Development of time projection chambers with micromegas for Rare Event Searches

The Rare Event Searches is a heterogeneous field from the point of view of their physical motivations: double betha neutrinoless decay experiments, direct detection of WIMPs as well as axions and other WISPs (candidates for the DM, but also motivated by other questions from Particle Physics). The field is rather defined by the requirements of these experiments, essentially a very sensitive detector with low background which is usually operated in underground laboratories. The availability of a rich description of the event registered by the detector is a powerful tool for the discrimination of the signal from the background. The topological description of the interaction that can be delivered by a gaseous TPC is a useful source of information about the event. The generic requirements for a gaseous TPC that is intended for rare event searches are very good imaging capabilities, high gain and efficiency, stability and reliabiligy and radiopurity, which could imply working with particular gases, in absence of quencher and at high pressure, high granularity and the use of state-of-the-art electronics, and everything must be scalable to higher detectors. Such requirements could be fulfilled by TPCs because they are equipped with Micro-Pattern Gas Detectors, like Micromegas. The phenomenology of TPCs is studied in detail and R&D activities to its application to rare event searches are reported, in particular regarding microbulk micromegas, the latest manufacturing technique. A big part of the work has been devoted to the development of libraries and programs for generic Monte Carlo simulations on low energy TPCs and micromegas specific processes (primary charge generation, drift processes, implementation of the readout, generation of the electronic signals) and associated tools for information management and interpretation of the results. The role micromegas detectors have played in the CAST (CERN Axion Solar Telescope) experiment is reviewed, describing the strategies followed to improve the background more than a factor of 50, since the beginning of the experiment to 2011. To provide more precise guidelines aiming to continue and accelerate the encouraging evolution of micromegas backgound in CAST and to deliver prospects for IAXO (International AXion Observatory) an study on the CAST micromegas background is carried out relying in both simulations and tests-benches. Underground operation of CAST detector with a heavy shielding (at least 10 cm lead thickness) and improved radiopurity produced a background about 30 times lower than CAST nominal background, demonstrating the potencial of the detectors. The success of the 2012 upgrade of two of the CAST micromegas detectors, leading to an improvement of a factor 5 in background level, has been the first application/confirmation of the conclusions from these studies. In conclusion, the prospects to the application of micromegas to rare event searchers are encouraging for the issues that were proposed. The tests on the different aspects of the micromegas operation that are demanded by rare event searches (high pressure, particular mixtures, absence of quench) produced encouraging results. Moreover the state-of-the-art micromegas manufacturing technique, microbulk, has been measured to be radiopure. The impressive progression of the background of CAST micromegas detectors may be the most significant milestone. There has been an important advance in the understanding of the background nature, the potential of the different applied strategies and the way the detector performance and the analysis methods interact with the different kinds of background events. It can be assured that this progression, which have improved more than two orders of magnitude from the first micromegas installation, will not stop in the present sunset background level, and the future IAXO helioscope will be provided with more sensitive micromegas detectors. The ultra-low background obtained in the LSC (which is only an upper bound, probably no a real limit for the micromegas) is one of the facts that support this assertion. But its significance goes beyond the application to helioscopes. It demonstrates the possibility of registering ultra-low background below 10 keV with a low energy threshold.