Ph.d. degree
CIEMAT, Spain, 2021

Edgar Sánchez García

Supervisor: Pablo Garcia Abia, Luciano Romero Barajas, Roberto Santorelli

Underground argon radio-purity studies for DarkSide-20k and R&D on noble gas detectors for rare-events investigation

Keywords:

• Noble liquid detectors (scintillation, ionization, double-phase)
• Dark Matter detectors (WIMPs, axions, etc.)
• Scintillators, scintillation and light emission processes (solid, gas and liquid scintillators)

Abstract:

Significant evidence suggests that ordinary matter, composed of baryons and leptons, accounts for only 5% of the energy-matter content of the Universe. To account for the full energy-matter density and explain astrophysical observations on a cosmological scale, it is necessary to include two new components. The first is a new, non-luminous, collisionless type of matter, called dark matter, whose interactions with ordinary matter are mainly through the gravitational force. The second is a uniformly distributed component called dark energy, which is believed to be responsible for the accelerating expansion of the Universe. According to the latest data from the PLANCK satellite, dark energy accounts for 69% of the content of our Universe and dark matter for 27%. The nature of dark matter and dark energy is one of the most relevant problems in physics today.
Although there are different possible explanations for the origin of dark matter, a very attractive hypothesis comes from the physics of particles in the form of thermal relics produced during the Big Bang, which decouple naturally, with the right abundance and are currently present in the halos surrounding galaxies. These are the so-called weakly interacting massive particles (WIMPs), which have no electromagnetic charge and only interact through the weak or gravitational force.
During the last years, different experiments have tried to detect this hypothetical particle, setting lower limits for the effective cross section and rejecting several proposed hypotheses. This thesis has been carried out inside the DarkSide-20K experiment, a time projection chamber with a detection volume of 50 tonnes of liquid argon, which aims to reach the best sensitivity limits for spin-independent searches in the mass range for WIMPs from 10 GeV to 100 TeV during the next decade.
One of the keys to obtaining these challenging results is the use of underground argon (UAr), which has much lower levels of the 39Ar isotope (0.73 mBq/kg) than atmospheric argon (1 Bq/kg). A crucial point is the characterisation of the different argon batches that will be used to fill DarkSide-20K. A novel detector called DArT will measure small samples of UAr (about 1~L) over short periods of time (a few days) with an error of less than 10%. The number of expected signal events is very small and it is necessary to reduce the number of background events dramatically. DArT will be installed inside Argon Dark Matter (ArDM), a 1 tonne volume of liquid argon, which will act as an active veto and at the same time provide the necessary cryogenic conditions for the experiment. DArT will be installed at the Canfranc Underground Laboratory (LSC) in summer 2021.
The main work of this thesis has consisted in the optimisation of the DArT design in order to achieve the highest possible sensitivity within the specifications required by DarkSide-20K. In addition, the first results obtained from the characterisation of the surface detector are presented.
Apart from the work carried out within the DarkSide collaboration, two other lines of research related to R&D for liquid argon detectors are shown. The first is the development of a wavelength-sensitive particle detector, with the aim of studying a novel technique for particle discrimination in noble gases. The second line of research is related to the development of a detector to study the effects produced by positive ions accumulated in large liquid argon detectors.