Resumen de Development of low-diffusion techniques for a high pressure xenon electroluminescent TPC
The neutrino is a particle that has seen a surge of interest in the past decades. Its existence was conjectured by W. Pauli in 1930 to explain the continuous spectrum of the beta decay. The unambiguous discovery of neutrino oscillation in 1998 proved that neutrinos have a non vanishing mass.
It is therefore necessary to accommodate a massive neutrino in the theory by expanding the Standard Model and that can be done by assuming that the neutrino is a Majorana particle, which is defined as a fermion being its own antiparticle. The mechanism giving its mass to a Majorana neutrino could explain the leptogenesis and part of the matter-antimatter asymmetry observed in the early universe.
Another implication of a Majorana neutrino is that processes violating the lepton number by two units become allowed. This has a major experimental implication: the double beta decay would have a mode in which it does not emit neutrinos.
Seeing an unambiguous neutrinoless double beta decay would definitively establish that the neutrino is a Majorana particle.
The NEXT collaboration has been developing high pressure electroluminescent Time Projection Chambers (TPCs) over the past decade to search for the neutrinoless double beta decay in xenon-136 where the detection volume is constituted of the target isotope. A TPC is a type of detector that can measure the energy deposited by a primary radiation within its volume.
The NEXT experiment uses a linear amplification process, electroluminescence, with negligible fluctuations to exploit the good intrinsic energy resolution of the xenon gas in order to maximize the sensitivity of the experiment.
On the other hand, operating on a gas at high pressure extends the tracks left by the electrons emitted by double beta decay to a few centimeters which turns their fine topological features into accessible information. A classification of events based on their topology allows performance of an extra step of background rejection.
The proposal to use a noble gas, helium, as an admixture to xenon is the central subject of the present thesis. The energy transfer through elastic collisions between electrons and helium atoms is about two orders of magnitude more efficient than that between electrons and xenon atoms. On the other hand, at the energy scale of the drifting electrons, the elastic collisions of electron-helium dominate those of electron-xenon. Those considerations together hint for a sizeable reduction of the thermal diffusion in helium-xenon (HeXe) mixtures.
Overall, the conclusion based on numerical simulations being that unlike molecular gas admixtures, helium lowers the diffusion without sacrificing the energy resolution of pure xenon. A proposal to protect the helium-sensitive photomultipliers from the helium-rich atmosphere consists of using a crystalline window along with metallic sealants.
In light of the enthusing predictions concerning helium-xenon (HeXe) mixtures the NEXT collaboration built and commissioned the prototype NEXT-DEMO++.
NEXT-DEMO++ is built following the SOFT (Separately-Optimized Functions TPC) concept. An array of photomultipliers (PMTs) is responsible for detecting the primary scintillation and performing the energy measurement by collecting the secondary scintillation emitted from the amplification region. The final assembly notably implemented the design proposal that ensures a safe operation of the helium-sensitive photomultipliers for mixtures containing a sizeable amount of helium.
Built primarily with the goal to test HeXe mixtures, NEXT-DEMO++ was also thought of more generally as a test bench for NEXT-100. NEXT-DEMO++ has tested different tracking sensor configurations and will test different possible amplification stages for NEXT-100. The detector was successful at operating safely a HeXe mixture but revealed a source of difficulty regarding the separation of helium and xenon by cryo-recovery.
Data were taken using NEXT-DEMO++ to compare a 15% HeXe mixture with a reference pure xenon mixture at 9.1 bar. In order to characterize the two mixtures, the same calibration source was used. This radioactive source produces 83mKr that are distributed in the active volume before decaying, leaving point-like energy depositions which are especially useful to characterize the electron transport properties.
Because the 83mKr events release a monochromatic energy deposition, it is possible to measure the energy resolution of the detector at 41.56 keV with each mixture as the energy is proportional to the light detected. After correction of the signal to account for the electron attachment to impurities, the energy resolution is measured at 7.42 +/- 0.04 % FWHM in HeXe and at 4.99 +/- 0.02 % FWHM in pure xenon.
The transverse diffusion was nonetheless confirmed experimentally to be reduced by a factor between 2 and 3 in HeXe. This demonstrates that helium can be used as an admixture to xenon to make a low-diffusion mixture for an electroluminescent TPC. The first tonne-scale module that will be built by the NEXT collaboration, NEXT-HD, will be designed in a way that will allow it to be operated with a low-diffusion HeXe mixture.
