Development of floating strip micromegas detectors

Micromegas are high-rate capable, high-resolution micro-pattern gaseous detectors. Square meter sized resistive strip Micromegas are foreseen as replacement of the currently used precision tracking detectors in the Small Wheel, which is part of the forward region of the ATLAS muon spectrometer. The replacement is necessary to ensure tracking and triggering performance of the muon spectrometer after the luminosity increase of the Large Hadron Collider beyond its design value of $10^{34}$\,cm$^{-2}$s$^{-1}$ around 2020. In this thesis a novel discharge tolerant floating strip Micromegas detector is presented and described. By individually powering copper anode strips, the effects of a discharge are confined to a small region of the detector. This reduces the impact of discharges on the efficiency by three orders of magnitude, compared to a standard Micromegas. The physics of the detector is studied and discussed in detail. Several detectors are developed: A $6.4\times6.4\,$cm$^2$ floating strip Micromegas with exchangeable SMD capacitors and resistors allows for an optimization of the floating strip principle. The discharge behavior is investigated on this device in depth. The microscopic structure of discharges is quantitatively explained by a detailed detector simulation. A $48\times50\,$cm$^2$ floating strip Micromegas is studied in high energy pion beams. Its homogeneity with respect to pulse height, efficiency and spatial resolution is investigated. The good performance in high-rate background environments is demonstrated in cosmic muon tracking measurements with a $6.4\times6.4\,$cm$^2$ floating strip Micromegas under lateral irradiation with 550\,kHz 20\,MeV proton beams. A floating strip Micromegas doublet with low material budget is developed for ion tracking without limitations from multiple scattering in imaging applications during medical ion therapy. Highly efficient tracking of 20\,MeV protons at particle rates of 550\,kHz is possible. The reconstruction of the track inclination in a single detector plane is studied and optimized. A quantitative description of the systematic deviations of the method is developed, that allows for correcting the reconstructed track inclinations. The low material budget detector is tested in therapeutic proton and carbon ion beams at particle rates between 2\,MHz and 2\,GHz. No reduction of the detector up-time due to discharges is observed. The measurable pulse height decreases by only 20\% for an increase of particle rate from 2\,MHz to 80\,MHz. Efficient single particle tracking is possible at flux densities up to 7\,MHz/cm$^2$. The good multi-hit resolution of floating strip Micromegas is shown.Micromegas sind mikrostrukturierte Gasdetektoren, die auch bei sehr hohen Raten Teilchenspuren pr\"azise vermessen k\"onnen. Micromegas Detektoren mit resistiven Streifen und einer aktiven Fl\"ache von mehreren Quadratmeter werden die im Moment im Small Wheel Be\-reich des ATLAS Myonspektrometers eingebauten Spurdetektoren ersetzen. Der Austausch ist notwendig, um die geforderte Triggerf\"ahigkeit und die genaue Spurvermessung im Myonspektrometer auch nach der f\"ur etwa 2020 geplanten Erh\"ohung der Luminosit\"at des Large Hadron Colliders auf Werte jenseits der Designluminosit\"at von $10^{34}$\,cm$^{-2}$s$^{-1}$ sicher zu stellen. In dieser Arbeit wird ein neuartiger floating strip Micromegas Detektor vorgestellt und beschrieben, dessen Verhalten von auftretenden Entladungen nur in geringem Umfang beeinflusst wird. Indem die Kupferstreifen, die die Auslesestruktur bilden, einzeln mit Hochspannung versorgt werden, k\"onnen die Auswirkungen von Entladungen auf einen kleinen Bereich des Detektors begrenzt werden. Dadurch vermindert sich der Effizienzverlust durch Entladungen im Vergleich zu normalen Micromegas um drei Gr\"o{\ss}enordnungen. Die im Detektor auftretenden physikalischen Prozesse werden untersucht und detailliert diskutiert. Mehrere unterschiedliche Detektoren werden entwickelt: Die Optimierung des floating strip Prinzips ist mithilfe eines kleinen floating strip Micromegas mit austauschbaren SMD Kondensatoren und Widerst\"anden m\"oglich. Entladungen werden mit diesem Detektor, der eine aktive Fl\"ache von $6.4\times6.4\,$cm$^2$ besitzt, ausf\"uhrlich untersucht. Eine detaillierte Detektorsimulation, erlaubt die quantitative Beschreibung der Struktur von Entladungen. Die Eigenschaften eines gro{\ss}en floating strip Micromegas mit einer aktiven Fl\"ache von $48\times50\,$cm$^2$ wird durch Bestrahlung mit hochenergetische Pionen vermessen. Dabei wird insbesondere die Homogenit\"at des Detektor im Bezug auf Pulsh\"ohe, Nachweiseffizienz und Orts\-aufl\"osung untersucht. Studien zum Nachweis von Spuren kosmischer Myonen in einem $6.4\times6.4\,$cm$^2$ gro{\ss}en floating strip Micromegas unter seitlicher Bestrahlung mit 20\,MeV Protonen bei einer Rate von 550\,kHz, demonstrieren die Leistungsf\"ahigkeit dieser Detektoren bei hohen Untergrundraten. Ein Detektormodul mit stark reduzierter Absorptionsl\"ange und einer aktiven Fl\"ache von $6.4\times6.4\,$cm$^2$, bestehend aus zwei floating strip Micromegas, wird entwickelt. Durch Verringerung der Vielfachstreuung erm\"oglicht es die genaue Spurbestimmung von niederenergetischen Ionen in medizinischen Bildgebungsverfahren. Seine Nachweiseffizienz f\"ur 20\,MeV Protonen bei einer Rate von 550\,kHz ist hoch. Ein Verfahren zur Bestimmung der Steigung von Teilchenspuren innerhalb einer Detektorlage wird untersucht und optimiert. Dabei auftretende systematische Abweichungen der rekonstruierten Spursteigungen, werden durch eine detaillierte Detektorsimulation quantitativ beschrieben und lassen sich so korrigieren. Das Verhalten dieses Detektormoduls in Proton- und Kohlenstoff-Ionenstrahlen wird bei Teilchenraten zwischen 2\,MHz und 2\,GHz untersucht. Eine gleichbleibend hohe Nachweisf\"ahigkeit wird beobachtet, die mittlere Pulsh\"ohe verringert sich nur um 20\% bei einer Erh\"ohung der Teilchenrate von 2\,MHz auf 80\,MHz. Der vollst\"andige Nachweis einzelner Teilchenspuren ist bis zu einer Teilchenflussdichte von 7\,MHz/cm$^2$ m\"oglich. Die Detektoren sind in der Lage, etliche Teilchenspuren gleichzeitig nachzuweisen

Integration of Spatial Distortion Effects in a 4D Computational Phantom for Simulation Studies in Extra-Cranial MRI-guided Radiation Therapy: Initial Results.

PurposeSpatial distortions in magnetic resonance imaging (MRI) are mainly caused by inhomogeneities of the static magnetic field, nonlinearities in the applied gradients, and tissue‐specific magnetic susceptibility variations. These factors may significantly alter the geometrical accuracy of the reconstructed MR image, thus questioning the reliability of MRI for guidance in image‐guided radiation therapy. In this work, we quantified MRI spatial distortions and created a quantitative model where different sources of distortions can be separated. The generated model was then integrated into a four‐dimensional (4D) computational phantom for simulation studies in MRI‐guided radiation therapy at extra‐cranial sites.MethodsA geometrical spatial distortion phantom was designed in four modules embedding laser‐cut PMMA grids, providing 3520 landmarks in a field of view of (345 × 260 × 480) mm3. The construction accuracy of the phantom was verified experimentally. Two fast MRI sequences for extra‐cranial imaging at 1.5 T were investigated, considering axial slices acquired with online distortion correction, in order to mimic practical use in MRI‐guided radiotherapy. Distortions were separated into their sources by acquisition of images with gradient polarity reversal and dedicated susceptibility calculations. Such a separation yielded a quantitative spatial distortion model to be used for MR imaging simulations. Finally, the obtained spatial distortion model was embedded into an anthropomorphic 4D computational phantom, providing registered virtual CT/MR images where spatial distortions in MRI acquisition can be simulated.ResultsThe manufacturing accuracy of the geometrical distortion phantom was quantified to be within 0.2 mm in the grid planes and 0.5 mm in depth, including thickness variations and bending effects of individual grids. Residual spatial distortions after MRI distortion correction were strongly influenced by the applied correction mode, with larger effects in the trans‐axial direction. In the axial plane, gradient nonlinearities caused the main distortions, with values up to 3 mm in a 1.5 T magnet, whereas static field and susceptibility effects were below 1 mm. The integration in the 4D anthropomorphic computational phantom highlighted that deformations can be severe in the region of the thoracic diaphragm, especially when using axial imaging with 2D distortion correction. Adaptation of the phantom based on patient‐specific measurements was also verified, aiming at increased realism in the simulation.ConclusionsThe implemented framework provides an integrated approach for MRI spatial distortion modeling, where different sources of distortion can be quantified in time‐dependent geometries. The computational phantom represents a valuable platform to study motion management strategies in extra‐cranial MRI‐guided radiotherapy, where the effects of spatial distortions can be modeled on synthetic images in a virtual environment

High-Rate Capable Floating Strip Micromegas

We report on the optimization of discharge insensitive floating strip Micromegas (MICRO-MEsh GASeous) detectors, fit for use in high-energy muon spectrometers. The suitability of these detectors for particle tracking is shown in high-background environments and at very high particle fluxes up to 60MHz/cm$^2$. Measurement and simulation of the microscopic discharge behavior have demonstrated the excellent discharge tolerance. A floating strip Micromegas with an active area of 48cm$\times$50cm with 1920 copper anode strips exhibits in 120GeV pion beams a spatial resolution of 50$\mu$m at detection efficiencies above 95%. Pulse height, spatial resolution and detection efficiency are homogeneous over the detector. Reconstruction of particle track inclination in a single detector plane is discussed, optimum angular resolutions below $5^\circ$ are observed. Systematic deviations of this $\mu$TPC-method are fully understood. The reconstruction capabilities for minimum ionizing muons are investigated in a 6.4cm$\times$6.4cm floating strip Micromegas under intense background irradiation of the whole active area with 20MeV protons at a rate of 550kHz. The spatial resolution for muons is not distorted by space charge effects. A 6.4cm$\times$6.4cm floating strip Micromegas doublet with low material budget is investigated in highly ionizing proton and carbon ion beams at particle rates between 2MHz and 2GHz. Stable operation up to the highest rates is observed, spatial resolution, detection efficiencies, the multi-hit and high-rate capability are discussed.Comment: Presented at ICHEP 2014, accepted for publication in Nuclear Physics B Proceedings Supplement

Proton Radiography for a Small-Animal Irradiation Platform Based on a Miniaturized Timepix Detector

Pre-treatment proton radiography and computed tomography can improve precision of proton therapy. A compact imaging setup for small-animal proton radiography, based on a miniaturized Timepix detector is presented along with results from proof-of-concept experiments. The MiniPIX detector was placed behind a µ-CT calibration phantom with 10 different tissue-equivalent inserts. The intensity of the 70 MeV proton beam was adjusted such that pixel signal clusters from individual protons on the detector could be resolved. Analysis and event filtering on various cluster properties were used to suppress unwanted events. The energy deposition of the selected clusters was converted to water-equivalent thickness (WET) of the traversed material using a conversion curve based on Monte Carlo simulations and measured clusters of protons after traversing PMMA slabs of known thickness. Despite a systematic underestimation of up to 3%, retrieved WET values are in good agreement with ground truth values from literature. The achieved spatial resolution ranges from 0.3 to 0.7 mm for phantom-detector-distances of 1 to 5 cm. Applicability to living animals is currently limited by the relatively long acquisition time of up to 20 minutes per radiography. This obstacle can however be overcome with the latest detector generation Timepix3, allowing to handle higher particle rates and thus requiring shorter irradiation times