Ultra-low background Micromegas X-ray detectors for Axion searches in IAXO and BabyIAXO

Ruiz Chóliz, Elisa
García Irastorza, Igor (dir.) ; Luzon Marco, Gloria (dir.)

Universidad de Zaragoza, 2019


Resumen: The main concept of a particle gaseous detector is that radiation passing through a gas can ionize atoms or molecules if the energy delivered is higher than the ionization potential of the gas. This gaseous detector concept has been adopted by many particle physics detection techniques for rare event searches, the most representative being the Time Projection Chambers (TPCs). These detectors consist of a gas chamber, where an electric field is applied, and some sort of patterned anode plane, where the charge amplification and detection occur. Specifically, Micromegas readouts are of interest due to their flexible designs in terms of patterning, high granularity, good energy resolution and potential radiopurity. Axions are hypothetical elementary particles that were proposed as the most compelling solution to the strong CP problem of Quantum Chromodynamics (QCD). And beyond the QCD predicted axions, it extends a whole category of particles called Axion-Like Particles (ALPs), that are well motivated by different extensions of the Standard Model (SM). Some axions and ALPs hints are found when they are invoked as a solution for unexplained astrophysical observations, like the intergalactic transparency to very high energetic (VHE) photons or the anomalous cooling of stellar objects. Moreover, due to their weakly interacting nature, they are suitable candidates to be part or the totality of the dark matter of the Universe. An interesting property from the experimental point of view is that axions and photons can oscillate in the presence of a magnetic field, which is called the Primakoff effect. Well known solar physics predict the emission of axions from the core of the Sun via plasma photons conversion. These axions would escape from the Sun and travel to Earth, where they could convert back to x-ray photons via inverse Primakoff effect inside a laboratory magnet. Helioscopes are experiments that use this idea to search for solar axions. A powerful magnet tracks the Sun so an axion-photon conversion can occur, and then, x-ray detectors would be able to measure an excess over the background at keV energies. The International AXion Observatory (IAXO) is a proposed helioscope with a dedicated magnet and x-ray optics specially built for axion searches, and also, with improved low background microbulk Micromegas x-ray detectors. The sensitivity of the experiment would allow IAXO to probe completely unexplored regions of the parameter space, having potential for discovery. In the context of the IAXO international Collaboration, a BabyIAXO helioscope has been proposed for a short-term commissioning in order to prove all the technologies required for IAXO. In this context, a deep understanding of the radioactive background of the detector is important in order to identify the most problematic sources and shield the detectors from them. To implement them and prove the IAXO background requirements, a IAXO detector prototype has been built in the University of Zaragoza, called IAXO-D0. The work of this thesis has consisted in two parts: the computational simulation of a background model of IAXO-D0, and the commissioning, data taking, analysis and background study of the prototype. To produce the IAXO-D0 background model, the new REST framework has been used. It is a C++/root based software that provides tools for acquisition, storage, simulation, treatment and analysis of data taken with gaseous TPCs, allowing direct comparison between experimental data and simulations. The IAXO-D0 detector and shielding geometry has been implemented in the REST software and a complete simulation of all the known background sources has been carried out. Also, a simulation of [0,10] keV x-rays has allowed characterizing x-ray induced events, and discrimination cuts have been defined from their observables. These discrimination cuts have been applied to all of the individual background source. From this work, a low background level has been obtained for the IAXO-D0 prototype, which is of the same order of the IAXO pathfinder background. Furthermore, it was learnt that cosmic neutrons are a very important source of background that persists after the discrimination process. This result is a motivation to revise the shielding designs for BabyIAXO. On the other hand, during 2017, the IAXO-D0 prototype was commissioned at the University of Zaragoza, along with the gas system installation, the electronics upgrade and the Micromegas characterization. The first data taking campaign was carried out during 2018, with approximately 400 hours of background data and daily calibrations with a 109Cd source. REST software has also been used for the data processing and analysis, and the same strategy has been followed: an x-ray characterization has been performed using the data of a long 109Cd calibration run, and a representative population of events produced by the 8 keV x-rays form the copper fluorescence has been chosen to define the discrimination cuts. Then, background runs have been calibrated in energy, and the discrimination cuts have been applied. The resulting experimental background of the IAXO-D0 prototype is in agreement with the simulations and expectations. Overall, both experimental and simulated background levels are in good agreement, and some better knowledge of the cosmic contribution to the background has been learnt. From this work, some paths are left to be explored towards the BabyIAXO commissioning, like the revision of the shielding to reject cosmic neutrons, the computation of a background model for xenon based mixtures, the implementation of the cosmic vetoes at the IAXO-D0 prototype or the optimization of the code to obtain even more realistic simulations and better statistics.

Pal. clave: fisica ; fisica atomica y nuclear ; fisica de particulas ; fisica nuclear experimental de bajas energias

Titulación: Programa de Doctorado en Física
Plan(es): Plan 488

Departamento: Física Teórica

Nota: Presentado: 30 10 2019
Nota: Tesis-Univ. Zaragoza, Física Teórica, 2019







 Registro creado el 2020-01-15, última modificación el 2021-05-20


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