Time Projection Chamber

The most striking feature of a xenon dual-phase time projection chamber (TPC) is the ability to independently measure light and charge signals, allowing for position reconstruction, particle identification, and energy reconstruction.

Typically, the cylindrical detector is made of a polytetrafluoroethylene (PTFE) frame for high reflectivity. The top and bottom planes are covered with arrays of light sensors: Photomultiplier Tubes (PMTs). The inner part is filled with liquid xenon to a certain level, with a gaseous xenon layer above. The animation highlights the two signal measurement processes for light and charge.

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

Xenon as Target Material

Liquid xenon features several favourable properties for detecting dark matter. The high atomic number (A = 131) guarantees a higher sensitivity for spin-independent WIMP interactions compared to other materials (cross-section is proportional to A2). The natural xenon isotope abundances are roughly composed of half even and half odd isotopes, enabling investigations into spin-dependent interactions.

Due to the high charge number (Z = 54), xenon has excellent stopping power for low-energy gammas. This self-shielding capability can be exploited for background reduction via fiducialization, where a quiet central detector region can be defined away from radioactive backgrounds in detector materials.

A high liquid density of about 3 g/cm3 (three times larger than water) combined with a boiling point of 178 K (-100°C) at a pressure of 2 bar allows for the construction of compact detectors with large target masses and simple cryogenic systems.

Xenon is a scintillator, emitting light from excited dimers with a wavelength of about 175 nm. Xenon is transparent to its scintillation light, and the wavelength is detectable by commercial photosensors, enabling good signal detection efficiency.

Xenon can be cleaned to extreme purity levels regarding electronegative impurities (such as oxygen and water), and 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 electron drift velocity, the depth of the interaction point within the detector can be calculated.

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

This spatial and temporal information facilitates the definition of an ultra-pure fiducial volume to exploit the self-shielding power of liquid xenon. Backgrounds from outside of detector or from the detector materials themselves can be suppressed.

Particle Identification

The two signals also allow for the differentiation between two different interaction processes with xenon atoms.

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 distinguished from nuclear recoil events, as depicted in the graph using XENON1T calibration data.