Ph.d. degree
Universidad de Zargoza, Spain 2014

Diana Carolina Herrera Muñoz

Supervisor: Igor García Irastorza

Development of a Micromegas Time Projection Chamber in Xe-based Penning Mixtures for Rare Event Searches

 Keywords:

• Time projection Chambers (TPC)
• Micropattern gaseous detectors (MSGC, GEM, THGEM, RETHGEM, MHSP, MICROPIC, MICROMEGAS, InGrid, etc)
• Dark Matter detectors (WIMPs, axions, etc.)
• Double-beta decay detectors

 Abstract:
The performance of a microbulk Micromegas High Pressure Time Projection Chamber (HP TPC) filled with Xe-based Penning mixtures towards Rare Event Searches, like the observation of the neutrinoless double beta decay or the direct detection of dark matter particles, was studied. Two Xe-based Penning mixtures were analyzed in a small TPC prototype (2.4 l of volume): a xenon+trimethylamine (Xe+TMA) mixture and a xenon+neon (Xe+Ne) mixture.
From X-rays measurements, it was found that a mixture of Xe (which should be enriched in Xe-136) with TMA concentration between 1-2% is very attractive for neutrinoless double decay searches. The most important advantage of this mixture is the improvement in energy resolution, reaching values that fulfill the requirements of an experiment looking for this process. In particular, values of energy resolution of 9.6% FWHM at 10 bar for 22.1 keV were attained, which extrapolate into 0.9% FWHM at the transition energy value. Gains up to a factor 100 higher and required amplification fields at least 40% lower than in pure Xe were achieved. On the other hand, the drift velocity in Xe+TMA mixtures is higher than in pure Xe in the range 50-400 V/cm/bar, reaching values up to an order of magnitude superior; for instance, for a reduced drift field (Ed/P) of 300 V/cm/bar the drift velocity in a Xe+2.2%TMA mixture is 1.3 cm/μs and for pure Xe 0.14 cm/μs. A reduction of the lon gitudinal diffusion coefficient by a factor about 3 respect to pure Xe was also observed.
On the other hand, columnar recombination was observed for alpha-particles in Xe+TMA mixtures. These results represent a first step towards the evaluation of the idea to use this mixture as medium gas for directional dark matter searches on virtue of the columnar recombination effect. The direction of the nuclear recoil of each event could be deduced thanks to the dependance of charge collection on the track angle, which would be an identifying signature of dark matter interactions.
Furthermore, the study of the Xe+Ne mixtures indicated that a microbulk Micromegas High Pressure TPC with Ne concentration between 20 and 30% may be optimum for application at low energy as for axions and dark matter searches, where the presence of a lighter noble gas is of interest.