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Characterization of CRYO ASIC for charge readout in the nEXO experiment

This paper demonstrates that the CRYO ASIC, a cryogenic application-specific integrated circuit designed for direct operation in liquid xenon, meets the nEXO experiment's charge readout requirements by achieving high gain stability, reliable in-situ calibration, and low noise performance while mitigating boiling effects through elevated pressure operation.

Original authors: Z. Li, M. Yu, E. Angelico, A. Atencio, A. Gupta, P. Knauss, A. Pena-Perez, B. G. Lenardo, P. Acharya, A. Amy, A. Anker, I. J. Arnquist, J. Bane, V. Belov, T. Bhatta, A. Bolotnikov, J. Breslin, P. A. B

Published 2026-09-10

📖 4 min read🧠 Deep dive

CC BY 4.0

Original authors: Z. Li, M. Yu, E. Angelico, A. Atencio, A. Gupta, P. Knauss, A. Pena-Perez, B. G. Lenardo, P. Acharya, A. Amy, A. Anker, I. J. Arnquist, J. Bane, V. Belov, T. Bhatta, A. Bolotnikov, J. Breslin, P. A. Breur, J. P. Brodsky, E. Brown, T. Brunner, B. Burnell, E. Caden, G. F. Cao, L. Q. Cao, D. Cesmecioglu, D. Chernyak, M. Chiu, R. Collister, M. Marquis, T. Daniels, L. Darroch, R. DeVoe, M. L. di Vacri, X. Defay, Y. Y. Ding, D. Doering, M. J. Dolinski, A. Dragone, B. Eckert, A. Emara, N. Fatemighomi, W. Fairbank, B. T. Foust, D. Gallacher, N. Gallice, A. Gaur, W. Gillis, F. Girard, A. Gorham, K. Gracequist, G. Gratta, C. A. Hardy, J. Hasi, S. Hedges, M. Heffner, E. Hein, H. Hernandez Herrera, J. D. Holt, A. Iverson, X. S. Jiang, A. Karelin, D. Keblbeck, C. Kenney, I. Kotov, A. Kuchenkov, K. S. Kumar, S. Lavoie, A. Larson, M. B. Latif, K. G. Leach, D. S. Leonard, G. Lessard, K. K. H. Leung, G. Li, X. Li, C. Licciardi, R. Lindsay, R. MacLellan, S. Majidi, C. Malbrunot, B. Markovic, J. Masbou, M. Medina-Peregrina, S. Mngonyama, B. Mong, D. C. Moore, K. Ni, I. Nitu, A. Nolan, S. C. Nowicki, J. C. Nzobadila Ondze, A. Odian, J. L. Orrell, G. S. Ortega, L. Pagani, H. Peltz Smalley, A. Piepke, A. Pocar, S. Prentice, V. Radeka, E. Raguzin, R. Rai, H. Rasiwala, D. Ray, B. Reese, S. Rescia, F. Retiere, G. Richardson, V. Riot, R. Ross, L. Rota, P. C. Rowson, R. Saldanha, S. Sangiorgio, S. Sekula, T. Shetty, L. Si, F. Spadoni, V. Stekhanov, X. L. Sun, S. Thibado, T. Totev, S. Triambak, R. H. M. Tsang, O. A. Tyuka, T. Vallivilayil John, E. van Bruggen, M. Vidal, S. Viel, M. Walent, H. Wang, Q. D. Wang, M. Watts, W. Wei, M. Wehrfritz, L. J. Wen, S. Wilde, X. M. Wu, H. Xu, H. B. Yang, L. Yang, O. Zeldovich, J. Zhao

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 authors. For technical accuracy, refer to the original paper. Read full disclaimer

Deep beneath the Earth's surface, in the quiet dark of a laboratory, scientists are hunting for a ghost. They are searching for a rare event called neutrinoless double beta decay, a process that, if found, would rewrite our understanding of the universe and explain why matter exists at all. To catch this fleeting event, researchers are building a massive detector filled with liquid xenon, a heavy, clear fluid cooled to temperatures far colder than any natural winter. When a rare atomic nucleus inside this liquid decays, it releases a tiny burst of electric charge. To see this signal, the detector must be equipped with incredibly sensitive electronic eyes that can sit directly inside the freezing liquid, listening for the faintest whisper of electricity without adding any noise of their own. The challenge is immense: the electronics must work perfectly in a hostile, super-cold environment, remain free of any radioactive contamination that could hide the signal, and be precise enough to distinguish a single event from the background hum of the universe.

A team of researchers has now demonstrated that they have built the right kind of eye for this job. They tested a custom-made computer chip, designed specifically to operate while submerged in liquid xenon, and found that it performs exactly as needed. In a series of experiments, the team placed the chip inside a chamber filled with the super-cold liquid and watched how it behaved. They discovered that the chip generates just enough heat to cause the liquid xenon to boil locally, creating tiny bubbles that would interfere with the delicate measurements. However, they also found a simple solution: by slightly increasing the pressure inside the chamber, they could stop the boiling entirely, allowing the chip to operate in a calm, stable liquid. Under these conditions, the chip proved to be remarkably quiet and stable, capable of detecting the tiny electrical signals from the detector without introducing errors.

The chip, known as the CRYO ASIC, is a small piece of silicon that acts as a translator, converting the invisible flow of electrons from the liquid into digital data that scientists can analyze. It is designed to be mounted directly on the sensor tiles inside the detector, eliminating the need for long wires that would pick up interference. To test it, the researchers first placed the chip in a cold chamber filled with nitrogen gas, simulating the freezing temperatures of the final detector. They verified that the chip could start up, read signals, and remain stable over many hours. They checked its ability to measure the strength of a signal with extreme precision, finding that its readings varied by less than two-tenths of a percent over a fourteen-hour period. This level of consistency is crucial, as it means the detector can trust its own measurements over the many years it will spend searching for the rare decay.

The real test came when the chip was submerged in actual liquid xenon. The researchers had to be careful because the chip, like any electronic device, produces a small amount of heat as it works. When they first turned it on in the liquid, the heat caused the xenon to boil, creating a layer of gas bubbles around the chip. These bubbles acted like static on a radio, creating noise that would have made it impossible to see the faint signals they were looking for. By monitoring the liquid with a camera and adjusting the pressure inside the tank, the team showed that raising the pressure to just above one-tenth of a megapascal stopped the boiling. Once the liquid settled, the noise dropped to a level that matched their computer simulations perfectly.

The measurements confirmed that the chip meets the strict requirements for the experiment. It achieved a noise level low enough to detect the tiny charge of a single event, with a performance that stayed consistent across all its sixty-four channels. The team also showed that the chip could calibrate itself using an internal signal generator, a feature that will allow the detector to check its own accuracy over time without needing external equipment. These results prove that the chip is a viable solution for the next generation of neutrinoless double beta decay experiments. It offers a way to listen to the liquid xenon with the clarity and silence required to hear the universe's most elusive secrets. With this technology validated, the path is clear to build the full-scale detector, where these chips will work together to watch for the moment a nucleus changes without a trace, revealing a fundamental truth about the nature of matter.

Technical Summary: Characterization of CRYO ASIC for Charge Readout in the nEXO Experiment

Problem Statement
The nEXO experiment is a proposed next-generation search for neutrinoless double beta decay ( $0\nu\beta\beta$ ) of $^{136}$ Xe, utilizing a tonne-scale liquid xenon (LXe) time projection chamber (TPC). A critical requirement for the experiment is a charge readout system capable of achieving an overall energy resolution better than 1% at the $Q_{\beta\beta}$ value of 2.458 MeV, while maintaining extremely low radioactivity and enabling millimeter-scale position reconstruction. To meet these goals, the readout electronics must be placed directly within the LXe to minimize input capacitance and pick-up noise. The system requires digitizing 3,580 independent channels with microsecond-scale time resolution while keeping electronic noise below 150 equivalent noise electrons ( $e^-$ ). The challenge lies in developing a custom cryogenic application-specific integrated circuit (ASIC) that can operate stably in LXe, manage heat dissipation without inducing boiling, and maintain low noise and high gain stability over long durations.

Methodology
The authors characterized the first prototype of the CRYO ASIC, a System-on-Chip (SoC) waveform digitizer implemented in TSMC's 130 nm CMOS process. The ASIC features 64 input channels, each sampled at 2 MSPS with 12-bit resolution, and includes on-chip supply regulation and an internal pulser system for in-situ calibration.

The testing methodology involved two primary setups:

Cryogenic Environmental Chamber: A dry nitrogen gas environment at 160 K was used to test the ASIC's functionality, gain stability, and noise performance without the complications of liquid immersion. Two test boards were utilized: a "Cold Board" for single-chip testing and a Front-End Mother Board (FEMB) for simultaneous dual-chip operation. Both boards were loaded with C0G ceramic capacitors (22 pF or 39 pF) to simulate detector input capacitance.
LXe Test Stand: A dedicated test stand at UC San Diego, consisting of an inner LXe chamber submerged in an HFE-7000 cryostat, was used to evaluate the ASIC's performance directly in liquid xenon. The system was monitored via a camera and temperature/pressure sensors.

Key experimental variables included varying the peaking time (0.6 $\mu$ s to 3.6 $\mu$ s) and gain settings, with a focus on the configuration of 28.6 mV/fC gain and 1.2 $\mu$ s peaking time. The study specifically investigated the onset of nucleate boiling caused by ASIC heat dissipation and the efficacy of pressurization as a mitigation strategy.

Key Contributions and Results

Cryogenic Stability and Calibration: In the nitrogen environment at 160 K, the CRYO ASIC demonstrated stable operation. Over a 14-hour interval, the relative response variations for nearly all channels remained below 0.2%. The internal pulser system was shown to track the gain variations of an external reference signal with high fidelity, validating its use for periodic in-situ calibration.
Boiling Mitigation: Initial operation in LXe at ~0.095 MPa and 170.45 K resulted in visible boiling around the ASIC, which correlated with increased noise in the power spectral density. By applying localized heating to the top flange to raise the chamber pressure to ~0.104 MPa (while minimizing bulk temperature changes), the boiling was suppressed. The authors note that nEXO is planned to operate at ~0.15 MPa, suggesting boiling will not be an issue under final detector conditions.
Noise Performance: Noise measurements in LXe (at pressures suppressing boiling) showed agreement with design simulations.
- For a 22 pF load capacitance (representative of the top bank channels), the measured Equivalent Noise Charge (ENC) was approximately 156 $e^-$ with a 1.2 $\mu$ s peaking time.
- For a 39 pF load (bottom bank), the ENC was higher, consistent with simulation trends.
- Extrapolating to the estimated nEXO charge tile capacitance of 18.2 pF, the expected ENC is approximately 137 $e^-$ .
Simulation Agreement: The measured noise power spectral densities and ENC values in LXe aligned well with simulation models, confirming the validity of the design assumptions.

Significance and Claims
The paper establishes the CRYO ASIC as a viable low-noise in-LXe charge readout solution for the nEXO experiment. The results demonstrate that the ASIC achieves stable operation and gain stability (<0.2% variation) when operated directly in LXe. Furthermore, the study confirms that the heat dissipation from the ASIC can be managed through system pressurization to prevent boiling-induced noise. Crucially, the agreement between measured noise in LXe and simulation indicates that the 150 $e^-$ design requirement can be satisfied, particularly when extrapolated to the expected nEXO charge tile capacitance. These findings validate the feasibility of the proposed in-LXe readout architecture. Future work will involve testing the ASICs coupled to sensor tiles for ionization charge readout, as well as integration tests with radiopure cables and mechanical assemblies required for deployment in the nEXO experiment.

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