arXiv subjects

Felix Hagemann

Publications and source records attributed to Felix Hagemann.

6 recordsLinked to original sources

arXiv2026

Compton imaging of undepleted volumes of germanium detectors

The shape of the undepleted volume of a p-type high-purity Broad Energy Germanium detector, dependent on the bias voltage, has been imaged by measuring spatially-resolved Compton-scattering efficiency. The bias voltage was raised stepwise from $-50\,\text{V}$ to the full-depletion voltage. The geometric acceptance was determined at full depletion. Below full depletion, the relative acceptance observed for $2\times2\times2\,\text{mm}^3$ voxels was used to create the image of the undepleted volume for each bias voltage. The images were used to extract the impurity density profile of the detector by fitting predictions of the open-source software package SolidStateDetectors$.$jl to the images. The result is shown and compared to the impurity density profile deduced from capacitance measurements. This is the first time that three-dimensional images of the undepleted volumes of a germanium detector have become available and have been used to deduce an impurity density profile.

physics.ins-det↗

arXiv2026

Depth Calibration of Double-sided Strip Germanium Detectors for the Compton Spectrometer and Imager Satellite

Double-sided strip high-purity germanium detectors with three-dimensional position reconstruction capability have been developed over three decades, with space-based applications in high-energy astrophysics and heliophysics. Position resolution in three dimensions is key to reconstruction of Compton scattering events, including for the upcoming Compton Spectrometer and Imager (COSI) satellite mission. Two-dimensional position reconstruction is enabled by segmentation of the two detector faces into orthogonal strip contacts, enabling a pixelized analysis. The depth of an interaction cannot be measured directly but must be inferred from the charge collection time difference between the two faces of the detector. Here, we demonstrate for the first time the depth calibration of a detector with the COSI satellite geometry read out using an application specific integrated circuit (ASIC) developed for the COSI mission. In this work, we map collection time difference to depth using the Julia-based simulation package SolidStateDetectors$.$jl and validate it with comparison to the timing distributions observed in data. We also use simulations and data to demonstrate the depth resolution on a per-pixel basis, with >90% of pixels having <0.9 mm (FWHM) resolution at 59.5 keV and <0.6 mm (FWHM) resolution at 122.1 keV.

astro-ph.IM↗

arXiv2022

Temperature Dependence of the Electron-Drift Anisotropy and Implications for the Electron-Drift Model

The electron drift in germanium detectors is modeled making many assumptions. Confronted with data, these assumptions have to be revisited. The temperature dependence of the drift of electrons was studied in detail for an n-type segmented point-contact germanium detector. The detector was mounted in a temperature controlled, electrically cooled cryostat. Surface events were induced with collimated 81 keV photons from a $^{133}$Ba source. A detailed analysis of the rise time of pulses collected in surface scans, performed at different temperatures, is presented. The longitudinal anisotropy of the electron drift decreases with rising temperature. A new approach, making use of designated rise-time windows determined by simulations using SolidStateDetectors$.$jl, was used to isolate the longitudinal drift of electrons along different axes to quantify this observation. The measured temperature dependence of the longitudinal drift velocities combined with the standard electron drift model as widely used in relevant simulation packages results in unphysical predictions. A first suggestion to modify the electron-drift model is motivated and described. The results of a first implementation of the modified model in SolidStateDetectors$.$jl are shown. They describe the data reasonably well. A general review of the model and the standard input values for mobilities is suggested.

physics.ins-det↗

arXiv2022

Bayesian inference of high-purity germanium detector impurities based on capacitance measurements and machine-learning accelerated capacitance calculations

The impurity density in high-purity germanium detectors is crucial to understand and simulate such detectors. However, the information about the impurities provided by the manufacturer, based on Hall effect measurements, is typically limited to a few locations and comes with a large uncertainty. As the voltage dependence of the capacitance matrix of a detector strongly depends on the impurity density distribution, capacitance measurements can provide a path to improve the knowledge on the impurities. The novel method presented here uses a machine-learned surrogate model, trained on precise GPU-accelerated capacitance calculations, to perform full Bayesian inference of impurity distribution parameters from capacitance measurements. All steps use open-source Julia software packages. Capacitances are calculated with SolidStateDetectors$.$jl, machine learning is done with Flux$.$jl and Bayesian inference performed using BAT$.$jl. The capacitance matrix of a detector and its dependence on the impurity density is explained and a capacitance bias-voltage scan of an n-type true-coaxial test detector is presented. The study indicates that the impurity density of the test detector also has a radial dependence.

physics.ins-det↗

arXiv2022

Identification and simulation of surface alpha events on passivated surfaces of germanium detectors and the influence of metalisation

Events from alpha interactions on the surfaces of germanium detectors are a major contribution to the background in germanium-based searches for neutrinoless double-beta decay. Surface events are subject to charge trapping, affecting their pulse shape and reconstructed energy. A study of alpha events on the passivated end-plate of a segmented true-coaxial n-type high-purity germanium detector is presented. Charge trapping is analysed in detail and an existing pulse-shape analysis technique to identify alpha events is verified with mirror pulses observed in the non-collecting channels of the segmented test detector. The observed radial dependence of charge trapping confirms previous results. A dependence of the probability of charge trapping on the crystal axes is observed for the first time. A first model to describe charge trapping effects within the framework of the simulation software SolidStateDetectors$.$jl is introduced. The influence of metalisation on events from low-energy gamma interactions close to the passivated surface is also presented.

physics.ins-det↗

arXiv2014

A new unified approach for modeling hot rolling of steel Part 1: Comparison of models for recrystallization

Models for the microstructure evolution during hot rolling are reviewed. The basic macroscopic phenomena related to recrystallization are summarized. Constitutive models based on semi empirical equations are compared to more sophisticated models based on cellular automata, vertex and Monte-Carlo-Potts methods. The applicability of each kind of model approach for online and offline process control in steel industry is discussed. While constitutive models are still state-of-the-art for online process control, mesoscale models with a spatial representation of the microstructure can provide better predictive capabilities at the cost of long computation times. To fill this gap a new approach based on modeling the interaction of an ensemble of multiple grains is outlined and first simulation results are presented. The proposed approach allows the unified modeling of dynamic, static and metadynamic recrystallization as well as grain growth.

cond-mat.mtrl-sci↗