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).