  NaI(Tl) large crystals are applied in the search for galactic dark matter
particles through their elastic scattering off the target nuclei in the
detector by measuring the scintillation signal produced. However, energies
deposited in the form of nuclear recoils are small, which added to the low
efficiency to convert that energy into scintillation, makes that events at or
very near the energy threshold, attributed either to radioactive backgrounds or
to spurious noise (non-bulk NaI(Tl) scintillation events), can compromise the
sensitivity goals of such an experiment. DAMA/LIBRA experiment, using 250 kg
NaI(Tl) target, reported first evidence of the presence of an annual modulation
in the detection rate compatible with that expected for a dark matter signal
just in the region below 6 keVee (electron equivalent energy). In the frame of
the ANAIS (Annual modulation with NaI Scintillators) dark matter search project
a large and long effort has been carried out in order to understand the origin
of events at very low energy in large sodium iodide detectors and develop
convenient filters to reject those non attributable to scintillation in the
bulk NaI(Tl) crystal. 40K is probably the most relevant radioactive contaminant
in the bulk for NaI(Tl) detectors because of its important contribution to the
background at very low energy. ANAIS goal is to achieve levels at or below 20
ppb natural potassium. In this paper we will report on our effort to determine
the 40K contamination in several NaI(Tl) crystals, by measuring in coincidence
between two (or more) of them. Results obtained for the 40K content of crystals
from different providers will be compared and prospects of the ANAIS dark
matter search experiment will be briefly reviewed.
