DarkSide-20k is a novel liquid argon dark matter detector currently under construction at the Laboratori Nazionali del Gran Sasso (LNGS) of the Istituto Nazionale di Fisica Nucleare (INFN) that will push the sensitivity for Weakly Interacting Massive Particle (WIMP) detection into the neutrino fog. The core of the apparatus is a dual-phase Time Projection Chamber (TPC), filled with 50 tonnes of low radioactivity underground argon (UAr) acting as the WIMP target. NUV-HD-cryo Silicon Photomultipliers (SiPM)s designed by Fondazione Bruno Kessler (FBK) (Trento, Italy) were selected as the photon sensors covering two $$10.5~\text {m}^2$$ 10.5 m 2 Optical Planes, one at each end of the TPC, and a total of $$5~\text {m}^2$$ 5 m 2 photosensitive surface for the liquid argon veto detectors. This paper describes the Quality Assurance and Quality Control (QA/QC) plan and procedures accompanying the production of FBK NUV-HD-cryo SiPM wafers manufactured by LFoundry s.r.l. (Avezzano, AQ, Italy). SiPM characteristics are measured at 77 K at the wafer level with a custom-designed probe station. As of March 2025, 1314 of the 1400 production wafers (94% of the total) for DarkSide-20k were tested. The wafer yield is $$93.2\pm 2.5$$ 93.2 ± 2.5 %, which exceeds the 80% specification defined in the original DarkSide-20k production plan.

DarkSide-20k (DS-20k) is a dark matter detection experiment under construction at the Laboratori Nazionali del Gran Sasso (LNGS) in Italy. It utilises similar to 100 t of low radioactivity argon from an underground source (UAr) in its inner detector, with half serving as target in a dual-phase time projection chamber (TPC). The UAr cryogenics system must maintain stable thermodynamic conditions throughout the experiment's lifetime of over 10 years. Continuous removal of impurities and radon from the UAr is essential for maximising signal yield and mitigating background. We are developing an efficient and powerful cryogenics system with a gas purification loop with a target circulation rate of 1000 slpm. Central to its design is a condenser operated with liquid nitrogen which is paired with a gas heat exchanger cascade, delivering a combined cooling power of more than 8 kW. Here we present the design choices in view of the DS-20k requirements, in particular the condenser's working principle and the cooling control, and we show test results obtained with a dedicated benchmarking platform at CERN and LNGS. We find that the thermal efficiency of the recirculation loop, defined in terms of nitrogen consumption per argon flow rate, is 95% and the pressure in the test cryostat can be maintained within +/-(0.1-0.2) mbar. We further detail a 5-day cool-down procedure of the test cryostat, maintaining a cooling rate typically within -2K/h, as required for the DS-20k inner detector. Additionally, we assess the circuit's flow resistance, and the heat transfer capabilities of two heat exchanger geometries for argon phase change, used to provide gas for recirculation. We conclude by discussing how our findings influence the finalisation of the system design, including necessary modifications to meet requirements and ongoing testing activities.

Silicon Photomultipliers (SiPMs) timing performance, in particular the Coincidence Time Resolution (CTR), is of paramount importance in applications like Time-of-Flight Positron Emission Tomography (ToF-PET). In turns, the CTR strongly depends on the Single Photon Time Resolution (SPTR) defined as the time jitter when a single photon is detected by the SiPM. In this study, we performed a characterization of the recently introduced FBK Near Ultra- Violet-High Density with Metal-filled Trenches (NUV-HD-MT) technology in terms of its timing performance by measuring the SPTR of single Single Photon Avalanche Diodes (SPADs) with different cell sizes. We used a femto-second laser with a wavelength of 390nm and a custom-made high frequency amplifier. The single SPADs show an excellent SPTR of $(17.63 \pm 0.04) \mathrm{ps}$, $(22.5 \pm 0.1) \mathrm{ps}$, $(24.4 \pm 0.1) \mathrm{ps}$ FWHM for the $30 \mu \mathrm{m}, 40 \mu \mathrm{m}, 50 \mu \mathrm{m}$ microcell respectively. We performed a signal shape analysis to study the influence of the avalanche point injection in the microcell active area on the SPTR.

The dual-phase liquid argon time projection chamber is presently one of the leading technologies to search for dark matter particles with masses below 10 GeV c(-2). This was demonstrated by the DarkSide-50 experiment with approximately 50 kg of low-radioactivity liquid argon as target material. The next generation experiment DarkSide-20k, currently under construction, will use 1,000 times more argon and is expected to start operation in 2027. Based on the DarkSide-50 experience, here we assess the DarkSide-20k sensitivity to models predicting light dark matter particles, including Weakly Interacting Massive Particles (WIMPs) and sub-GeV c(-2) particles interacting with electrons in argon atoms. With one year of data, a sensitivity improvement to dark matter interaction cross-sections by at least one order of magnitude with respect to DarkSide-50 is expected for all these models. A sensitivity to WIMP-nucleon interaction cross-sections below 1x10(-42) cm(2) is achievable for WIMP masses above 800 MeV c(-2). With 10 years exposure, the neutrino fog can be reached for WIMP masses around 5 GeV c(-2).

Experiments aimed at direct searches for WIMP dark matter require highly effective reduction of backgrounds and control of any residual radioactive contamination. In particular, neutrons interacting with atomic nuclei represent an important class of backgrounds due to the expected similarity of a WIMP-nucleon interaction, so that such experiments often feature a dedicated neutron detector surrounding the active target volume. In the context of the development of DarkSide-20k detector at INFN Gran Sasso National Laboratory (LNGS), several R D projects were conceived and developed for the creation of a new hybrid material rich in both hydrogen and gadolinium nuclei to be employed as an essential element of the neutron detector. Thanks to its very high cross-section for neutron capture, gadolinium is one of the most widely used elements in neutron detectors, while the hydrogen-rich material is instrumental in efficiently moderating the neutrons. In this paper results from one of the R Ds are presented. In this effort the new hybrid material was obtained as a poly(methyl methacrylate) (PMMA) matrix, loaded with gadolinium oxide in the form of nanoparticles. We describe its realization, including all phases of design, purification, construction, characterization, and determination of mechanical properties of the new material.

Recent developments in PET instrumentation, including advances in silicon photomultipliers and electronics, are fueling the development of novel PET systems improving flexibility and reducing costs. Among the improvements, ultra-high resolution time-of-flight (TOF) enables the use of limited angle geometry without sacrificing image quality. In this work, we present simulation results for a PET system under development, with ultra-high TOF resolution and depth-of-interaction (DOI). The proposed system is arranged for long axial field-of-view scanning using flat detector panels. We evaluate the performance of the system for varying TOF and DOI resolutions while incorporating geometric corrections during reconstruction. Results demonstrate the promising performance of a 2-panel, 70 ps TOF resolution system with 2.5 mm DOI resolution, as evidenced by traditional and task-based image quality metrics.

The performance of PET detectors is significantly influenced by the reflectors used to fabricate the crystal arrays, especially in high spatial resolution detectors that utilize dual-ended readout for depth-of-interaction (DOI) encoding. This study compared the performance of dual-ended readout PET detectors based on LYSO arrays that had the same 0.5 mm pitch and 20 mm thickness but employed five different reflectors: $80 \mu \mathrm{m}$ thick barium sulfate (BaSO4), $50 \mu \mathrm{m}$ thick Toray E60, and Toray E20 with thicknesses of $38 \mu \mathrm{m}, 50 \mu \mathrm{m}$, and $75 \mu \mathrm{m}$, respectively. Performance comparisons focused on flood histograms, energy resolution, timing resolution, and DOI resolution.

BGO has been highlighted as a cost-effective option for hybrid scintillator/Cherenkov radiator. However, effectively utilizing its improved timing resolution is hindered by challenges in managing the high detection fluctuation of Cherenkov photons and their spectral overlap with scintillation photons. Our previous study showed that the dual-ended readout and an adaptive timestamp method could enhance the detection of quasi-isotropically and simultaneously emitted Cherenkov photons in BGO. The OctaSiPM, developed by FBK, features a single pixel active area of 2.5 × 1.4 mm 2 and a 2 × 4 SiPM arrangement, offering a total active area of 5.3 × 5.8 mm 2 . In our recent investigation, when one end of a BGO crystal was coupled to two segmented pixels of the OctaSiPM, additional information about the initial photon density of the corresponding event could be provided. Thus, we hypothesize that the combination of the segmentation scheme and the dual-ended readout can provide multidimensional information on gamma events in BGO TOF-PET detectors. In the experimental setup, a Teflon-wrapped polished BGO crystal was read out from both ends using OctaSiPMs, followed by low-noise high-frequency amplification. Coincidence detection of gamma rays was conducted between a BGO detector and an LYSO:Ce:Mg reference detector. Relationship between segmented pixel time differences and top-bottom time differences revealed a unique pattern correlating with Cherenkov photon events. By selectively adjusting time windows, time difference distribution converged towards a single Gaussian distribution, enabling the selection of pure Cherenkov-triggered events. This marks the first instance of achieving a single Gaussian fitting from BGO data, underscoring the novelty and importance of our findings. Moreover, the analysis of initial photon counts provided insights into the likelihood of prompt Cherenkov photon detection and its temporal density.

The need to push the timing performances to the limit finds breeding ground in several fields from high energy physics to biomedical applications such as Time of Flight Positron Emission Tomography (ToF-PET). In the last years, excellent results have been achieved thanks to the improvement of the scintillator crystal materials, the electronics readout and the detector development. In this context, SiPM Coincidence Time Resolution (CTR) is a key parameter in order to assess the device timing performance. In this work we will present the CTR of the recently introduced FBK NUV-HD-MT Silicon Photomultiplier (SiPM) technology. Thanks to the addition of the optically insulating material inside the trenches, FBK NUV-HDMT devices show an extremely low CrossTalk (CT) of about ≃ 5% at 47.5V (≃ 15V excess bias). The Photon Detection Efficiency (PDE) reaches the ≃ 65% at the same excess bias at 420nm. The CTR was measured using a 4mm × 4mm SiPM to match the 3mm × 3mm × 5mm LYSO:Ce:Ca crystal and comparing different microcell sizes. By using a standard readout electronics we achieved a CTR of ~ 95ps FWHM for all the devices thanks to the extremely low CT of the technology that allows to push the voltage bias to high values. We also compare the CTR between the SiPM version with a metal mask outside the active area (capacitive coupling) and the SiPM without it in order to asses the role of the masking in the timing performance, to discuss about limitations and further improvements.

Progress in 3D interconnecting technologies paved the way for a new generation of Silicon Photomultipliers (SiPM) and Single Photon Avalanche Diode (SPAD): hybrid devices which combine the integrated functional-ities of the digital SiPM with the high performance of custom technologies, like low noise and high detection efficiency. Recently, Fondazione Bruno Kessler (FBK) has been working on the implementation of recently developed 3D integration technologies, on SiPMs devices, to improve both performances and functionalities by creating backside-illuminated (BSI) devices and Through Silicon Vias (TSV) interconnections. Two different technology platforms have been investigated: a BSI design for near-infrared (NIR) sensitive SiPMs and TSV interconnections for near-and vacuum-ultraviolet (NUV/VUV) sensitive detectors. For NIR applications, electrical characterization of ultra-thin (about 10 mu m) SiPM wafers with a metal reflector on the frontside has shown an improved photon detection efficiency (PDE) when operated in BSI configuration compared with non-thinned front-side illuminated (FSI) devices, allowing at the same time full high-segmentation access to the SiPM output from the front-side. Instead, for NUV/VUV applications, a FSI stacked approach is considered more suitable since the junction depth needs to be shallower. In this case, TSV interconnections using two different approaches (named Via-Mid and Via-Last) have been implemented allowing the placement of the contacts on the backside of the wafer.

Position-sensitive SiPMs (PS-SiPMs) are promising photodetectors for high-resolution small animal positron emission tomography (PET) scanners. With the ultimate goal of developing a 0.5 mm resolution small-animal PET scanner, we developed 10 x 10 mm 2 linearly graded SiPMs (LG-SiPMs), a type of PS-SiPMs. In this paper, the performance of depth-of-interaction encoding dual-ended readout detectors based on these LG-SiPMs and LYSO arrays with a pitch of 0.5 mm and a thickness of 20 mm were evaluated. The flood histogram shows the crystal elements were clearly resolved. The average energy resolution and timing resolution across the LYSO array are 22.3 ± 9.4% and 0.99 ± 0.02 ns, respectively.

Silicon photomultipliers (SiPMs) are single-photon sensitive large-area detectors widely used in many applications. Among them, they are used in several radiation-harsh applications, like high-energy physics and experiments in space, where they receive a significant radiation dose. The effect of ionizing and non-ionizing radiation dose on their performance is very interesting for those applications.We irradiated several Silicon Photomultipliers with protons and X-ray. We investigated the noise increment and directly compared per performance worsening on several SiPM technologies produced sat FBK (Trento, Italy). We also characterized the temperature dependence of the noise down to cryogenic levels, extracting the activation energy. We investigated in depth the defects created by protons within the microcells of the SiPMs, with emission microscopy measurements. We also investigated the effect of ionizing-energy-loss in the SiPM microcells, showing a relevant effect of charge accumulation in the dielectrics and in the trenches. Some technologies demonstrated a worse radiation tolerance with an internal modification of electric fields that increases the primary noise and afterpulsing. This have been directly confirmed with a dedicated irradiation campaign comparing SiPMs with different materials inside the deep trenches between microcells.All these considerations are useful to develop new SiPM technologies that are more radiation hard, both in terms of bulk damage and ionizing-energy-loss effects.

Positron emission tomography (PET) has become standard practice in many clinical applications including oncology, cardiology and neurology. Recent developments in PET scanners have pushed the limits of these applications, thanks to long axial field of view scanners which can improve system sensitivity and the use of time-of-flight (TOF) information which can improve spatial resolution. However, the cost and space requirements for total-body PET systems has created a need for flexible, low-cost yet high-sensitivity systems with large axial field of view. Additionally, currently achievable TOF resolutions (around 200 ps for current clinical scanners) limit the gains in image resolution. In this work, we present the first reconstructions for a flexible two-panel ultra-high TOF resolution PET system expected to achieve 75 ps full-width at half maximum (FWHM). The scanner relies on pixelated L(Y)SO scintillators with novel detectors with dual readout measuring depth-of-interaction (DOI) information. The performance of the new system is demonstrated on Monte-Carlo simulations of an anthropomorphological numerical phantom reconstructed using a newly developed TOF+DOI-enabled reconstruction engine. The presented reconstructions exhibit high image quality, demonstrating the promise of the proposed PET system.

Glass-free Silicon Photomultipliers (SiPMs) optimized for Vacuum Ultraviolet (VUV) light detection have been developed and characterized at FBK, including Through Silicon Vias (TSVs) that bring the top contact to the backside of the device tier to be able to perform 3D integration with the readout electronics. R&D activity was carried out on SiPMs on Vacuum Ultraviolet-High Density (VUV-HD) SiPMs with a 1x1 cm 2 active area and a Photon Detection Efficiency of about 23% at 175 nm. Two different TSV fabrication strategies have been developed: "Via-Mid", in which the TSV is formed during the processing of the frontside of the wafer, and "Via-Last", in which the TSV is fabricated with backside-only processing. Both approaches provided functional TSVs, showing good characteristics, such as high connection yield, good insulation from the substrate and no modification of the SiPM characteristics for the "Via-Last" approach. A notable feature of the TSVs fabricated at FBK and customized for the VUV-HD SiPMs is the absence of the glass support wafer, which allows preserving sensitivity to VUV light as well as a reduction of the external component of the optical crosstalk. In this work, we discuss the different microfabrication strategies and report on the detailed electro-optical characterization of 1x1 cm 2 VUV-HD SiPMs with "Via-Last" TSVs.

Functional imaging techniques like positron emission tomography (PET) are an essential tool in an aging society. Despite impressive advances in microelectronics, photodetectors and scintillation materials, PET is still awaiting a breakthrough in terms of reduced cost and increased performance. Large potential is seen in ultraprecise time-of-flight (TOF), aiming at coincidence time resolutions (CTRs) better than 30 ps. However, state-of-the-art TOF-PET systems are still far away from this goal, achieving typical CTRs of 214 ps (FWHM). Several proposals have been put forth, whereas the most promising is to use prompt photon emission, e.g. Cherenkov radiation in BGO crystals, which are cheap to produce, thus contributing to drastic cost cutting. However, Cherenkov detection is challenging due to its limited photon yield, which in turn requires a very high photon detection efficiency (PDE), low dark count rate (DCR) and extremely fast and innovative electronic readout schemes. Recent analog silicon photomultipliers (aSiPMs) meet the first two targets, but not the latter.In the Digilog project we envisage to unite the best of these two worlds, combining high PDE, low DCR and an exceptional SPTR. To reach this goal, we will segment state-of-the-art aSiPMs into smaller clusters, called µSiPMs. A balanced segmentation of the electronic readout will make it possible to efficiently detect the first scintillation and Cherenkov photons, with a manageable granularity at system level. The µSiPM signals will feature photon-density time walk correction and photon counting. We envision to create 3D-stacked sensors where the electronics will be housed in a CMOS bottom-tier and the µSiPMs in the top-tier chip. Preliminary measurements on first µSiPM test-structures already reached PDE and DCR close to their commercial counterparts, while an SPTR of 25 ps FWHM, close to our sub-20 ps goal, has been achieved.

The emerging field of ultra-fast timing has enormous potential in high-energy physics, nuclear medicine, and imaging, and is driving advances in detector and readout electronics. To achieve improved time resolution, significant efforts have been devoted to this field, resulting in impressive coincidence time resolution (CTR) values for silicon photomultiplier (SiPM) and scintillator crystals using state-of-the-art high-frequency (HF) readout. However, scaling this readout method for system-level applications is not yet feasible, hence requiring dedicated application-specific integrated circuits (ASICs). In this study, we evaluate the performance of the FastIC, an 8-channel readout ASIC suitable for TOP-PET application, in terms of CTR. The study employs Cherenkov photons produced in bismuth germanium oxide (BGO) scintillator crystals and novel SiPMs with metal-trenching. We obtain CTR values of 257±5 ps and 152±4 ps, respectively, for 2x2x20mm³ and 2x2x3mm³ BGO crystals using HF readout as a reference, with minimal electronic interference, by exploiting the full potential of SiPMs with metal trenching. In comparison, the CTR values obtained using the FastIC for the same crystals were 490±3 ps and 330±4 ps, respectively. We also assess the limits of the FastIC and suggest areas for further improvements.

The shift towards early diagnosis and prevention in medicine necessitates high-performing PET imaging systems with improved sensitivity and specificity. To achieve this, additional time-of-flight measurements in PET enhance the signal-to-noise ratio and enable the construction of better-performing systems. However, the timing resolution of PET systems is limited by several factors, such as slow scintillation response time, optical photon travel time spread, number of detected scintillation photons, and timing precision of the photo-sensor and fast readout electronics. Although there have been efforts to develop new scintillators for PET, this contribution focuses on optimizing photo sensors and readout electronics. The authors integrated high-performing HD-NUV-MT silicon photomultipliers from FBK with novel low-power FastIC ASICs for fast-timing applications. They constructed a pair of 16-channel gamma detector modules and placed them in a coincidence setup, demonstrating very high coincidence timing resolution below 100 ps FWHM. This achievement enables the construction of high-performing PET detectors with incomplete sampling and simplified systems consisting of flat panel detectors that can be easily combined in larger systems. The authors simulated the performance of an imager comprising two 120 cm × 60 cm panels of segmented 20 mm thick LYSO crystal arrays read by dual-sided readout with fast timing and imaged a human XCAT phantom. They demonstrated that such a system, consisting of much less scintillator material compared to a total body PET imager with a standard opening, exhibits excellent performance. Overall, this approach provides a promising path toward developing highly sensitive and specific PET imaging systems that can aid in early disease diagnosis and prevention.

Silicon photomultipliers (SiPMs) are highly-sensitive photodetectors emerging as the technology of choice for many applications, including among the others, large high-energy physics experiments and detectors for space instruments, where they are often exposed to a large amount of radiation. In recent years, there has been an increasing interest in assessing the performance deterioration of such detectors after ionizing and non-ionizing radiation, such as protons, neutrons and X or gamma rays. It is therefore interesting to characterize the effect of irradiation on such Geiger-mode detectors, differentiating between the ionizing and non-ionizing energy-loss effects. Moreover, it is interesting to compare the radiation damage effects on several types of SiPMs, to assess the main phenomena and the deterioration mechanisms, aiming to a more radiation tolerant SiPM design.In this work we irradiated several types of SiPM structures, produced in FBK (Trento, Italy), with 40 keV X-rays, at several doses, up to 100 kGy (in silicon), performing both online measurements (after each irradiation step) and offline functional characterization, after one month of room temperature annealing. The SiPMs are made with many different technologies, in particular different layouts, junction polarities, internal structures and starting materials. We studied the variation in the reverse current-voltage curves, distinguishing the effects on multiplied and not-multiplied current components, the primary dark count rate, the correlated noise probabilities and photon detection efficiency. Comparing all the measurement results, knowing the internal structure and the fabrication processes, we were able to extract and distinguish different deterioration mechanisms, also supported by TCAD simulations on the different effects of ionizing radiation inside the microcells.

Silicon photomultipliers (SiPMs) are single-photon sensitive large-area detectors widely used in many applications. Among them, they are used in several radiation-harsh applications, like high-energy physics and experiments in space, where they receive a significant radiation dose. The effect of ionizing and nonionizing radiation dose on their performance is very interesting for those applications. In this contribution we irradiated several silicon photomultipliers technologies produced at FBK with 74MeV protons and with 40 keVX-rays (up to to 100kGy dose in silicon). We monitored the reverse current after each proton or X-ray irradiation step. We also characterized the SiPMs functionally after the irradiation and after few weeks of room temperature annealing. We studied the main degradation effects and the main difference in the functional performances modification among the SiPM technologies. Having a complete knowledge of the internal structures of the SiPMs and being able to compare directly the effects on performances degradation of different SiPM technologies is very interesting for a future development of a more radiation-tolerant SiPM technology.

This paper describes a new method for optical readout of Time Projection Chambers (TPCs), based on the Linearly Graded Silicon Photomultiplier (LG-SiPM). This is a single photon-sensitive detector with excellent timing and 2D position resolution developed at Fondazione Bruno Kessler, Trento (FBK). The LG-SiPM produces time-varying voltage signals that are used to reconstruct the 3D position and energy of ionisation tracks generated inside the TPC. The TPC used in this work contained room-temperature CF4 gas at a pressure of 100 mbar, with two THGEMs to produce secondary scintillation light. A collimated 241Am source (Qα = 5.486 MeV) was used to produce the ionisation tracks. The successful reconstruction of these tracks is demonstrated, and the consistency of the methodology characterised through varying the geometry of the tracks within the TPC. Energy reconstruction and deposition studies are also described, demonstrating the feasibility of the LG-SiPM as a potential option for optical TPC readout.