Ph.D. degree thesis
accepted by EPFL, Switzerland

Alessandro Mapelli

Supervisor: Philippe Renaud

Scintillation Particle Detectors Based on Plastic Optical Fibres and Microfluidics

 Keywords:

• Scintillating fibres
• Liquid scintillators
• Microfabrication
• Microfluidics

 Abstract:
This thesis presents the design, development, and experimental validation of two types of scintillation particle detectors with high spatial resolution. The first one is based on the well established scintillating fibre technology. It will complement the ATLAS (A Toroidal Large ApparatuS) detector at the CERN Large Hadron Collider (LHC). The second detector consists in a microfabricated device used to demonstrate the principle of operation of a novel type of scintillation detector based on microfluidics.
The first part of the thesis presents the work performed on a scintillating fibre tracking system for the ATLAS experiment. It will measure the trajectory of protons elastically scattered at very small angles to determine the absolute luminosity of the CERN LHC collider at the ATLAS interaction point. The luminosity of an accelerator characterizes its performance. It is a process- independent parameter that is completely determined by the properties of the colliding beams and it relates the cross section of a given process to the corresponding event rate. Detector modules will be placed above and below the LHC beam in roman pot units at a distance of 240 m on each side of the ATLAS interaction point. The roman pots are vessels allowing the detectors to approach the beam axis at distances of the order of a millimetre. Overlap detectors, also based on the scintillating fibre technology, will measure the precise relative position of the two detector modules. Results obtained duri ng beam tests at DESY and at CERN validate the detectors design and demonstrate the achievable spatial resolution.
The second part of the thesis introduces a novel type of scintillation detector based on microfluidics, describing its main features and their experimental validation. Mi- crofluidic devices can be fabricated in a single photolithographic step with dimensional resolutions of the order of a micrometre. Microchannels can be easily filled with scin- tillating fluid, overcoming the difficulties encountered with previous liquid scintillation detectors made of capillary bundles. The possibility to circulate and to replace the irradiated liquid scintillator makes the active medium of the detector intrinsically radiation hard. Moreover, by changing the scintillator in the microchannels, the same device can be used to detect different types of particles.
Prototype detectors have been fabricated by using a photosensitive resin (SU-8) as structural element. The SU-8 negative-tone photoresist exhibits outstanding proper- ties such as good adhesion on different types of substrates, high mechanical strength and chemical stability. Moreover, its high level of resistance to radiation damage, com- parable to Kapton film, makes it a good candidate for novel radiation detectors. A standard SU-8 process has been optimized to fabricate dense arrays of hollow optical waveguides filled with liquid scintillator and coupled to external photodetectors. The photoelectric yield of this assembly is in good agreement with theoretical predictions and it is comparable to the yield of commercial small diameter scintillating fibres. Microfluidic scintillation detectors can be designed and processed to meet the require- ments of a wide range of applications like dosimetry, beam profiling, particle tracking, and calorimetry for high energy physics experiments, medical imaging, hadrontherapy and security devices. Miniaturized detectors as well as large devices can be fabricated with the same microfabrication techniques.