M.Sc. degree
University of Coimbra, Portugal, 2018

Rita Joana da Cruz Roque

Supervisor: Fernando Domingues Amaro

X-ray imaging using 100 μm thick Gas Electron Multipliers operating in Kr-CO2

Keywords:

• Electron multipliers (gas)
• Gaseous imaging and tracking detectors
• Micropattern gaseous detectors (MSGC, GEM, THGEM, RETHGEM, MHSP, MICROPIC, MICROMEGAS, InGrid, etc)
• X-ray radiography and digital radiography (DR)

Abstract:

Krypton is known to have the best value of position resolution amongst the noble gases within the range 14 − 34 keV, which makes it a good candidate for imaging applications. Also, the choosing of 100 μm thick Gas Electron Multipliers (GEM) over the standard GEM plates presents an undeniable advantage as the former is more robust to sparking while achieving similar multiplication coefficients. By taking these factors into account, higher charge gains and better values of position resolution can be achieved to produce cleaner imaging data in the energy range 14−34 keV.
A cascade of two non-standard GEM plates (twice the thickness of a standard GEM) fabricated at CERN was coupled to a 2D resistive readout with an active area of 10x10 cm². This setup allows event energy and interaction position information to be recorded using only four channels, simplifying the electronic system and the image reconstruction process. This detection system was operated in krypton-based mixtures and irradiated by a 55Fe and a continuous X-ray source.
Whenever possible, the results were compared to the ones achieved in a Ar-CO2 (70:30) mixture.
Parameters such as the charge gain, energy resolution, image signal-to-noise ratio, position resolution and contrast response were measured under the described conditions. For krypton-based mixtures, the improvement in position resolution and in contrast response was recorded for radiation energies higher than 18 keV.