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Andrea Castoldi

Publications and source records attributed to Andrea Castoldi.

9 recordsLinked to original sources
arXiv2025

ASTAROTH: A Novel Detector for Dark Matter Direct Detection Using Cryogenic SiPMs

The DAMA experiment's long-standing claim of dark matter detection remains a key open issue in astroparticle physics. Independent verification requires NaI(Tl)-based detectors with enhanced low-energy sensitivity. Current detectors rely on photomultiplier tubes (PMTs) which features limited detection efficiency, intrinsic radioactivity, and high noise at keV energies. ASTAROTH is an R&D project that developed a proof of concept NaI(Tl) detector where siliconphotomultipliers (SiPMs) have been used instead of PMTs, offering higher photon detection efficiency, negligible radioactivity, and, most of all, a reduction of two orders of magnitude in the dark noise. The setup includes a custom cryostat operating at approximately 80 K. We report the first characterization of an approximately 360 g NaI(Tl) crystal coupled to a 5 x 5 cm SiPM matrix, yielding 4.5 photoelectrons\keV after crosstalk correction. This promising result demonstrates the feasibility of SiPM-based readout for NaI(Tl) and paves the way for future large-scale dark matter experiments.

physics.ins-det↗
arXiv2025

First use of large area SiPM matrices coupled with NaI(Tl) scintillating crystal for low energy dark matter search

The long-standing claim of dark matter detection by the DAMA experiment remains a crucial open question in astroparticle physics. A key step towards its independent verification is the development of NaI(Tl)-based detectors with improved sensitivity at low energies. The majority of NaI(Tl)-based experiments rely on conventional photomultiplier tubes (PMTs) as single photon detectors, which present technological limitations in terms of light collection, intrinsic radioactivity and high noise contribution at keV energies. ASTAROTH is an R&D project developing a NaI(Tl)-based detector where the scintillation light is read out by silicon photomultipliers (SiPMs) matrices. SiPMs exhibit high photon detection efficiency, negligible radioactivity, and, most importantly, a dark noise nearly two orders of magnitude lower than PMTs, when operated at cryogenic temperature. To this end, ASTAROTH features a custom-designed cryostat based on a bath of cryogenic fluid, able to safely operate the detector and the read-out electronics down to about 80 K. This work reports the first experimental characterization of an approximately 360 g NaI(Tl) detector read out by a large area (5 cm x 5 cm) SiPM matrix. The net photoelectron yield obtained with a preliminary configuration is approximately 4.5 photoelectrons/keV after crosstalk correction, which is rather promising in light of several planned developments. The signal-to-noise ratio and the energy threshold attainable with SiPMs is expected to improve the sensitivity for dark matter searches beyond the reach of current-generation PMT-based detectors. This result is the first proof of the viability of this technology and sets a milestone toward the design of future large-scale experiments.

physics.ins-det↗
arXiv2025

Introducing Combined Effects of Filtering and ASE Noise in Optical Links Supposing Different Equalization Algorithms

This paper develops and validates a discrete-time modeling framework for the joint impact of cascaded optical filtering, distributed ASE noise, and transceiver noise in coherent optical links. The work focuses on the post-equalization signal-to-noise ratio, which is the central quantity used to quantify filtering penalty under receiver DSP. Starting from an optical-link abstraction with arbitrary filter transfer functions and colored noise spectra, we derive analytical expressions for the matched-filter bound, Zero-Forcing Equalization, Minimum Mean Square Error Equalization, Fractionally Spaced Equalization, and finite-length equalizers with and without explicit colored-noise treatment. The model is coupled to a measurement-based transceiver SNR characterization, so that optical-link penalties and receiver impairments can be evaluated within the same formulation. Time-domain simulations with LMS equalization validate the analytical predictions over severe filtering conditions, different tap lengths, and different ASE-noise positions along the link. Experimental results with commercial transceivers and ROADMs further confirm the accuracy of the MMSE and FSE models, while highlighting the role of realistic filter modeling and equalizer implementation limits. The resulting framework provides a tractable basis for quality-of-transmission estimation and optical-network digital-twin implementations.

eess.SP↗
arXiv2024

Analysis of long-lived effects in high-repetition-rate stroboscopic transient X-ray absorption experiments on thin films

Time-resolved X-ray absorption spectroscopy (tr-XAS) has been shown to be a versatile measurement technique for investigating non-equilibrium dynamics. Novel X-ray free electron laser (XFEL) facilities like the European XFEL offer increased repetition rates for stroboscopic XAS experiments through a burst operation mode, which enables measurements with up to 4.5 MHz. These higher repetition rates lead to higher data acquisition rates but can also introduce long-lived excitations that persist and thus build up during each burst. Here, we report on such long-lived effects in Ni and NiO thin film samples that were measured at the European XFEL. We disentangle the long-lived excitations from the initial pump-induced change and perform a detailed modelling-based analysis of how they modify transient X-ray spectra. As a result, we link the long-lived effects in Ni to a local temperature increase, as well as the effects in NiO to excited charge carrier trapping through polaron formation. In addition, we present possible correction methods, as well as discuss ways in which the effects of these long-lived excitations could be minimized for future time-resolved X-ray absorption spectroscopy measurements.

cond-mat.mtrl-sci↗
arXiv2022

Photon shot-noise limited transient absorption soft X-ray spectroscopy at the European XFEL

Femtosecond transient soft X-ray Absorption Spectroscopy (XAS) is a very promising technique that can be employed at X-ray Free Electron Lasers (FELs) to investigate out-of-equilibrium dynamics for material and energy research. Here we present a dedicated setup for soft X-rays available at the Spectroscopy & Coherent Scattering (SCS) instrument at the European X-ray Free Electron Laser (EuXFEL). It consists of a beam-splitting off-axis zone plate (BOZ) used in transmission to create three copies of the incoming beam, which are used to measure the transmitted intensity through the excited and unexcited sample, as well as to monitor the incoming intensity. Since these three intensity signals are detected shot-by-shot and simultaneously, this setup allows normalized shot-by-shot analysis of the transmission. For photon detection, the DSSC imaging detector, which is capable of recording up to 800 images at 4.5 MHz frame rate during the FEL burst, is employed and allows approaching the photon shot-noise limit. We review the setup and its capabilities, as well as the online and offline analysis tools provided to users.

physics.ins-det↗
arXiv2022

The ASTAROTH project

The most discussed topic in direct search for dark matter is arguably the verification of the DAMA claim. In fact, the observed annual modulation of the signal rate in an array of NaI(Tl) detectors can be interpreted as the awaited signature of dark matter interaction. Several experimental groups are currently engaged in the attempt to verify such a game-changing claim with the same target material. However, all present-day designs are based on a light readout via Photomultiplier Tubes, whose high noise makes it challenging to achieve a low background in the 1-6 keV energy region of the signal. Even harder it would be to break below 1 keV energy threshold, where a large fraction of the signal potentially awaits to be uncovered. ASTAROTH is an R\&D project to overcome these limitations by using Silicon Photomultipliers (SiPM) matrices to collect scintillation light from NaI(Tl). The all-active design based on cubic crystals is operating in the 87-150 K temperature range where SiPM noise can be even a hundred times lower with respect to PMTs. The cryostat was developed following an innovative design and is based on a copper chamber immersed in a liquid argon bath that can be instrumented as a veto detector. We have characterized separately the crystal and the SiPM response at low temperature and we have proceeded to the first operation of a NaI(Tl) crystal read by SiPM in cryogeny.

physics.ins-det↗
arXiv2022

The interplay of local electron correlations and ultrafast spin dynamics in fcc Ni

The complex electronic structure of metallic ferromagnets is determined by a balance between exchange interaction, electron hopping leading to band formation, and local Coulomb repulsion. The interplay between the respective terms of the Hamiltonian is of fundamental interest, since it produces most, if not all, of the exotic phenomena observed in the solid state. By combining high energy and temporal resolution in femtosecond time-resolved X-ray absorption spectroscopy with ab initio time-dependent density functional theory we analyze the electronic structure in fcc Ni on the time scale of these interactions in a pump-probe experiment. We distinguish transient broadening and energy shifts in the absorption spectra, which we demonstrate to be caused by electron repopulation and correlation-induced modifications of the electronic structure, respectively. Importantly, the theoretical description of this experimental result hence requires to take the local Coulomb interaction into account, revealing a temporal interplay between band formation, exchange interaction, and Coulomb repulsion.

cond-mat.mtrl-sci↗
arXiv2022

Megahertz-rate Ultrafast X-ray Scattering and Holographic Imaging at the European XFEL

The advent of X-ray free-electron lasers (XFELs) has revolutionized fundamental science, from atomic to condensed matter physics, from chemistry to biology, giving researchers access to X-rays with unprecedented brightness, coherence, and pulse duration. All XFEL facilities built until recently provided X-ray pulses at a relatively low repetition rate, with limited data statistics. Here, we present the results from the first megahertz repetition rate X-ray scattering experiments at the Spectroscopy and Coherent Scattering (SCS) instrument of the European XFEL. We illustrate the experimental capabilities that the SCS instrument offers, resulting from the operation at MHz repetition rates and the availability of the novel DSSC 2D imaging detector. Time-resolved magnetic X-ray scattering and holographic imaging experiments in solid state samples were chosen as representative, providing an ideal test-bed for operation at megahertz rates. Our results are relevant and applicable to any other non-destructive XFEL experiments in the soft X-ray range.

cond-mat.mes-hall↗