Title: Results of the first NaI scintillating calorimeter prototypes by COSINUS,
Abstract: Over almost three decades the TAUP conference has seen a remarkable momentum
gain in direct dark matter search. An important accelerator were first
indications for a modulating signal rate in the DAMA/NaI experiment reported in
1997. Today the presence of an annual modulation, which matches in period and
phase the expectation for dark matter, is supported at > 9$\sigma$ confidence.
The underlying nature of dark matter, however, is still considered an open and
fundamental question of particle physics. No other direct dark matter search
could confirm the DAMA claim up to now; moreover, numerous null-results are in
clear contradiction under so-called standard assumptions for the dark matter
halo and the interaction mechanism of dark with ordinary matter. As both bear a
dependence on the target material, resolving this controversial situation will
convincingly only be possible with an experiment using sodium iodide (NaI) as
target. COSINUS aims to even go a step further by combining NaI with a novel
detection approach. COSINUS aims to operate NaI as a cryogenic calorimeter
reading scintillation light and phonon/heat signal. Two distinct advantages
arise from this approach, a substantially lower energy threshold for nuclear
recoils and particle identification on an event-by-event basis. These key
benefits will allow COSINUS to clarify a possible nuclear recoil origin of the
DAMA signal with comparatively little exposure of O(100kg days) and, thereby,
answer a long-standing question of particle physics. Today COSINUS is in R&D
phase; in this contribution we show results from the 2nd prototype, albeit the
first one of the final foreseen detector design. The key finding of this
measurement is that pure, undoped NaI is a truly excellent scintillator at low
temperatures: We measure 13.1% of the total deposited energy in the NaI crystal
in the form of scintillation light (in the light detector).