M.Sc. degree
University of Coimbra, Portugal 2014

Marina Rodrigues Jorge

Supervisor: Luís Manuel Panchorrinha Fernandes

Experimental measurement of the electroluminescence yield of pure xenon and mixtures of xenon with percentage quantities of CH4

 Keywords:

• Charge transport, multiplication and electroluminescence in rare gases and liquids
• Electron multipliers (gas)
• Gaseous detectors
• Double-beta decay detectors

 Abstract:
Electroluminescence (EL), as signal amplification method of primary ionisation of noble gases in Gas Proportional Scintillation Counters (GPSC), developed in the 1970's, has played an important role in applications to many fields such as astronomy, plasma physics and high-energy physics, up to rising of solid state detectors in the mid 1990's. However, in the last years EL amplification recovered importance in experiments for rare event detection, such as direct dark matter and double beta decay searches.
The Neutrino Experiment with a xenon TPC, NEXT, is an international collaboration that aims at measure the neutrinoless double beta decay, 0ββ, of the isotope 136Xe.
Recent Monte Carlo simulation studies performed in Coimbra pointed out different trends for several gases added to xenon. While the presence of CH4 in levels of the order of the percent or below does not have important impact on the EL reduction, the addition of CF4 reduces significantly the EL, even for amounts of few decimals percent. A compromise must be found between the reduction of EL and the amount of molecular additive.
It is of great importance for the NEXT Collaboration to have these R&D studies performed, both experimentally and by simulation. The Atomic and Nuclear Instrumentation Group (GIAN) of the Instrumentation Centre at the University of Coimbra, has large expertise in the field, assuming the responsibility to carry out the detailed studies on this topic.
In the present work, relative measurements have been performed for the EL yields of xenon and xenon-mixtures in a uniform electric field driftless GPSC. The operational parameters of the detector, including amplitude, energy resolution and drift velocity, were measured as a function of the reduced electric field in the scintillation region. The results obtained agree with those from Monte Carlo simulations.