  Recently there is a flourishing and notable interest in the crystalline
scintillator material sodium iodide (NaI) as target for direct dark matter
searches. This is mainly driven by the long-reigning contradicting situation in
the dark matter sector: the positive evidence for the detection of a dark
matter modulation signal claimed by the DAMA/LIBRA collaboration is (under
so-called standard assumptions) inconsistent with the null-results reported by
most of the other direct dark matter experiments. We present the results of a
first prototype detector using a new experimental approach in comparison to
\textit{conventional} single-channel NaI scintillation light detectors: a NaI
crystal operated as a scintillating calorimeter at milli-Kelvin temperatures
simultaneously providing a phonon (heat) plus scintillation light signal and
particle discrimination on an event-by-event basis. We evaluate energy
resolution, energy threshold and further performance parameters of this
prototype detector developed within the COSINUS R&D project.
