Xenon

Xenon is an element which can be easily kept in the liquid state, becoming an excellent scintillator, meaning it emits light if a particles interacts with it. It is heavy and dense, which increases the probability of interaction with dark matter. It absorbs very well the radiations from outside and its intrinsic radioactivity, already very low, can be reduced at minimum through further purification.

Xenon

Xenon is a colorless and odorless noble gas that is present in traces in the atmosphere. It is used in fluorescent lamps or even as a propeller in spacecraft vehicles and is obtained as a byproduct of the oxygen and hydrogen separation from air.

Xenon in the search for dark matter

In particle physics xenon is used to detect interactions due to the passage of possible dark matter particles (seedark matter). This is achieved efficiently in the course of the XENON1T experiment at the INFN Gran Sasso National Laboratory; currently being upgraded and to be called XENONnT. The reason why xenon is well suited for the detection of Dark Matter is because of its capability to emit a light pulse, i.e. scintillation (see scintillator) , a consequence of particle interactions with either its nuclei or electrons. The xenon boiling point is relatively high with respect to other cryogenic elements (~ -100 oC), which means that it can maintain a liquid state longer with less requirement for cooling. Furthermore, it is a heavy element (54 protons and 77 neutrons, i.e. 131 nucleons) thus offering a large number of target nucleons per unit length. Different xenon isotopes (same number of protons and different number of neutrons), which could interact differently with Dark Matter, allow us to test different models of Dark Matter interactions. Xenon does not come with long-lived unstable isotopes whose decays are sources of undesired events which might mimic the Dark Matter signal. The only unstable atom comes from Kr (captured from air during the xenon separation process, in particular 85Kr) which is beta unstable and removed efficiently in a distillation process that will be described in the Distillation Column section (see distillazion column). The high density naturally provides a very efficient self-shielding capability against possible fake events generated by environmental radioactivity, which are stopped in few centimeters of xenon i.e. on the external shell. This feature gives rise to a greater number of interactions with dark matter at the core.

Other possible targets for dark matter detection

Le Even though the xenon is well suited for the detection of dark matter, it is not the only target material which can be exploited as a sensitive target in dark matter experiments. In fact, another good target material is argon, another noble gas, lighter than xenon, which is used in the LNGS DARKSIDE detector. Other experiments at the LNGS are currently collecting data making use of different target material such as sodium iodide crystals (DAMA/CRESST experiment), and Calcium Tungstate crystals (CRESST/ DAMA experiment). There are also detectors exploiting semi-conductors like germanium and silicon, or super-heated liquid, i.e. liquid kept in a metastable state in between liquid and gas phase.