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Poster Summary: The Commissioning of the CUORE Experiment: the Mini-Tower Run

March 15, 2017

(by Simone Copello) CUORE is a ton-scale experiment approaching the data taking phase in Gran Sasso National Laboratory. Its primary goal is to search for the neutrinoless double-beta decay in 130Te using 988 Te02 crystals. The crystals are operated as bolometers at ˜10 mK taking advantage of one of the largest dilution cryostat ever built. The success of the CUORE experiment mainly depends on three factors: the capability to reach the base temperature and to maintain it for a long time, the low background rate in the region of interest around the 0?ßß Q-value of 2527 keV (the target is < 0.01 counts/keV/yr) and the small detectors energy resolution (the target is < 5 keV FWHM @ 2527 keV).

Concluded in March 2016, the cryostat commissioning consisted in a sequence of cool down runs each one integrating new parts of the apparatus. In total four commissioning runs were performed. In the first one, with the empty cryostat, the innermost vessel (connected to the dilution unit mixing chamber) reached the temperature of 5.9 mK. In the second run the readout wiring and the Detector Calibration System were added without affect the cryostat performance. In the third run the top lead shield was inserted into the cryostat increasing the mass of the 50 mK stage by more than two tons.

cuore Fig1_Temperature

Figure 1 (see text for details)

The last run was performed with the fully configured cryostat apart from the detectors array. The side and the bottom lead shields were added so that the mass to be cooled at 4 K reached the value of about 14 tons. During that run the cryogenic system, involving a Helium-based Fast Cooling System and five Pulse Tubes, was able to cool the cryostat down to 4 K in less than three weeks. The base temperature of 6.3 mK was reached and maintained for more than 60 days within an RMS range of 0.2 mK (see figure 1). The dilution unit cooling power was measured to be 3 µW at 10 mK.

cuore Fig2_TlPeak

Figure 2 (see text for details)

During that run an array of 8 crystals, called mini-tower, was installed in place of the final detectors array. It was used to check bolometers operation, as well as electronics and DAQ performances. The average energy resolution of the five best bolometers was measured to be 10 keV FWHM at 2615 keV (see figure 2), demonstrating that the CUORE resolution target of 5 keV can be likely achieved after the detector optimization phase. Moreover a very preliminary measurement didn’t show unaccounted background sources.

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