Micromegas operation in high pressure xenon: charge and scintillation readout

Abstract

The operational characteristics of a Micromegas operating in pure xenon at the pressure range of 1 to 10 bar are investigated. The maximum charge gain achieved in each pressure is approximately constant, around 4 x 10 2 , for xenon pressures up to 5 bar and decreasing slowly above this pressure down to values somewhat above 10 2 at 10 bar.The MM presents the highest gains for xenon pressures above 4 bar, when compared to other micropattern gaseous multipliers. The lowest energy resolution obtained for Xrays of 22.1 keV exhibits a steady increase with pressure, from 12% at 1bar to about 32% at 10 bar. The effective scintillation yield, defined as the number of photons exiting through the MM mesh holes per primary electron produced in the conversion region was calculated. This yield is about 2x10 2 photons per primary electron at 1 bar, increasing to about 6x10 2 at 5 bar and, then, decreasing again to 2x10 2 at 10 bar. The readout of this scintillation by a suitable photosensor will result in higher gains but with increased statistical fluctuations. IntroductionThe "NEXT -Neutrino Experiment with a Xenon TPC" collaboration proposes a new concept for neutrino-less double-beta decay search, based on a Time Projection Chamber (TPC) filled with high pressure gaseous Xenon (100 kg at 10 bar and room temperature operation) [1]. The TPC could be based on Electroluminescence for signal amplification, with excellent energy resolution and tracking capabilities.The noble gas (Xe) has two important functions, as source and detector, both acting simultaneously. Xenon is the only one, among the noble gases, that has a ββ decaying isotope, 136 Xe, and whose natural abundance is rather high, 9%, which can be enriched by centrifugation at a reasonable cost. There are no other long-lived radioactive isotopes and the Q ββ value, 2480 keV, is acceptably high. In addition, xenon can be easily re-circulated and continuously purified through getters.The TPC energy resolution is essential not only to reduce the tail of the 2νββ spectrum from overlapping the region of interest of the 0νββ spectrum, but also to prevent the contamination of the region of interest by the most severe background from 2.6 MeV gamma rays from 206 Tl and 2.4 MeV gamma rays from 214 Bi. On the other hand, the tracking capability aims to further reduce external backgrounds by identifying the unique topological signature of the 0νββ events, double electron track, in opposition to a single electron track resulting from gamma interactions. The optimization of these two features is crucial for such experiment with very low event rates and high background levels.The proposed detector design, called SOFT approach, is based on a specific readout for both tracking and energy measurement [2]. Primary ionization signals are amplified by means of electroluminescence amplification in a confined region of the TPC, the scintillation region. Electroluminescence photons emitted towards the hemisphere of the anode can be used for tracking, while photons emitted in the opposite dire...

Rare event searches based on Micromegas detectors: the T-REX project

“…Representative of the measurements with the 5.5 MeV alpha peak of the 241 Am, energy resolutions down to ∼2% (FWHM) in pure Xe, roughly independent of pressure, have been measured. When blocking the alphas, the energy resolution achieved for the 59.5 keV photon peak of the same source is of 9.3% (FWHM) at 3.5 bar, which are consistent with other measurements performed [19]. Tests adding a quencher to the Xe are being performed as well, as they are of potential interest because of the higher gains achieved with these mixtures.…”

Gaseous and dual-phase time projection chambers for imaging rare processes

“…58). 47 However, in the 46 For small Micromegas ( 10 cm 2 ) manufactured with the microbulk technique, the maximum gains obtained for 5-20 keV x-rays at 10 bar were in the range of m = 400-500 in Xe/TMA mixtures [254], to be compared with m 100 in pure xenon [112]. For the large Micromegas detectors used in NEXT-MM the maximum operating gain decreased by around a factor of two.…”

“…The proposal was to use a Xe/TMA Penning-fluorescent mixture (∼ 99/1), that allows a reduced operating voltage in the multiplication region and some residual (potentially usable) scintillation in the TMA band [93]. The addition of TMA resulted in a charge gain of m = 200 at 10 bar, greatly improving on previous results obtained in pure xenon [112]. 46 This provided sufficient signal to noise ratio for a calorimetric measurement of 30 keV x-rays ( Fig.…”

Microscopic simulation of xenon-based optical TPCs in the presence of molecular additives

“…As shown in [73], recombination light amounts at most to 10% of the total scintillation light observed for α particles at 10 bar, as long as E/P > 30 V/cm/bar. For electrons and X-rays the effect is even smaller [33]. In those conditions, the average energy to create a VUV photon in xenon depends just on the population of excited states, N ex , that are directly promoted by the impinging particle (of energy ε):…”