Performance of the EDELWEISS-III experiment for direct dark matter searches

We present the results of measurements demonstrating the efficiency of the EDELWEISS-III array of cryogenic germanium detectors for direct dark matter searches. The experimental setup and the FID (Fully Inter-Digitized) detector array is described, as well as the efficiency of the double measurement of heat and ionization signals in background rejection. For the whole set of 24 FID detectors used for coincidence studies, the baseline resolutions for the fiducial ionization energy are mainly below 0.7 keV(ee) (FHWM) whereas the baseline resolutions for heat energies are mainly below 1.5 keV(ee) (FWHM). The response to nuclear recoils as well as the very good discrimination capability of the FID design has been measured with an AmBe source. The surface beta- and alpha-decay rejection power of R-surf < 4 x 10(-5) per alpha at 90% C. L. has been determined with a 210Pb source, the rejection of bulk gamma-ray events has been demonstrated using gamma-calibrations with (133B)a sources leading to a value of R gamma(-mis-fid) < 2 : 5 x 10(-6) at 90% C. L.. The current levels of natural radioactivity measured in the detector array are shown as the rate of single gamma background. The fiducial volume fraction of the FID detectors has been measured to a weighted average value of (74.6 +/- 0.4)% using the cosmogenic activation of the Zn-65 and Ge-68,Ge-71 isotopes. The stability and uniformity of the detector response is also discussed. The achieved resolutions, thresholds and background levels of the upgraded EDELWEISS-III detectors in their setup are thus well suited to the direct search of WIMP dark matter over a large mass range.