light, which serves as a complementary channel for particle identification and background discrimination (distinguishing nuclear recoils from electron recoils). However, PbWO4 crystals exhibit negligible scintillation light yield at room temperature; their light output increases significantly only as temperatures decrease, reaching a plateau around liquid nitrogen temperature (77 K). Consequently, a suitable photosensor capable of operating at cryogenic temperatures (down to the 10 mK scale of the calorimeter) is required. Silicon Photo-Multipliers (SiPMs) are promising candidates due to their compact size, low operating voltage, and high photon detection efficiency. However, their performance parameters—specifically breakdown voltage, gain, dark count rate, and noise characteristics—must be thoroughly characterized at cryogenic temperatures to ensure stability and reliability in detectors operating well below 1 K.

Methodology
The study focuses on characterizing Hamamatsu S13360-1350CS SiPMs under controlled thermal conditions. The experimental setup involves two SiPMs mounted on a custom copper support, positioned at variable distances from an optical fiber used for light injection. The system is connected to external readout electronics via RG178 coaxial cables and dedicated cryostat feedthroughs.

The characterization process involved three distinct stages:

Climatic Chamber Tests: Preliminary measurements were conducted at the University of Pavia using a climatic chamber over a temperature range of -50°C to +25°C. This stage validated the setup and established baseline electrical and optical properties.
Liquid Nitrogen (77 K) Tests: The assembly was immersed in a liquid nitrogen bath to investigate I-V characteristics and mechanical behavior at 77 K.
Dilution Refrigerator (3 K) Tests: The final and most critical tests were performed at the University of Milano-Bicocca using an Oxford Instruments cryogen-free dilution refrigerator (Proteox LX). This system, designed for the RES-NOVA project, provided stable operating conditions down to 10 mK. The SiPMs were cooled to an operating temperature of 3 K.

Key parameters measured included:

Breakdown Voltage ( $V_{bd}$ ): Determined via two methods: (1) extrapolating the gain-to-voltage curve to zero gain using single-photon spectra, and (2) analyzing I-V characteristics using a Keithley 6487 picoammeter.
Gain: Calculated from the distance between adjacent peaks in the photon spectrum, calibrated against the digitizer specifications and amplification gain.
Dark Count Rate (DCR): Evaluated by counting pulses exceeding a 0.5 photoelectron threshold as a function of the trigger threshold (staircase plot).

Key Results

Breakdown Voltage Evolution: The breakdown voltage was observed to decrease significantly as temperature dropped. At room temperature (302 K), $V_{bd}$ was approximately 51.5 V. At 77 K, it decreased to approximately 41.5 V. Crucially, the measurements at 3 K showed that the breakdown voltage remained stable at approximately 41.5 V, indicating that the voltage plateaued between 77 K and 3 K.
Gain and Linearity: The gain was found to be linearly dependent on the bias voltage. The extrapolation of the gain curve to zero allowed for the precise determination of $V_{bd}$ .
Dark Count Rate: As expected, the dark count rate decreased with decreasing temperature. Staircase plots confirmed that both the dark count rate (single photon triggers) and optical crosstalk rate (double photon triggers) visibly decreased as the temperature was lowered from +25°C to -50°C.
Mechanical Stability: The setup demonstrated mechanical robustness at cryogenic temperatures. Specific attention was paid to the optical fiber clamping, where the ferrule was left floating to avoid mechanical stress on the fiber during thermal contraction.

Significance and Claims
The paper presents a preliminary but successful characterization of SiPMs for use in the RES-NOVA experiment. The primary significance of this work lies in validating the experimental facility and methodology for optical characterization of PbWO4 crystals at cryogenic temperatures.

The authors claim that the observed behavior of the breakdown voltage—specifically the linear decrease from room temperature to 150 K followed by a plateau below 42 V at lower temperatures—is consistent with previous studies on Hamamatsu S13370 (VUV4) SiPMs and theoretical models of avalanche formation in silicon. This agreement validates the experimental setup used in the Oxford Instruments cryostat.

The study concludes that the setup is operational and yields promising results for the intended application. The authors state that this work contributes to the development of photosensor solutions for cryogenic scintillating detectors. While the paper notes that the breakdown voltage stabilizes at 3 K, it modestly frames the current work as a "preliminary characterisation" and a step toward setting up a facility for the "quick optical characterisation" of the PbWO4 crystals. Future work is planned to include measurements at intermediate temperatures (specifically 38 K) to further refine the temperature dependence profile, but the paper does not claim that the SiPMs are fully qualified for the final detector deployment without these further campaigns.

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