The DAMA measurements of Xe quenching factor in pure LXe scintillators

  • In order to avoid the activation of low background set-ups operating deep underground, the routine energy calibrations have to be performed with gamma sources. In case the WIMP class of Dark Matter particle candidates is investigated by their scattering on target nuclei, the knowledge of the quenching factor for the given nucleus in the given detector is necessary. It strongly depends for whatever kind of detector on its specific features; see for example the summary table X of Riv. N. Cim. 26 n.1 (2003)1.
  • The quenching factor for nuclear recoils in a pure liquid Xenon (LXe) scintillator is the ratio of the amount of light induced by a recoil nucleus to the amount of light induced by an electron of the same kinetic energy. It can be enhanced with respect to the one in LXe detectors where an electric field is applied since the recombination effect - expected from the scintillation mechanism itself in pure LXe and experimentally verified several times - can occur. In particular, alpha/e light ratio equal to 1.1 - 1.2 has been measured in pure liquid Xenon scintillators by several authors.
  • It depends in operating conditions on various specific experimental features, such as e.g. on the initial purity of the used Xenon gas, on the inner surfaces treatment, on the reached vacuum before filling, on the used purification line components and on the degassing/release characteristics of all the materials of - and inside - the inner vessel (that is on the presence and amount of impurities able to act as quenchers in LXe). For example, for this reason (and for radiopurity purposes) DAMA uses in the final set-up a pure liquid Xenon scintillator with Cu inner vessel without any internal material and initial vacuum before filling not worse than 10-6 mbar (for details see NIMA482(2002)728).