The DAMA/LIBRA apparatus
Introduction
While running the first generation DAMA/NaI apparatus ( highly radiopure NaI(Tl)) [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], DAMA proposed to realize a ton apparatus in 1996 [13]. Thus, a second generation R&D project for highly radiopure NaI(Tl) detectors was funded and carried out over several years with leader companies in order to realize, as a new step, a second generation experiment with an exposed mass of about 250 kg. The highly radiopure NaI(Tl) DAMA/LIBRA (Large sodium Iodide Bulk for RAre processes) apparatus has been designed and built mainly to further investigate the Dark Matter particle component(s) in the galactic halo by exploiting the annual modulation signature; it also aims to improve the investigation of several rare processes. DAMA/NaI completed its data taking in July 2002 and the installation of DAMA/LIBRA started (see Fig. 1). DAMA/LIBRA has begun first operations in March 2003.
In the present paper, the DAMA/LIBRA apparatus and, in particular, the features of its new detectors, developed by means of the second generation R&D, are described.
Section snippets
The target material
Highly radiopure NaI(Tl) scintillators have been chosen as target—detector material for particle Dark Matter investigations since they offer many competitive aspects; among them we remind: (1) well-known technology; (2) high radiopurity reachable by material selections and protocols, by chemical/physical purifications, etc.; (3) large mass feasible; (4) high duty cycle; (5) routine calibrations feasible down to keV range in the same conditions as the production runs; (6) well controlled
Layout of the experimental apparatus
The main parts of the experimental apparatus are: (i) the installation; (ii) the multicomponent passive shield; (iii) the 25 highly radiopure NaI(Tl) detectors; (iv) the glove-box for calibrations; (v) the electronic chain and the monitoring/alarm system; (vi) the data acquisition (DAQ) system.
All the materials constituting the apparatus have been selected for low radioactivity by measuring samples with low background Ge detector deep underground in the Gran Sasso National Laboratory and/or by
The shield from environmental radioactivity
Fig. 4(a) shows a schematic view of the Cu/Pb/Cd/polyethylene-paraffin low background hard shield against environmental radioactivity made of very high radiopure materials, which are underground since at least about 15 years. Moreover, paraffin/polyethylene fill the space surrounding the plexiglass box as much as possible depending on the available space and, as mentioned, mostly outside the building, the DAMA/LIBRA apparatus is almost fully surrounded by about 1 m concrete, made from the Gran
The new highly radiopure NaI(Tl) detectors
The NaI(Tl) DAMA/LIBRA apparatus uses 25 NaI(Tl) highly radiopure detectors with 9.70 kg mass each one ( volume) placed in five rows by five columns. The granularity of the apparatus is an interesting feature for Dark Matter particle investigations since Dark Matter particles can just contribute to events where only one of the 25 detectors fires (single-hit events) and not to whose where more than one detector fire in coincidence (multiple-hit events).
The new DAMA/LIBRA
The photomultipliers
The photomultipliers used in this experiment have been built by Electron Tubes Limited and longly stored underground. The PMTs are made of ultra-low background glass. The materials entering in the construction of the PMT have been selected by various kind of measurements and, in particular, measurements on samples have also been carried out with low background DAMA/Ge detector deep underground.
The PMTs have flying leads (instead of a hard socket) and are directly connected to suitable voltage
The electronic chain
Each crystal of the DAMA/LIBRA apparatus is viewed by two PMTs with grounded cathode and supplied by positive high voltage. The HV power supply for the PMTs is given by a CAEN multichannel voltage supply with voltage stability of 0.1%.
Fig. 14 shows the scheme of the electronic devices of a single detector. In particular, the part of the chain regarding the analog signals and their processing are shown. It is also reported the logic devices providing the trigger of the single detector. The
Uniformity of the light collection
The absence of dead spaces in the light collection has been carefully investigated by performing suitable calibrations in different positions of the detector; in fact, by irradiating the whole detector with high-energy sources (e.g., 137Cs) from different positions, no significant variations of the peak position and energy resolution have been observed. Moreover, the peaks at high energy (see Fig. 7) and their energy resolutions are well compatible with those expected for calibration (see
Conclusions
In this paper, the highly radiopure NaI(Tl) DAMA/LIBRA apparatus, running at the Gran Sasso National Laboratory of the INFN, has been described. The main components and performances have been discussed.
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