3 FIG. 1: (Color online) Principle of operation of the XENON10 two-phase TPC. A particle interaction in the liquid produces primary scintillation light ( S 1) and ionization electrons. The electrons drift under an electric field ε$_{d}$ (about 1 kV/cm) un- til they are extracted from the liquid into the gas where they are accelerated by an electric field ε$_{e}$ (about 10 kV/cm) pro- ducing proportional scintillation light ( S 2). Two arrays of photomultipliers, one in the liquid and one in the gas, detect simultaneously the S 1 and S 2 light signals. !"#$%&’"#()* +,& ! -./&(0 1$2%2(3 4"05 67"0)78&9’-: 672127%"2()*&9’;: <*5=%72(&>7"?% !$_{;&00@A3}$ !$_{;B&(3&C"D%$}$ !$_{-&A3&C"D%$}$ ’- E(=20"(# 6)7%"=*5 F)%$2D5&G7"D 5H 5H 5H 5H 5H ’; I(2D5&G7"D G)%5&G7"D G)3 !"JK"D !$_{!}$ !$_{D}$ B. Electrodes Assembly and Electric Fields electrons extracted from the liquid to the gas phase, N$_{γ}$ is the number of proportional scintillation photons pro- duced. α (the amplification factor) and β (the threshold of the reduced field for proportional light production) are experimentally determined values. For the case of Xe, the reported values for α vary from 70 to 140 and β values are between 0.8 and 1.0 kV/cm/bar [2, 18, 19]. The number of secondary photons generated by one drift- ing electron is sufficiently large to be detected by PMTs, thus a two-phase detector is single-electron sensitive. In XENON10 operating conditions, the values of p , ε$_{e}$ and x were 2.1 bar, 12 kV/cm and 0.25 cm, respectively, yield- ing ∼ 100 photons per extracted electron. The different amplitude of the S 1 and S 2 signals asso- ciated with nuclear recoils, such as produced by WIMP or neutron interactions, and with electron recoils, such as produced by background beta and gamma-rays, provides the basis for background discrimination in a two phase TPC such as XENON10. Since electron diffusion in LXe is small [20], the proportional scintillation pulse is pro- duced in a small volume with the same XY coordinates as the interaction site, allowing 2D localization. The Z coordinate is inferred from the drift time measurement, and the known electron drift velocity at the operating field. The TPC approach and the strong self-shielding property of LXe ( ρ ≈ 3 g/cc, Z = 54), make the XENON concept very powerful for background identification and rejection based on event topology: multiple scatter events from the overwhelming majority of gamma rays and from neutrons can be identified and rejected. The sensitive target can be “fiducialized” to keep only the inner core free of background. The capability to accurately identify the 3D position on an event-by-event basis, also allows to correct the po- sition dependence of both the direct and proportional scintillation signals, which in turn results in improved energy resolution. For a recent review of the properties of LXe and its response to radiation we refer to [20]. One of the challenges of a two-phase detector is to ef- ficiently detect the small number of primary scintillation photons associated with the low energy events of interest in a dark matter search. Because of the large refractive index of LXe [21] and consequent total internal reflection at the liquid/gas interface, the presence of PMTs at the bottom of the active drift volume is essential for efficient S 1 light collection. When the XENON concept was proposed, the devel- opment of PMTs capable to operate directly in LXe had just began. The typical quantum efficiency of these early PMTs at 178 nm was in the 10% range. To achieve high light collection of the S 1 signal and thus a low energy threshold required for a dark matter search, the baseline XENON detector design used a CsI photocath- ode in place of a common cathode, and photomultipliers or other type of photodetectors in the gas region [22]. During the XENON R&D phase, different photodetec- tors were tested[23, 24] and also charge-sensitive readouts based on the Gas Electron Multiplier concept [25]. The operation of CsI photocathodes to detect the direct light in two phase prototypes confirmed [26] the high QE first measured by the Columbia group [27]. However, the com- plexity of operating a TPC under the very strong electric field required by the CsI photocathode, while suppress- ing positive photon feedback, and the fast improvement of the performance of Hamamatsu PMTs for LXe opera- tion, led us to abandon the CsI photocathode approach. Prior to adopting the Hamamatsu one-inch square R8520 PMT for XENON10, a two-inch diameter R9288 PMT was extensively tested [2, 26]. Fig. 2 shows a 2D mechanical drawing of the XENON10 detector assembly, and Fig. 4 shows a pic- ture of the detector mounted on the movable door of the passive shield (see section III). A closer view of the in- ner TPC structure is shown in Fig. 3. The TPC active volume is defined by a polytetrafluoroethylene (PTFE) cylinder with an inner diameter of 20 cm and a height of 15 cm for a total active mass of about 14 kg of LXe. PTFE is used as UV light reflector [28] and as electrically insulating support for the TPC structure. To produce the electric fields in the liquid and gas re- gions, four wire meshes, also referred to as grids, are used; two in the liquid (cathode and gate grid) and two in the gas (anode and top grid). The 0.203 mm thick electro- formed meshes, are made of electropolished 304 stain-