Abstract
Axion helioscopes aim at the detection of solar axions through their conversion into x-rays in laboratory magnetic fields. The use of low background x-ray detectors is an essential component contributing to the sensitivity of these searches. Here we review the recent advances on Micromegas detectors used in the CERN Axion Solar Telescope (CAST) and proposed for the future International Axion Observatory (IAXO). The most recent Micromegas setups in CAST have achieved background levels of 1.5 × 10−6 keV−1 cm−2 s−1, a factor of more than 100 lower than the ones obtained by the first generation of CAST detectors. This improvement is due to the development of active and passive shielding techniques, offline discrimination techniques allowed by highly granular readout patterns, as well as the use of radiopure detector components. The status of the intensive R&D to reduce the background levels will be described, including the operation of replica detectors in test benches and the detailed Geant4 simulation of the detector setup and the detector response, which has allowed the progressive understanding of background origins. The best levels currently achieved in a test setup operating in the Canfranc Underground Laboratory (LSC) are as low as ∼ 10−7 keV−1 cm−2 s−1, showing the good prospects of this technology for application in the future IAXO.
Highlights
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The shielding approach described here was applied to the CERN Axion Solar Telescope (CAST) Micromegas detectors in 2007, and lasted until the progresses reported in this work motivated a new upgrade in 2012
Aluminium Cathode Copper Cathode Aluminium Cahtode used to reproduce the background spectrum induced by th intrinsic radioactivity of an aluminium cathode that was used in a specific test bench
Summary
The segmentation of the mesh will provide simplification of the microbulk production and mass minimization, allowing to pave large surfaces with high radiopurity As it was already explained, the implementation of a high efficient cosmic muon veto will be essential to achieve background levels in CAST similar to underground ones. The energy dependence of analysis observables is an open field for optimization In this direction, an electron beam based on PIXE (Particle Induced x-rays Emission) has been installed and tested at CAST detector laboratory at CERN [10]. An electron beam based on PIXE (Particle Induced x-rays Emission) has been installed and tested at CAST detector laboratory at CERN [10] This beam generates different fluorescence lines in the RoI depending on the target material like Al (1.49 keV), Ti (4.41 keV) or Cu (8.9 keV). These lines are complementary to the ones of usual CAST calibration runs at 3 and 5.9 keV
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