CERN Accelerating science

Thesis
Report number	CERN-THESIS-2018-106
Related title	Applications of gas scintillation properties in optically read out GEM-based detectors
Author(s)	Brunbauer, Florian Maximilian (Vienna, Tech. U.)
Publication	147.
Thesis note	PhD : Vienna, Tech. U. : 2018-05-08
Thesis supervisor(s)	Streli, Christina ; Ropelewski, Leszek
Note	Presented 25 May 2018
Subject category	Detectors and Experimental Techniques
Abstract	Strong signal amplification, high achievable spatial resolution and low material budget as well as applicability in high-rate environments as key advantages of micropattern gaseous detectors make them an attractive candidate for radiation detection and imaging. Reading out scintillation light emitted during electron avalanche multiplication with modern imaging sensors provides accurate visualisations of incident radiation. The adjustable gain of ampli- fication structures such as Gaseous Electron Multipliers (GEMs) enables radiation detection over a wide range of energies from minimum ionising particles to single low-energy X-ray photons and highly ionising radiation. Scintillation characteristics of gas mixtures for opti- cally read out GEM-based detectors were investigated. Light yield and scintillation spectra of the emitted scintillation light in a range of operating conditions with variable amplification fields and different gas mixtures were studied to determine optimum operation conditions for optically read out detectors. Long term sealed mode operation of optically read out GEM- based detectors was achieved with a minimal degradation of signal strength. The presented gas scintillation studies and detector concepts based on optically read out GEMs effectively pave the way for applications ranging from radiation imaging to high energy physics and hadron therapy. Optically read out GEM-based detectors optimised and employed for X-ray radiography and tomography were developed. Operating in a proportional high-sensitivity regime, single X-ray photon sensitivity could be used for X-ray fluorescence imaging and material distinction with 2D resolution. Augmenting images of particle tracks with timing information, which can be obtained with fast photon detectors or complementary electronic readout, 3D reconstructed trajectories in an optically read out Time Projection Chamber (TPC) could be obtained. A transparent multi-pad anode was developed to combine si- multaneous optical and electronic readout to extend the track reconstruction capabilities of optically read out TPCs. A planispherical GEM-based detector employing radially focused field lines in the conversion volume to minimise parallax-induced broadening was developed and shown to permit significantly improved spatial resolution for X-ray fluorescence applica- tions of gaseous detectors with thick conversion layers. Taking advantage of the high spatial resolution achievable with optical readout and the low material budget of gaseous detectors, a proton beam monitoring detector was developed. 2D dose imaging as well as accurate beam profile and intensity monitoring were demonstrated at a clinical proton therapy facility.

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