Abstract
High pressure xenon Time Projection Chambers (TPC) based on secondary scintillation (electroluminescence) signal amplification are being proposed for rare event detection such as directional dark matter, double electron capture and double beta decay detection. The discrimination of the rare event through the topological signature of primary ionisation trails is a major asset for this type of TPC when compared to single liquid or double-phase TPCs, limited mainly by the high electron diffusion in pure xenon. Helium admixtures with xenon can be an attractive solution to reduce the electron diffu- sion significantly, improving the discrimination efficiency of these optical TPCs. We have measured the electroluminescence (EL) yield of Xe–He mixtures, in the range of 0 to 30% He and demonstrated the small impact on the EL yield of the addition of helium to pure xenon. For a typical reduced electric field of 2.5 kV/cm/bar in the EL region, the EL yield is lowered by ∼ 2%, 3%, 6% and 10% for 10%, 15%, 20% and 30% of helium concentration, respectively. This decrease is less than what has been obtained from the most recent simulation framework in the literature. The impact of the addition of helium on EL statistical fluctuations is negligible, within the experimental uncertainties. The present results are an important benchmark for the simulation tools to be applied to future optical TPCs based on Xe-He mixtures.
Highlights
In the conversion region, the sensitive volume, exciting and/or ionising the gas atoms/molecules and leading to the emission of primary scintillation from the gas de-excitation and, in the case of highly ionising particles, from electron/ion recombination
The present results are an important benchmark for the simulation tools to be applied to future optical Time Projection Chambers (TPC) based on Xe-He mixtures
The statistical fluctuations in the EL produced at electric fields below the onset of electron multiplication are negligible when compared to those associated with the primary ionisation formation, while the statistical fluctuations of the EL produced in electron avalanches are dominated by the much larger variance of the total number of electrons produced in the avalanches [11, 28]
Summary
In the conversion (drift) region, the sensitive volume, exciting and/or ionising the gas atoms/molecules and leading to the emission of primary scintillation (providing the t0 signal of the event, i.e. the start-of-event time-stamp) from the gas de-excitation and, in the case of highly ionising particles, from electron/ion recombination. The EL region is defined by two parallel electrodes, being the electric field intensity set between the gas excitation and the gas ionisation thresholds Upon crossing this region, each electron attains, from the electric field, enough kinetic energy to excite but not ionise the gas atoms/molecules, by electron impact, leading to high scintillation-output (electroluminescence) ensuing the gas deexcitation processes, without charge avalanche formation. Recent studies have demonstrated that the addition of molecular gases, such as CO2, CH4 and CF4, to pure xenon, a√t sub-percent concentration levels, reduces the electron diffusion to the level of ∼2 mm/ m, without jeopardizing the performance of the TPC in terms of EL yield and energy resolution, with CH4 being the most suitable candidate [20, 30, 31]. CH4, at the same time, presents some degree of excimerquenching [30, 31], which could limit the primary scintillation yield and, the calibration for low-energy events
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