arXiv subjects

Rafael Ballabriga

Publications and source records attributed to Rafael Ballabriga.

13 recordsLinked to original sources

arXiv2026

Development and characterization of hybrid MCP-PMT with embedded Timepix4 ASIC used as pixelated anode

We present a novel single-photon detector based on a vacuum tube incorporating a photocathode, a microchannel plate (MCP), and a Timepix4 CMOS ASIC functioning as a pixelated anode. Designed to handle photon rates up to 1 billion per second across a 7 cm$^2$ active area, the detector achieves outstanding spatial and temporal resolutions of 5-10 $μ$m and below 50 ps r.m.s., respectively. The Timepix4 ASIC comprises approximately 230,000 pixels, each integrating analog and digital front-end electronics. This enables data-driven acquisition and supports data transmission rates up to 160 Gb/s. External FPGA-based electronics manage both configuration and readout. In order to test the timing performance of the Timepix4 ASIC we performed preliminary characterization of an assembly bonded to a 100 $μ$m thick n-on-p silicon sensor using a pulsed infrared laser, which demonstrated a per-pixel timing resolution of 110 ps, with cluster-based averaging methods improving to below 50 ps. Six prototype detectors incorporating different MCP stack configurations and end-spoiling depths were produced by Hamamatsu Photonics. We report on their characterization, including dark count rates, gain, and spatial and timing resolutions, assessed both in laboratory conditions and during a test-beam campaign at CERN's SPS facility.

physics.ins-det↗

arXiv2026

Characterization of the H2M Monolithic CMOS Sensor

The H2M (Hybrid-to-Monolithic) is a monolithic pixel sensor manufactured in a modified \SI{65}{\nano\meter}~CMOS imaging process with a small collection electrode. Its design addresses the challenges of porting an existing hybrid pixel detector architecture into a monolithic chip, using a digital-on-top design methodology, and developing a compact digital cell library. Each square pixel integrates an analog front-end and digital pulse processing with an 8-bit counter within a \SI{35}{\micro\meter}~pitch. This contribution presents the performance of H2M based on laboratory and test beam measurements, including a comparison with analog front-end simulations in terms of gain and noise. A particular emphasis is placed on backside thinning in order to reduce material budget, down to a total chip thickness of \SI{21}{\micro\meter} for which no degradation in MIP detection performance is observed. For all investigated samples, a MIP detection efficiency above \SI{99}{\%} is achieved below a threshold of approximately 205 electrons. At this threshold, the fake-hit rate corresponds to a matrix occupancy of fewer than one pixel per the \SI{500}{\nano\second}~frame. Measurements reveal a non-uniform in-pixel response, attributed to the formation of local potential wells in regions with low electric field. A simulation flow combining technology computer-aided design, Monte Carlo, and circuit simulations is used to investigate and describe this behavior, and is applied to develop mitigation strategies for future chip submissions with similar features.

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arXiv2025

The H2M Monolithic Active Pixel Sensor -- characterizing non-uniform in-pixel response in a 65 nm CMOS imaging technology

The high energy physics community recently gained access to the TPSCo 65 nm ISC (Image Sensor CMOS), which enables a higher in-pixel logic density in monolithic active pixel sensors (MAPS) compared to processes with larger feature sizes. To explore this novel technology, the Hybrid-to-Monolithic (H2M) test chip has been designed and manufactured. The design followed a digital-on-top design workflow and ports a hybrid pixel-detector architecture, with digital pulse processing in each pixel, into a monolithic chip. The chip matrix consists of 64$\times$16 square pixels with a size of 35$\times$35 um2, and a total active area of approximately 1.25 um2. The chip has been successfully integrated into the Caribou DAQ system. It is fully functional, and the measured threshold dispersion and noise agree with the expectation from front-end simulations. However, a non-uniform in-pixel response related to the size and location of the n-wells in the analog circuitry has been observed in test beam measurements and will be discussed in this contribution. This asymmetry in the pixel response, enhanced by the 35 um pixel pitch - larger than in other prototypes - and certain features of the readout circuit, has not been observed in prototypes with smaller pixel pitches in this technology.

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arXiv2025

Impact of the circuit layout on the charge collection in a monolithic pixel sensor

CERN's strategic R&D programme on technologies for future experiments recently started investigating the TPSCo 65nm ISC CMOS imaging process for monolithic active pixels sensors for application in high energy physics. In collaboration with the ALICE experiment and other institutes, several prototypes demonstrated excellent performance, qualifying the technology. The Hybrid-to-Monolithic (H2M), a new test-chip produced in the same process but with a larger pixel pitch than previous prototypes, exhibits an unexpected asymmetric efficiency pattern. This contribution describes a simulation procedure combining TCAD, Monte Carlo and circuit simulations to model and understand this effect. It proved able to reproduce measurement results and attribute the asymmetric efficiency drop to a slow charge collection due to low amplitude potential wells created by the circuitry layout and impacting efficiency via ballistic deficit.

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arXiv2024

Timing resolution performance of Timepix4 bump-bonded assemblies

The timing performance of the Timepix4 application-specific integrated circuit (ASIC) bump-bonded to a $100\;μ\textrm{m}$ thick n-on-p silicon sensor is presented. A picosecond pulsed infrared laser was used to generate electron-hole pairs in the silicon bulk in a repeatable fashion, controlling the amount, position and time of the stimulated charge signal. The timing resolution for a single pixel has been measured to $107\;\textrm{ps}$ r.m.s. for laser-stimulated signals in the silicon sensor bulk. Considering multi-pixel clusters, the measured timing resolution reached $33\;\textrm{ps}$ r.m.s. exploiting oversampling of the timing information over several pixels.

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arXiv2024

Timepix3: single-pixel multi-hit energy-measurement behavior

The event-driven hybrid-pixel detector readout chip, Timepix3, has the ability to simultaneously measure the time of an event on the nanosecond timescale and the energy deposited in the sensor. However, the behaviour of the system when two events are recorded in quick succession of each other on the same pixel was not studied in detail previously. We present experimental measurements, circuit simulations, and an empirical model for the impact of a preceding event on this energy measurements, which can result in a loss as high as 70~\%. Accounting for this effect enables more precise compensation, particularly for phenomena like timewalk. This results in significant improvements in time resolution -- in the best case, multiple tens of nanoseconds -- when two events happen in rapid succession.

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arXiv2023

High-rate, high-resolution single photon X-ray imaging: Medipix4, a large 4-side buttable pixel readout chip with high granularity and spectroscopic capabilities

The Medipix4 chip is the latest member in the Medipix/Timepix family of hybrid pixel detector chips aimed at high-rate spectroscopic X-ray imaging using high-Z materials. It can be tiled on all 4 sides making it ideal for constructing large-area detectors with minimal dead area. The chip is designed to read out a sensor of 320 x 320 pixels with dimensions of 75 μm x 75 μm or 160 x 160 pixels with dimensions of 150 μm x 150 μm. The readout architecture features energy binning of the single photons, which includes charge sharing correction for hits with energy spread over adjacent pixels. This paper presents the specifications, architecture, and circuit implementation of the chip, along with the first electrical measurements.

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arXiv2023

Ultrafast Radiographic Imaging and Tracking: An overview of instruments, methods, data, and applications

Ultrafast radiographic imaging and tracking (U-RadIT) use state-of-the-art ionizing particle and light sources to experimentally study sub-nanosecond dynamic processes in physics, chemistry, biology, geology, materials science and other fields. These processes, fundamental to nuclear fusion energy, advanced manufacturing, green transportation and others, often involve one mole or more atoms, and thus are challenging to compute by using the first principles of quantum physics or other forward models. One of the central problems in U-RadIT is to optimize information yield through, e.g. high-luminosity X-ray and particle sources, efficient imaging and tracking detectors, novel methods to collect data, and large-bandwidth online and offline data processing, regulated by the underlying physics, statistics, and computing power. We review and highlight recent progress in: a.) Detectors; b.) U-RadIT modalities; c.) Data and algorithms; and d.) Applications. Hardware-centric approaches to U-RadIT optimization are constrained by detector material properties, low signal-to-noise ratio, high cost and long development cycles of critical hardware components such as ASICs. Interpretation of experimental data, including comparisons with forward models, is frequently hindered by sparse measurements, model and measurement uncertainties, and noise. Alternatively, U-RadIT make increasing use of data science and machine learning algorithms, including experimental implementations of compressed sensing. Machine learning and artificial intelligence approaches, refined by physics and materials information, may also contribute significantly to data interpretation, uncertainty quantification, and U-RadIT optimization.

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arXiv2023

Active Personal Eye Lens Dosimetry with the Hybrid Pixelated Dosepix Detector

Eye lens dosimetry has been an important field of research in the last decade. Dose measurements with a prototype of an active personal eye lens dosemeter based on the Dosepix detector are presented. The personal dose equivalent at $3\,$mm depth of soft tissue, $H_\text{p}(3)$, was measured in the center front of a water-filled cylinder phantom with a height and diameter of $20\,$cm. The energy dependence of the normalized response is investigated for mean photon energies between $12.4\,$keV and $248\,$keV for continuous reference radiation fields (N-series) according to ISO 4037. The response normalized to N-60 ($\overline{E}=47.9\,\text{keV}$) at $0^\circ$ angle of irradiation stays within the approval limits of IEC 61526 for angles of incidence between $-75^\circ$ and $+75^\circ$. Performance in pulsed photon fields was tested for varying dose rates from $0.1\,\frac{\text{Sv}}{\text{h}}$ up to $1000\,\frac{\text{Sv}}{\text{h}}$ and pulse durations from $1\,\text{ms}$ up to $10\,\text{s}$. The dose measurement works well within the approval limits (acc. to IEC 61526) up to $1\,\frac{\text{Sv}}{\text{h}}$. No significant influence of the pulse duration on the measured dose is found. Reproducibility measurements yield a coefficient of variation which does not exceed $1\,\%$ for two tested eye lens dosemeter prototypes.

physics.med-ph↗

arXiv2022

Personal Dosimetry in Direct Pulsed Photon Fields with the Dosepix Detector

First investigations regarding dosimetric properties of the hybrid, pixelated, photon-counting Dosepix detector in the direct beam of a pulsed photon field (RQR8) for the personal dose equivalent $H\mathrm{_p(10)}$ are presented. The influence quantities such as pulse duration and dose rate were varied, and their responses were compared to the legal limits provided in PTB-A 23.2. The variation of pulse duration at a nearly constant dose rate of about 3.7$\,$Sv/h shows a flat response around 1.0 from 3.6$\,$s down to 2$\,$ms. A response close to 1.0 is achieved for dose rates from 0.07$\,$Sv/h to 35$\,$Sv/h for both pixel sizes. Above this dose rate, the large pixels (220$\,μ$m edge length) are below the lower limit. The small pixels (55$\,μ$m edge length) stay within limits up to 704$\,$Sv/h. The count rate linearity is compared to previous results, confirming the saturating count rate for high dose rates.

physics.med-ph↗

arXiv2022

Comparison of different sensor thicknesses and substrate materials for the monolithic small collection-electrode technology demonstrator CLICTD

Small collection-electrode monolithic CMOS sensors profit from a high signal-to-noise ratio and a small power consumption, but have a limited active sensor volume due to the fabrication process based on thin high-resistivity epitaxial layers. In this paper, the active sensor depth is investigated in the monolithic small collection-electrode technology demonstrator CLICTD. Charged particle beams are used to study the charge-collection properties and the performance of devices with different thicknesses both for perpendicular and inclined particle incidence. In CMOS sensors with a high-resistivity Czochralski substrate, the depth of the sensitive volume is found to increase by a factor two in comparison with standard epitaxial material and leads to significant improvements in the hit-detection efficiency and the spatial and time resolution.

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arXiv2022

Transient Monte Carlo Simulations for the Optimisation and Characterisation of Monolithic Silicon Sensors

An ever-increasing demand for high-performance silicon sensors requires complex sensor designs that are challenging to simulate and model. The combination of electrostatic finite element simulations with a transient Monte Carlo approach provides simultaneous access to precise sensor modelling and high statistics. The high simulation statistics enable the inclusion of Landau fluctuations and production of secondary particles, which offers a realistic simulation scenario. The transient simulation approach is an important tool to achieve an accurate time-resolved description of the sensor, which is crucial in the face of novel detector prototypes with increasingly precise timing capabilities. The simulated time resolution as a function of operating parameters as well as the full transient pulse can be monitored and assessed, which offers a new perspective on the optimisation and characterisation of silicon sensors. In this paper, a combination of electrostatic finite-element simulations using 3D TCAD and transient Monte Carlo simulations with the Allpix Squared framework are presented for a monolithic CMOS pixel sensor with a small collection diode, that is characterised by a highly inhomogeneous, complex electric field. The results are compared to transient 3D TCAD simulations that offer a precise simulation of the transient behaviour but long computation times. Additionally, the simulations are benchmarked against test-beam data and good agreement is found for the performance parameters over a wide range of different operation conditions.

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arXiv2019

Sub-100 nanosecond temporally resolved imaging with the Medipix3 direct electron detector

Detector developments are currently enabling new capabilities in the field of transmission electron microscopy (TEM). We have investigated the limits of a hybrid pixel detector, Medipix3, to record dynamic, time varying, electron signals. Operating with an energy of 60keV, we have utilised electrostatic deflection to oscillate electron beam position on the detector. Adopting a pump-probe imaging strategy we have demonstrated that temporal resolutions three orders of magnitude smaller than are available for typically used TEM imaging detectors are possible. Our experiments have shown that energy deposition of the primary electrons in the hybrid pixel detector limits the overall temporal resolution. Through adjustment of user specifiable thresholds or the use of charge summing mode, we have obtained images composed from summing 10,000s frames containing single electron events to achieve temporal resolution less than 100ns. We propose that this capability can be directly applied to studying repeatable material dynamic processes but also to implement low-dose imaging schemes in scanning transmission electron microscopy.

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