Time Projection Chamber

The striking feature of a xenon dual-phase time projection chamber (TPC) is an independent measurement of a light as well as a charge signal allowing for position reconstruction, particle identification and energy reconstruction.

The typical cylindrical detector is made of polytetrafluoroethylene (PTFE), as a frame for high reflectivity. The top and bottom plane are covered with arrays of light sensors, the Photomultiplier Tubes (PMT). The inner part is filled with liquid xenon to a certain level, with a gaseous xenon phase above. The animation on the right highlights the two signal measurement processes of light and charge.

When a particle deposits energy in form of a recoil within the liquid xenon, light (white flash) and charge (red e) are produced. The light is promptly detected by top and bottom PMTs, this is the S1 signal. The electrons are drifted upwards in an electric field, reaching the liquid and gas interface. Here the electrons are extracted by a second electric field and are accelerated into the gaseous xenon where secondary scintillation light is produced, this is the S2 signal.

Xenon as Target Material

Liquid xenon features several favourable properties for a dark matter detector. The high atomic number of A = 131 garantees an higher sensitivity for a spin-independent WIMP interaction compared to other materials (cross-section is proportional to A2). The natural xenon isotope composition is made of about half even and half odd isotopes, giving access to investigate spin-dependent interactions.

Due to the high charge number of Z = 54, xenon has a high stopping power for low-energy gammas. This self-shielding capability can be exploited for background reduction by fiducialization, where a quiet central detector region can be defined.

A high liquid density of about 3 g/cm3 in combination with a boiling point of 178 K (-100°C) at a pressure of 2 bar allows for building compact detectors with large target masses and simple cryogenic systems.

Scintillation light in liquid xenon is produced in two processes. In both cases light is emitted by an excited dimer. Xenon is transparent for its own scintillation light with a wavelength of about 175 nm.

Xenon can be cleaned to extreme purity levels in terms of electronegative impurities (such as oxygen and water), but also in terms of radioactive contaminants such as Kr-85 and Rn-222. XENONnT contains the purest xenon on Earth!

3D Position Reconstruction

By exploiting the time difference between the S1 and S2 signals for a constant drift velocity of the electrons, the depth and, therefore, the interaction point within the detector can be calculated.

Additionally, the  horizontal position of the interaction can be determined using the localized hit pattern of the top PMT array created by the S2 signal. Consequently, a full 3D position reconstruction along with the time stamp of the interaction can be achieved.

This gives access to the definition of a fiducial volume exploiting the self-shielding power of liquid xenon.

The background induced from the outside of the detector or from the detector materials themselves can be suppressed.

Particle Identification

The dual signal also allows for the differentiation between two different recoil processes of the xenon atom.

Most of the background components such as gamma- and beta-particles scatter off the electronic shell of the xenon atom, referred to as electronic recoil (ER) interactions. On the other hand, heavier particles such as WIMPs or neutrons can interact with the nucleus itself, referred to as nuclear recoil (NR) interactions.

ER and NR create different signatures for the light and charge signal. This difference can be exploited using the ratio of the S2 over S1 signals where one has:

(S2/S1)NR < (S2/S1)ER

Thus, electronic recoils can be discriminated from nuclear recoil events, which is depicted on the left using XENON1T calibration data.