AG Kröninger

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http://hdl.handle.net/2003/39249

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Now showing 1 - 14 of 14

A Bayesian tune of the Herwig Monte Carlo event generator
(2023-10-26) La Cagnina, Salvatore; Kröninger, Kevin; Kluth, Stefan; Verbytskyi, Andrii
The optimisation (tuning) of the free parameters of Monte Carlo event generators by comparing their predictions with data is important since the simulations are used to calculate experimental efficiency and acceptance corrections, or provide predictions for signatures of hypothetical new processes in experiments. We present a tuning procedure that is based on Bayesian reasoning and that allows for a proper statistical interpretation of the results. The parameter space is fully explored using Markov Chain Monte Carlo. We apply the tuning procedure to the Herwig7 event generator with both the cluster and the string hadronization models and a large set of measurements from hadronic Z-boson decays produced at LEP in e+e- collisions. Furthermore, we introduce a coherent propagation of uncertainties from the realm of parameters to the realm of observables and we show the effects of including experimental correlations of the measurements. To allow comparison with the approaches of other groups, we repeat the tuning considering weights for individual measurements.
Characterisation of silicon detectors using the two photon absorption – transient current technique
(2023) Pape, Sebastian; Kröninger, Kevin; Iván Vila, Álvarez
Modern high energy physics experiments have increasing demands on particle detectors in terms of their spatial and temporal resolution, as well as their ability to withstand higher radiation levels. To meet these demands, increasingly complex detectors with ever smaller device segmentations are being developed that require precise device characterisation. This work is dedicated to a newly developed characterisation technique: the two photon absorption - transient current technique (TPA-TCT); a method to characterise particle detectors with micrometer-scale three-dimensional spatial resolution. Femtosecond laser light with a wavelength in the quadratic absorption regime is focused to generate excess charge by two photon absorption in a volume of about 75 µm3 around the focal point. The drift of the excess charge carriers is studied to obtain information about the device under test. In this work, silicon detectors are used to explore and further develop the TPA-TCT. The technique is applied to pad detectors in order to study the technique and to strip and monolithic detectors to demonstrate the potential of TPA-TCT for the characterisation of state-of-the-art detector technologies. The applicability of the TPA-TCT in neutron, proton, and gamma irradiated devices is shown and radiation damage related effects on the technique are systematically studied. The reduction of charge multiplication in a low gain avalanche detector for increasing excess charge densities is observed and the role of diffusion to partially recover the gain is investigated. New techniques to investigate the electric field in complex segmented devices are developed and applied to strip and monolithic detectors. This work paves the way for the TPA-TCT as a tool to characterise detectors with three-dimensional micrometer-scale spatial resolution.
Development of a tool for Bayesian data analysis and its application in Monte Carlo tuning
(2023) La Cagnina, Salvatore; Kröninger, Kevin; Kluth, Stefan
In this thesis, a novel approach to Monte Carlo event generator tuning, grounded in Bayesian reasoning, is presented. The Bayesian Analysis Toolkit (BAT.jl) is introduced as a modern tool for performing Bayesian inference. A numerical test suite that verifies the validity and performance of the BAT.jl package is developed. The test suite is used to evaluate the performance of the Markov chain Monte Carlo (MCMC) sampling algorithms implemented in BAT.jl, utilizing a selection of test functions and different metrics to quantify the quality of the samples. The results show that the MCMC algorithms are able to sample the posterior distributions of the test functions accurately. Utilizing the BAT.jl toolkit, two hadronization models within the Herwig Monte Carlo event generator (MCEG) are successfully tuned to data from the LEP experiments. Several aspects of the tuning procedure are investigated, such as parameter and observable selection and parametrization quality. Samples generated using the tuned parameters, obtained from the global mode of the posterior, are compared to data through a χ2 test. The resulting p-values for the tuned simulations significantly outperform those from the nominal MCEG samples, indicating a successful tune and an improved description of the data. The posterior is also used to present a method for propagating the parameter uncertainties to the realm of the observables, providing a measure for the tuning uncertainty. Studies on the impact of assigning weights to the observables and the impact of correlations between measurements on the tuning are also presented. These show that weights can alter the tuning results, especially in cases with multiple modes in the posterior. However, their influence on the quality of the tune is minimal in this case. The correlation of measurements has less of an impact on the position of the global mode but substantially affects the associated parameter uncertainties estimates. Finally, a comparison of the two tuned hadronization models is presented, which indicates that the Lund string model describes the data slightly better than the cluster hadronization model for this set of observables.
Fast and accurate dose predictions for novel radiotherapy treatments in heterogeneous phantoms using conditional 3D‐UNet generative adversarial networks
(2022-02-19) Mentzel, Florian; Kröninger, Kevin; Lerch, Michael; Nackenhorst, Olaf; Paino, Jason; Rosenfeld, Anatoly; Saraswati, Ayu; Tsoi, Ah Chung; Weingarten, Jens; Hagenbuchner, Markus; Guatelli, Susanna
Purpose: Novel radiotherapy techniques like synchrotron X-ray microbeam radiation therapy (MRT) require fast dose distribution predictions that are accurate at the sub-mm level, especially close to tissue/bone/air interfaces. Monte Carlo (MC) physics simulations are recognized to be one of the most accurate tools to predict the dose delivered in a target tissue but can be very time consuming and therefore prohibitive for treatment planning. Faster dose prediction algorithms are usually developed for clinically deployed treatments only. In this work, we explore a new approach for fast and accurate dose estimations suitable for novel treatments using digital phantoms used in preclinical development and modern machine learning techniques. We develop a generative adversarial network (GAN) model, which is able to emulate the equivalent Geant4 MC simulation with adequate accuracy and use it to predict the radiation dose delivered by a broad synchrotron beam to various phantoms. Methods: The energy depositions used for the training of the GAN are obtained using full Geant4 MC simulations of a synchrotron radiation broad beam passing through the phantoms. The energy deposition is scored and predicted in voxel matrices of size 140 × 18 × 18 with a voxel edge length of 1 mm. The GAN model consists of two competing 3D convolutional neural networks, which are conditioned on the photon beam and phantom properties. The generator network has a U-Net structure and is designed to predict the energy depositions of the photon beam inside three phantoms of variable geometry with increasing complexity. The critic network is a relatively simple convolutional network, which is trained to distinguish energy depositions predicted by the generator from the ones obtained with the full MC simulation. Results: The energy deposition predictions inside all phantom geometries under investigation show deviations of less than 3% of the maximum deposited energy from the simulation for roughly 99% of the voxels in the field of the beam. Inside the most realistic phantom, a simple pediatric head, the model predictions deviate by less than 1% of the maximal energy deposition from the simulations in more than 96% of the in-field voxels. For all three phantoms, the model generalizes the energy deposition predictions well to phantom geometries, which have not been used for training the model but are interpolations of the training data in multiple dimensions. The computing time for a single prediction is reduced from several hundred hours using Geant4 simulation to less than a second using the GAN model. Conclusions: The proposed GAN model predicts dose distributions inside unknown phantoms with only small deviations from the full MC simulation with computations times of less than a second. It demonstrates good interpolation ability to unseen but similar phantom geometries and is flexible enough to be trained on data with different radiation scenarios without the need for optimization of the model parameter. This proof-of-concept encourages to apply and further develop the model for the use in MRT treatment planning, which requires fast and accurate predictions with sub-mm resolutions.
Enhancing precision radiotherapy: image registration with deep learning and image fusion for treatment planning
(2023) Ratke, Alexander; Kröninger, Kevin; Bäumer, Christian
Artificial intelligence is advancing in everyday life and supports its user by generating fast results in areas like communication or image recognition. This thesis aims at exploiting the abilities of deep-learning techniques for deformable image registration (DIR) to improve image alignment in medicine. An unsupervised registration and fusion workflow is developed and evaluated for 39 head scans, produced with computed tomography (CT) and magnetic resonance imaging (MRI). The three-part workflow starts by preprocessing the scans to unify the image formats and to perform affine transformation and rigid registration. Then, a deep-learning model trained for DIR is applied to these images. To obtain an appropriate configuration of the model, parameter tuning is required. The evaluation with the mutual-information metric indicates an improvement in image alignment of up to 14 % when using deep-learning-based DIR. Lastly, image fusion combines the registered CT and MRI scans with a wavelet-based method to merge the information of decomposed images. The workflow is designed for unimodal, e.g. T1- and T2-weighted MRI scans, and multimodal, e.g. CT and MRI scans, image pairs. Since medical imaging is an important basis of treatment-planning processes, the registered and fused images obtained from this workflow are expected to enhance precision radiotherapy.
From high energy physics to hospital
(2023) Schilling, Isabelle; Kröninger, Kevin; Bäumer, Christian
The effort to obtain continual progress in treatment quality in proton therapy facilities implies new technical requirements, mainly for the irradiation machines and the detector systems. For example, the collimation of proton spots generates stepper dose gradients and, thereby, the need for detectors with a high spatial resolution. Besides this, beam currents around 2 nA (≈ 1.2 ⋅ 1010 "protons" /"s" ) during patient treatment set challenging requirements on the detectors’ readout electronics for single particle tracking or counting. The knowledge gained in detector development in High Energy Physics (HEP) during the past decades is transferred to proton therapy applications in this work to address the upcoming detector requirements. It provides studies investigating the usage of a pixel detector designed for individual particle tracking in the high-radiation environment of the ATLAS experiment at LHC, namely the ATLAS IBL Pixel Detector, for proton beam measurements at proton therapy facilities. Due to the small pixel size of the detector under study, the shape of single pencil beam proton spots is determined with precision in the smaller pixel direction of 28 μm. The timing information of the particle hits on the detector allows the distinction between the single spots of scanned proton fields. Dose linearity checks reveal that the detector meets the requirement of an output dose consistency of ± 3 % for the daily quality assurance (QA) in the chosen dose range. Additionally, further studies lead to the conservative assumption that hit rates up to (73.85 ± 0.95) "clusters" /"25 ns" sampled with a frequency of 1 kHz feature a linear dependency on the beam current. Furthermore, the provided information on the deposited energy in the detector is utilized for range verification. Range differences of 1 mm required for the daily QA can be detected for proton energies impinging the sensor in the range of (30 − 44) MeV. Finally, an example of using the detector under study in the field of proton therapy is given by supporting a study investigating the energy deposition of platinum nanoparticles on a macroscopic scale. This work offers a characterization of the ATLAS IBL Pixel detector for proton therapy application and points out improvement opportunities for further detector development.
Room-temperature on-spin-switching and tuning in a porphyrin-based multifunctional interface
(2021-10-12) Sturmeit, Henning Maximilian; Cojocariu, Iulia; Windischbacher, Andreas; Puschnig, Peter; Piamonteze, Cinthia; Jugovac, Matteo; Sala, Alessandro; Africh, Cristina; Comelli, Giovanni; Cossaro, Albano; Verdini, Alberto; Floreano, Luca; Stredansky, Matus; Vesseli, Erik; Hohner, Chantal; Kettner, Miroslav; Libuda, Jörg; Schneider, Claus Michael; Zamborlini, Giovanni; Cinchetti, Mirko; Feyer, Vitaliy
Molecular interfaces formed between metals and molecular compounds offer a great potential as building blocks for future opto-electronics and spintronics devices. Here, a combined theoretical and experimental spectro-microscopy approach is used to show that the charge transfer occurring at the interface between nickel tetraphenyl porphyrins and copper changes both spin and oxidation states of the Ni ion from [Ni(II), S = 0] to [Ni(I), S = 1/2]. The chemically active Ni(I), even in a buried multilayer system, can be functionalized with nitrogen dioxide, allowing a selective tuning of the electronic properties of the Ni center that is switched to a [Ni(II), S = 1] state. While Ni acts as a reversible spin switch, it is found that the electronic structure of the macrocycle backbone, where the frontier orbitals are mainly localized, remains unaffected. These findings pave the way for using the present porphyrin-based system as a platform for the realization of multifunctional devices where the magnetism and the optical/transport properties can be controlled simultaneously by independent stimuli.
Tuning spin and charge at a metal-organic hybrid interface
(2021) Sturmeit, Henning; Cinchetti, Mirko; Westphal, Carsten
Metal-organic interfaces are key constituents of the various functional building blocks that can be found in molecular electronics and spintronics. Long electron and spin relaxation times of organic molecules make them superior to inorganic materials for many technological devices. When put into contact with a metal electrode, the hybridization of molecular orbitals and metallic states can lead to several intriguing effects, which strongly affect the electronic and magnetic properties of the system. In this regard, the interface obtained by depositing nickel tetraphenyl porphyrin onto the copper (100) surface (NiTPP/Cu(100)) can be seen as an interesting model system. Previous experiments reported an unexpected high charge transfer leading to a partial filling of the molecular orbitals up to the LUMO+3 and a reduction of the central nickel atom. Considering this observation as the point of departure, this thesis aims to develop different approaches to alter the hybridization at this interface. The results of this thesis can be divided into three main topics. First, it is shown that a pre-oxidation of the copper substrate leads to a substantial quenching of the charge transfer from the metal to the molecule and, thereby, weakens the interaction. In a second step, the spin configuration of the chelated nickel ion is changed by an on-top adsorbed NO2 molecule. The third part addresses the temperature-induced changes at the NiTPP/Cu(100) interface upon annealing. Up to the limit of thermal decomposition, the NiTPP molecules do not undergo chemical changes but only conformational modifications.
Exchange-mediated magnetic blue-shift of the band-gap energy in the antiferromagnetic semiconductor MnTe
(2020-08-12) Bossini, Davide; Terschanski, M.; Mertens, F.; Springholz, G.; Bonanni, A.; Uhrig, Götz S.; Cinchetti, Mirko
In magnetic semiconductors the optical spectrum and, in particular, the absorption edge representing the band-gap are strongly affected by the onset of the magnetic order. This contribution to the band-gap energy has hitherto been described theoretically in terms of a Heisenberg Hamiltonian, in which a delocalized conduction carrier is coupled to the localized magnetic moments by the exchange interaction. Such models, however, do not take into account the strong correlations displayed in a wide variety of magnetic semiconductors, which are responsible for the formation of the local moments. In particular, the itinerant carrier itself contributes to the spin moment. Here, we overcome this simplification in a combined experimental and theoretical study of the antiferromagnetic semiconductor α-MnTe. First, we present a spectroscopic optical investigation as a function of temperature, from which we extract the magnetic contribution to the blue-shift of the band-gap. Second, we formulate a minimal model based on a Hubbard–Kondo Hamiltonian. In this model, the itinerant charge is one of the electrons forming the localized magnetic moment, which properly captures correlation effects in the material. Our theory reproduces the experimental findings with excellent quantitative agreement, demonstrating that the magnetic contribution to the band-gap energy of α-MnTe is mediated solely by the exchange interaction. These results describe an intrinsic property of the material, independent of the thickness, substrate and capping layer of the specimen. One of the key findings of the model is that the basic effect, namely a blue-shift of the band-gap due to the establishment of the magnetic order, is a general phenomenon in charge-transfer insulators. The identification of the relevant magnetic interaction discloses the possibility to exploit the effect here discussed to induce a novel regime of coherent spin dynamics, in which spin oscillations on a characteristic time-scale of 100 fs are triggered and are intrinsically coupled to charges.
Modification of Pb quantum well states by the adsorption of organic molecules
(2019-02-06) Stadtmüller, Benjamin; Grad, Lisa; Seidel, Johannes; Haag, Florian; Haag, Norman; Cinchetti, Mirko; Aeschlimann, Martin
The successful implementation of nanoscale materials in next generation optoelectronic devices crucially depends on our ability to functionalize and design low dimensional materials according to the desired field of application. Recently, organic adsorbates have revealed an enormous potential to alter the occupied surface band structure of tunable materials by the formation of tailored molecule-surface bonds. Here, we extend this concept of adsorption-induced surface band structure engineering to the unoccupied part of the surface band structure. This is achieved by our comprehensive investigation of the unoccupied band structure of a lead (Pb) monolayer film on the Ag(1 1 1) surface prior and after the adsorption of one monolayer of the aromatic molecule 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA). Using two-photon momentum microscopy, we show that the unoccupied states of the Pb/Ag(1 1 1) bilayer system are dominated by a parabolic quantum well state (QWS) in the center of the surface Brillouin zone with Pb p orbital character and a side band with almost linear dispersion showing Pb p orbital character. After the adsorption of PTCDA, the Pb side band remains completely unaffected while the signal of the Pb QWS is fully suppressed. This adsorption induced change in the unoccupied Pb band structure coincides with an interfacial charge transfer from the Pb layer into the PTCDA molecule. We propose that this charge transfer and the correspondingly vertical (partially chemical) interaction across the PTCDA/Pb interface suppresses the existence of the QWS in the Pb layer. Our results hence unveil a new possibility to orbital selectively tune and control the entire surface band structure of low dimensional systems by the adsorption of organic molecules.
Strong modiﬁcation of the transport level alignment in organic materials after optical excitation
(2019-04-01) Stadtmüller, Benjamin; Emmerich, Sebastian; Jungkenn, Dominik; Haag, Norman; Rollinger, Markus; Eich, Steffen; Manira, Mahalingam; Aeschlimann, Martin; Cinchetti, Mirko; Mathias, Stefan
Organic photovoltaic devices operate by absorbing light and generating current. These two processes are governed by the optical and transport properties of the organic semiconductor. Despite their common microscopic origin—the electronic structure—disclosing their dynamical interplay is far from trivial. Here we address this issue by time-resolved photoemission to directly investigate the correlation between the optical and transport response in organic materials. We reveal that optical generation of non-interacting excitons in a fullerene ﬁlm results in a substantial redistribution of all transport levels (within 0.4eV) of the non-excited molecules. As all observed dynamics evolve on identical timescales, we conclude that optical and transport properties are completely interlinked. This ﬁnding paves the way for developing novel concepts for transport level engineering on ultrafast time scales that could lead to novel functional optoelectronic devices.
A case study for the formation of stanene on a metal surface
(2019-02-01) Maniraj, Mahalingam; Stadtmüller, Benjamin; Jungkenn, Dominik; Düvel, Marten; Emmerich, Sebastian; Shi, Wujun; Stökl, Johannes; Lyu, L.; Kollamana, J.; Wei, Z.; Jurenkow, A.; Jakobs, S.; Yan, B.; Steil, Sabine; Cinchetti, Mirko; Mathias, Stefan; Aeschlimann, Martin
The discovery and realization of graphene as an ideal two-dimensional (2D) material has triggered extensive efforts to create similar 2D materials with exciting spin-dependent properties. Here, we report on a novel Sn 2D superstructure on Au(111) that shows similarities and differences to the expected electronic features of ideal stanene. Using spin- and angle-resolved photoemission spectroscopy, we ﬁnd that a particular Sn/Au superstructure reveals a linearly dispersing band centered at the Γ-point and below the Fermi level with antiparallel spin polarization and a Fermi velocity of vF ≈ 1×106 m/s, the same value as for graphene. We attribute the origin of the band structure to the hybridization between the Sn and the Au orbitals at the 2D Sn-Au interface. Considering that free-standing stanene simply cannot exist, our investigated structure is an important step towards the search of useful stanene-like overstructures for future technological applications.
Molecular spectroscopy in a solid-state device
(2019-03-18) Atxabal, Ainhoa; Arnold, Thorsten; Parui, Subir; Zuccatti, Elisabetta; Cinchetti, Mirko; Casanova, Felix; Ortmann, Frank; Hueso, Luis E.
The quantification of the electronic transport energy gap of a molecular semiconductor is essential for pursuing any challenge in molecular optoelectronics. However, this remains largely elusive because of the difficulties in its determination by conventional spectroscopic methods. This communication presents an in-device molecular spectroscopy (i-MOS) technique, which permits measuring this gap seamlessly, in real device operative conditions, at room temperature and without any previous knowledge of the material's parameters. This result is achieved by determining the occupied and unoccupied molecular orbitals of an organic semiconductor thin-film by using a single three terminal solid-state device.
Tuning the charge flow between Marcus regimes in an organic thin-film device
(2019-05-07) Atxabal, Ainhoa; Arnold, Thorsten; Parui, Subir; Hutsch, Sebastian; Zuccatti, Elisabetta; Llopis, Roger; Cinchetti, Mirko; Casanova, Felix; Ortmann, Frank; Hueso, Luis E.
Marcus’s theory of electron transfer, initially formulated six decades ago for redox reactionsin solution, is now of great importance for very diverse scientific communities. The molecularscale tunability of electronic properties renders organic semiconductor materials in principlean ideal platform to test this theory. However, the demonstration of charge transfer indifferent Marcus regions requires a precise control over the driving force acting on the chargecarriers. Here, we make use of a three-terminal hot-electron molecular transistor, which letsus access unconventional transport regimes. Thanks to the control of the injection energy ofhot carriers in the molecular thinfilm we induce an effective negative differential resistancestate that is a direct consequence of the Marcus Inverted Region.

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