Claudio R. Chavez

The development of novel instrumentation requires an iterative cycle with three stages: design, prototyping, and testing. Recent advancements in simulation and nanofabrication techniques have significantly accelerated the design and prototyping phases. Nonetheless, detector characterization continues to be a major bottleneck in device development. During the testing phase, a significant time investment is required to characterize the device in different operating conditions and find optimal operating parameters. The total effort spent on characterization and parameter optimization can occupy a year or more of an expert's time. In this work, we present a novel technique for automated imaging sensor characterization that aims to accelerate the testing stage of the development cycle. This technique leverages closed-loop Bayesian optimization (BO), using real-time measurements to guide parameter selection and identify optimal operating states. We demonstrate the method with a novel low-noise CCD, showing that the machine learning-driven technique can efficiently characterize and optimize operation of the sensor without supervision of a device expert.

This work presents an X-ray characterization of a fully depleted, 725 μm thick p-channel SiSeRO-CCD. Measurements with a Fe55 source yield an energy resolution of 128.3±2.2 eV FWHM at 5.9 keV for single-pixel events, indicating that the SiSeRO amplifier preserves the intrinsic charge resolution of the CCD under multi-sample non-destructive readout. Characterization with a Am241 source extends the response to higher-energy photons, with reconstructed spectral features observed between 9 and 26 keV and the 59.5 keV γ emission. These measurements, together with a muon-derived diffusion calibration, show that charge transport and diffusion are consistent with interactions spanning the full sensor depth. These results demonstrate that the SiSeRO-CCD simultaneously achieves sub-electron noise performance and efficient charge collection in a thick, fully depleted silicon detector. This combination enables X-ray spectroscopy across a broad energy range while maintaining sensitivity to faint signals.

The Multi-channel Integrating Dual-slope Noise-Aware (MIDNA) Application Specific Integrated Circuits (ASICs) are a series of skipper-CCD readout chips fabricated in a 65nm low-power CMOS process that implement a Correlated Double Sampling (CDS) signal processing chain based on dual-slope integrators. They are designed to operate from room temperature to cryogenic temperatures, down to 84K. The present iteration of the ASIC has been fabricated with several design updates and the addition of an on-chip voltage reference, resulting in improved performance. This work presents the main vulnerabilities solved, the changes carried out, and the resulting performance benefits. Measurements with a skipper-CCD and the ASIC at 140K showed that the single-electron resolution can be reached by averaging the measured charge in the analog domain using the analog pile-up technique with a readout noise as low as 0.11 e⁻_rms of equivalent charge for 1200 averaged samples. The channel-to-channel crosstalk was characterized, showing an improvement from −38dB in the previous iteration to better than −62dB in the present ASIC. When using the analog pile-up technique, one limiting factor was the residual offset after CDS. In the updated ASIC, improvements to the integrator offset have decreased the CDS residue from 3.2mV to 0.2mV, enabling additional signal pile-up.

In this work, we characterize the radiation-induced damage in two thick, p-channel skipper-CCDs irradiated unbiased and at room temperature with 217-MeV protons. We evaluate the overall performance of the sensors and demonstrate their single-electron/single-photon sensitivity after receiving a fluence on the order of 10¹⁰ protons/cm². Using the pocket-pumping technique, we quantify and characterize the proton-induced defects from displacement damage. We report an overall trap density of 0.134 traps/pixel for a displacement damage dose of 2.3 × 10⁷ MeV/g. Three main proton-induced trap species were identified, V₂, C _i O _i and V _n O _m , and their characteristic trap energies and cross sections were extracted. We found that while divacancies are the most common proton-induced defects, C _i O _i defects have a greater impact on charge integrity at typical operating temperatures because their emission-time constants are comparable or larger than typical readout times. To estimate ionization damage, we measure the characteristic output transistor curves. We found no threshold voltage shifts after irradiation. Our results highlight the potential of skipper-CCDs for applications requiring high-radiation tolerance and can be used to find the operating conditions in which effects of radiation-induced damage are mitigated.

The multiple-amplifier sensing charge-coupled device (MAS-CCD) has emerged as a promising technology for astronomical observation, quantum imaging, and low-energy particle detection due to its ability to reduce the readout time for the same readout noise level compared with its predecessor, the skipper-CCD, by reading out the same charge packet through multiple inline amplifiers. Previous works identified a new parameter in this sensor, called node removal inefficiency (NRI), related to inefficiencies in charge transfer and residual charge removal from the sense node of each amplifier after readout. These inefficiencies can lead to distortions in the measured signals similar to those produced by the charge transfer inefficiencies in standard CCDs. We introduce more details in the mathematical model of the NRI mechanism and provide techniques to quantify its magnitude from the measured data. It also proposes a new operation strategy that significantly reduces its effect with minimal alterations of the timing sequences or voltage settings for the other signals of the sensor. The proposed technique is demonstrated experimentally on a 16-amplifier MAS-CCD. At the same time, the experimental data demonstrate that this approach minimizes the NRI effect to levels comparable with other sources of distortion such as the charge transfer inefficiency in scientific devices.

We present a candidate sensor for future spectroscopic applications, such as a Stage-5 Spectroscopic Survey Experiment or the Habitable Worlds Observatory. This type of charge-coupled device (CCD) sensor features multiple in-line amplifiers at its output stage allowing multiple measurements of the same charge packet, either in each amplifier or in the different amplifiers. Recently, the operation of an eight-amplifier sensor has been experimentally demonstrated, and we present the operation of a 16-amplifier sensor. This new sensor enables a noise level of ∼1 erms− with a single sample per amplifier. In addition, it is shown that sub-electron noise can be achieved using multiple samples per amplifier. In addition to demonstrating the performance of the 16-amplifier sensor, we aim to create a framework for future analysis and performance optimization of this type of detectors. New models and techniques are presented to characterize specific parameters, which are absent in conventional CCDs and Skipper CCDs: charge transfer between amplifiers and independent and common noise in the amplifiers and their processing.

A novel readout architecture that uses multiple nondestructive floating-gate amplifiers (FGAs) to achieve subelectron readout noise in a thick, fully depleted silicon detector is presented. This multiple-amplifier sensing charge-coupled device (MAS-CCD) can perform multiple independent charge measurements with each amplifier; measurements with multiple amplifiers can then be combined to further reduce the readout noise. This allows getting subelectron noise operation in less time compared with single-nondestructive-amplifier CCDs. The performance of this detector is demonstrated, emphasizing the ability to resolve individual quanta and to combine measurements across amplifiers to reduce readout noise. These characteristics make it a candidate technology for astronomical observations, quantum imaging, and low-energy interacting particles.

In this paper, a digital optimal-filtering method is proposed for reducing readout noise in charge-coupled devices (CCDs). The technique includes a number of noise sources that other methods omit but may restrict the sensor’s effectiveness. A procedure is described to obtain the noise directly from the video signal under standard readout sequence, instead of using an indirect noise measurements approach that do not capture all the interference sources. The proposed research method underwent extensive experimental testing on a large number of scientific sensors, reducing noise in comparison to standard techniques. A significant improvement in the performance comes from using a non-stationary noise model for the video signal and the calibration of the dynamics of the charge transfer signal for each sensor. The average reduction in the noise standard deviation of all tested devices was approximately 40%. Moreover, the spread of the standard deviation values was reduced compared to the standard technique. These two aspects indicate that this technique may increase the selection yield for the construction of multiple-sensors instruments.

This paper presents the implementation of a multiplexed analog readout electronics system that can achieve single-electron counting using Skipper-CCDs with non-destructive readout. The proposed system allows the best performance of the sensors to be maintained, with sub-electron noise-level operation, while maintaining low-bandwidth data transfer, a minimum number of analog-to-digital converters (ADC) and low disk storage requirement with zero added multiplexing time, even for the simultaneous operation of thousands of channels. These features are possible with a combination of analog charge pile-up, sample and hold circuits and analog multiplexing. The implementation also aims to use the minimum number of components in circuits to keep compatibility with high-channel-density experiments using Skipper-CCDs for low-threshold particle detection applications. Performance details and experimental results using a sensor with 16 output stages are presented along with a review of the circuit design considerations.