The DAMA/LIBRA experiment is a particle detector experiment designed to detect dark matter using the direct detection approach, by using a matrix of NaI(Tl) scintillation detectors to detect dark matter particles in the galactic halo. The experiment aims to find an annual modulation of the number of detection events, caused by the variation of the velocity of the detector relative to the dark matter halo as the Earth orbits the Sun. It is located underground at the Laboratori Nazionali del Gran Sasso in Italy.
It is a follow-on to the DAMA/NaI experiment which observed an annual modulation signature over 7 annual cycles (1995-2002).
The detector is made of 25 highly radiopure scintillating thallium-doped sodium iodide (NaI(Tl)) crystals placed in a 5 by 5 matrix. Each crystal is coupled to two low background photomultipliers. The detectors are placed inside a sealed copper box flushed with highly pure nitrogen; to reduce the natural environmental background the copper box is surrounded by a low background multi-ton shield. In addition, 1 m of concrete, made from the Gran Sasso rock material, almost fully surrounds this passive shield. The installation has a 3-level sealing system which prevents environmental air reaching the detectors. The whole installation is air-conditioned and several operative parameters are continuously monitored and recorded.
DAMA/LIBRA was upgraded in 2008 and in 2010. In particular, after the upgrade in 2010 the experiment entered in its phase 2, with an increase of the set-up’s sensitivity thanks to the lowering of the energy threshold. The DAMA/LIBRA-phase 2 is in data taking.
Operation and results
DAMA/LIBRA phase 1 data collection started in September 2003. The DAMA/LIBRA released data correspond to 7 annual cycles. Considering these data together with those by DAMA/NaI, a total exposure (1.33 ton × yr) has been collected over 14 annual cycles. This experiment has further confirmed the presence of a model-independent annual modulation effect in the data in 2-6 keV range that satisfy all the features expected for a dark matter signal with high statistical significance.
Interpretation and comparisons
The obvious criticism of the seasonal variation of events recorded in the DAMA/LIBRA experiment is that it is in fact due to some purely seasonal effect unconnected with WIMPs. Although the deep underground location minimizes temperature swings and other direct sunlight effects, there are annual humidity fluctuations and other non-obvious effects. At moment, all these criticisms are taken in account by DAMA collaboration in analysis of the experimental data and they have been excluded, as discussed in published results. A repetition of this experiment in the Southern Hemisphere with the variation in phase with DAMA/LIBRA would discount this objection; if on the other hand variation was detected in the Southern Hemisphere that was 6 months out of phase with DAMA/LIBRA, then the seasonal variation objection would be upheld.
Improved versions of DAMA/LIBRA, named SABRE (Sodium-iodide with Active Background REjection) are under construction in two places. One is at LNGS, and the other is in Australia at the Stawell Underground Physics Laboratory, a laboratory being constructed 1025 m below the surface in a gold mine in Stawell, Victoria. First results are expected in 2017.
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