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Sub-ppb xenon purity control and study of PTFE surface contamination towards the first results of XENONnT


Pizzella,  Veronica
Division Prof. Dr. Manfred Lindner, MPI for Nuclear Physics, Max Planck Society;

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Pizzella, V. (2021). Sub-ppb xenon purity control and study of PTFE surface contamination towards the first results of XENONnT. PhD Thesis, Ruprecht-Karls-Universität, Heidelberg.

Cite as: https://hdl.handle.net/21.11116/0000-0009-952B-F
The XenonnT experiment, successor of Xenon1T, aims at probing the cross
sections of the interaction between a WIMP, a well-motivated dark matter candidate,
and a xenon nucleus down to 1.4 x 1048 cm2; in addition, it will allow to distinguish if the electronic event excess observed by Xenon1T was due to new physics or a new
standard model background. XenonnT employs 8.5 tonnes of xenon in a dual-phase
Time Projection Chamber (TPC) and it needs both ultra-pure xenon and an ultralow
background for increasing the sensitivity. Some of the impurities of concern are:
radioactive impurities such as 85Kr, 222Rn and its progenies, and -3H, since they increase
the background rate; electronegative impurities such as oxygen, since they reduce the
amount of electrons in the TPC.
In the first part of this thesis, we present an offline purity monitor, able to detect
trace impurities at sub-ppb level. The setup uses a combination of Atmospheric Pressure
Ionization Mass Spectrometry (APIMS), with a commercial APIX dQ from ThermoFisher,
and a custom-made gas chromatography setup. The setup is used to measure xenon
samples from the start of the science run of XenonnT. First results are given on oxygen
and hydrogen contamination of the gaseous phase. In the second part, a study of
deposition of 222Rn daughters on Polytetrafluoroethylene (PTFE) is presented. The
study was conducted in the framework of the assembly of the XenonnT TPC, as PTFE
is the most abundant material. Results are given for several PTFE samples that were
deployed during the Xenon1T TPC construction phase.