Th. Gebhart

Bio: Th. Gebhart is an academic researcher from Max Planck Society. The author has contributed to research in topics: Tokamak & Physics. The author has an hindex of 3, co-authored 4 publications receiving 166 citations.

Strip detector design for ATLAS and HERA-B using two-dimensional device simulation

Abstract: Irradiation scenarios were simulated in order to evaluate different technology and design options for silicon strip detectors exposed to a high luminosity environment. Two-dimensional process and device simulations were performed to get an insight into the device behaviour. The boundary condition of the free oxide regions between the strips was evaluated thoroughly to obtain correct field distributions. Using these results the formation of electron accumulation layers on the surface of the p-side and the depletion voltage dependence on the strip geometry can be explained. We investigated the “blocking implant” and the “spray implant” techniques as promising candidates for the n-side isolation of irradiated detectors. The main drawback of the “blocking implanted” devices is the increase of the electric field with increasing oxide charges. This implies the danger of impact ionization in irradiated devices. A “spray implanted” isolation layer leads to the highest electric field in the non-irradiated state which has the advantage of a better testability. Small gaps between strips as used in charge division readout reduce electric fields and leakage currents.

Siggle sided $p+n$ and double-sided silicon strip detectors exposed to fluences up to $2 \times 10^{14}$/cm$^{2}$ 24 GeV protons

Abstract: Single-sided p+n and double-sided detectors have been designed for surviving the drastic changes of material properties expected from their use in the harsh radiation environment at the LHC. Detectors optimized for capacitive charge division readout have been exposed to a fluence of 2×1014/cm2 24 GeV protons. Their principal design characteristics and properties after irradiation are described. An explanation for the hitherto not understood survival of single-sided p+n detectors is given. First results with single-sided p+n detectors optimized for binary readout are presented.

Enhanced performance in fusion plasmas through turbulence suppression by megaelectronvolt ions

Abstract: Alpha particles with energies on the order of megaelectronvolts will be the main source of plasma heating in future magnetic confinement fusion reactors. Instead of heating fuel ions, most of the energy of alpha particles is transferred to electrons in the plasma. Furthermore, alpha particles can also excite Alfvénic instabilities, which were previously considered to be detrimental to the performance of the fusion device. Here we report improved thermal ion confinement in the presence of megaelectronvolts ions and strong fast ion-driven Alfvénic instabilities in recent experiments on the Joint European Torus. Detailed transport analysis of these experiments reveals turbulence suppression through a complex multi-scale mechanism that generates large-scale zonal flows. This holds promise for more economical operation of fusion reactors with dominant alpha particle heating and ultimately cheaper fusion electricity. Experiments at the Joint European Torus tokamak show improved thermal ion confinement in the presence of highly energetic ions and Alfvénic instabilities in the plasma.

A Fully Depleted pn-Junction CCD for Infrared-, UV- and X- ray detection

Abstract: This paper describes the performance of the Fully Depleted pn-junction CCD (pn-CCD) system, developed for ESA's XMM-satellite mission for soft x-ray imaging and spectroscopy in the single photon counting mode in the 100 eV to 10 keV photon range The 58 mm x 60 mm large pn-CCD array, designed and fabricated at the Semiconductor Lab (Halbleiterlabor) of the Max-Planck-Institut, uses pn-junctions for registers and as backside structure This concept naturally enables full depletion of the detector volume independent of the silicon wafer's resistivity and thickness, and as such make it an efficient detector for the x-ray region and the infrared For high detection efficiency in the soft x-ray region and UV, an ultrathin pn-CCD backside deadlayer has been realized Each pn-CCD-channel is equipped with its own on-chip JFET amplifier which, in combination with the CAMEX-amplifier and multiplexing chip, facilitates parallel readout and fast data rate: the cooled pn-CCD system can be read out at a data rate up to 3 MHz with an electronic noise floor of ENC < 5 e-

6 x 6-cm fully depleted pn-junction CCD for high-resolution spectroscopy in the 0.1- to 15-keV photon energy range

Abstract: The concept and performance of the fully depleted pn- junction CCD system, developed for the European XMM- and the German ABRIXAS-satellite missions for soft x-ray imaging and spectroscopy in the 0.1 keV to 15 keV photon range, is presented. The 58 mm X 60 mm large pn-CCD array uses pn- junctions for registers and for the backside instead of MOS registers. This concept naturally allows to fully deplete the detector volume to make it an efficient detector to photons with energies up to 15 keV. For high detection efficiency in the soft x-ray region down to 100 eV, an ultrathin pn-CCD backside deadlayer has been realized. Each pn-CCD-channel is equipped with an on-chip JFET amplifier which, in combination with the CAMEX-amplifier and multiplexing chip, facilitates parallel readout with a pixel read rate of 3 MHz and an electronic noise floor of ENC < e-. With the complete parallel readout, very fast pn-CCD readout modi can be implemented in the system which allow for high resolution photon spectroscopy of even the brightest x-ray sources in the sky.

ATLAS pixel detector electronics and sensors

Abstract: The silicon pixel tracking system for the ATLAS experiment at the Large Hadron Collider is described and the performance requirements are summarized. Detailed descriptions of the pixel detector electronics and the silicon sensors are given. The design, fabrication, assembly and performance of the pixel detector modules are presented. Data obtained from test beams as well as studies using cosmic rays are also discussed.

Radiation hardness of silicon detectors — a challenge from high-energy physics

Abstract: An overview of the radiation-damage-induced problems connected with the application of silicon particle detectors in future high-energy physics experiments is given. Problems arising from the expected hadron fluences are summarized and the use of the nonionizing energy loss for normalization of bulk damage is explained. The present knowledge on the deterioration effects caused by irradiation is described leading to an appropriate modeling. Examples are given for a correlation between the change in the macroscopic performance parameters and effects to be seen on the microscopic level by defect analysis. Finally possible ways are out-lined for improving the radiation tolerance of silicon detectors either by operational conditions, process technology or defect engineering.

The silicon microstrip sensors of the ATLAS semiconductor tracker

Abstract: This paper describes the AC-coupled, single-sided, p-in-n silicon microstrip sensors used in the Semiconductor Tracker (SCT) of the ATLAS experiment at the CERN Large Hadron Collider (LHC). The sensor requirements, specifications and designs are discussed, together with the qualification and quality assurance procedures adopted for their production. The measured sensor performance is presented, both initially and after irradiation to the fluence anticipated after 10 years of LHC operation. The sensors are now successfully assembled within the detecting modules of the SCT, and the SCT tracker is completed and integrated within the ATLAS Inner Detector. Hamamatsu Photonics Ltd. supplied 92.2% of the 15,392 installed sensors, with the remainder supplied by CiS.

The ATLAS silicon pixel sensors

Abstract: Prototype sensors for the ATLAS silicon pixel detector have been developed. The design of the sensors is guided by the need to operate them in the severe LHC radiation environment at up to several hundred volts while maintaining a good signal-to-noise ratio, small cell size, and minimal multiple scattering. The ability to be operated under full bias for electrical characterization prior to attachment of the readout integrated circuit electronics is also desired.

Evaluation of Energy Loss and Charge Sharing in Cadmium Telluride Detectors for Photon-Counting Computed Tomography

Abstract: We present estimates of energy loss and charge sharing for a pixelated cadmium telluride (CdTe) detector used for photon-counting spectral computed tomography (CT). In a photon-counting pixelated CdTe detector, several physical effects lead to detected events with reduced energies, including Compton scattering, fluorescence emission, charge diffusion, trapping of charge carriers and slow-hole-motion-induced incomplete charge collection. Charge sharing is the result of the lost energy being collected by adjacent pixels. We simulated the photon transport and the charge-collection process with a Monte Carlo-based simulation and evaluated these effects on the detector performance. The trapping effect and poor hole collection have been studied together using an analytical model. We also investigated the detector response under the influence of only the fluorescence effect. We conclude that the charge sharing effects should be taken into account when the pixel is smaller than 1 mm2. A straightforward way to decrease the double counting of X-rays from events with charge sharing is to increase the electronic threshold. However, increasing the threshold comes at the cost of losing low-energy events, which is undesirable, at least in applications such as pediatric imaging.