Microfluidic Scintillation Detectors for High Energy Physics

This thesis deals with the development and study of microfluidic scintillation detectors, a technology of recent introduction for the detection of high energy particles. Most of the interest for such devices comes from the use of a liquid scintillator, which entails the possibility of changing the active material in the detector, leading to increased radiation resistance. A first part of the thesis focuses on the work performed in terms of design and modelling studies of novel prototype devices, hinting to new possibilities and applications. In this framework, the simulations performed to validate selected designs and the main technological choices made in view of their fabrication are addressed. The second part of this thesis deals with the microfabrication of several prototype devices. Two different materials were studied for the manufacturing of microfluidic scintillation detectors, namely the SU-8 photosensitive epoxy and monocrystalline silicon. For what concerns the former, an original fabrication approach based on successive bonding and selective release steps of resin layers patterned over sacrificial metal films is detailed. This approach was used to fabricate monolithic, free-standing devices embedding one or two layers of microfluidic channels, with a material budget corresponding to only 0.03% and 0.06% of the radiation length of SU-8. A first experimental validation of these devices is presented as well. Concerning silicon devices, studies on the fabrication of microchannel arrays by both dry and wet etching are reported. Adaptations of these standard techniques to the specific needs of microfluidic scintillation detectors are addressed, specifically the smoothing of scalloped sidewalls resulting from deep reactive ion etching as well as the mask design methodology applied to KOH etching in order to yield microchannels with smooth vertical sidewalls on wafers with the standard <100> crystalline orientation. The anisotropic characteristics of wet etching were also exploited to demonstrate the fabrication of arrays of microfluidic channels having slanted reflective facets at their extremities, which can act as micromirrors that deviate the scintillation light in the out of plane direction, thus introducing new possibilities for the planar integration of the devices. Experimental results on the characterization of the light yield and attenuation length of silicon prototype devices performed using electrons from a radioactive source are presented. A brief study on the accelerated ageing of the detector in which the liquid scintillator was damaged by intense UV irradiation is reported. Such study provides encouraging results on how the capability of recirculating the active material in microfluidic scintillation detectors can be used to extend their lifetime or increase the stability of their performance in time.