5 FIG. 4: (Color online) Photo of the XENON10 detector and shield, with the shield door open. FIG. 6: (Color online) Simulated electrostatic field in XENON10. Arrows and colors indicate the field direction and strength, respectively. The field cage was optimized for field uniformity in the sensitive region. The region below the cathode shows a strong reversed field. FIG. 5: (Color online) Photo of the assembled XENON10 TPC structure. and the mean XY position from the proportional scin- tillation was negligible ( < 0 . 6 mm). The impact of the field distribution in the gas on the energy resolution of the detector was also studied. For the inner region of the meshes, it was found that the relative path length vari- ation of electrons traversing the gap from the liquid-gas surface to the anode mesh is ≤ 20% FWHM. The active LXe volume is viewed by 89 PMTs: the bottom array of 41 PMTs is located 1.2 cm below the cathode, fully immersed in LXe. The 48 PMTs of the top array are in the gas. The XENON10 PMTs are 1” × 1” square Hamamatsu R8520-06-AL [31] designed to work in LXe and optimized for low radioactivity. They have a bialkali photocathode and a quartz window, with a typical quantum efficiency > 20% at 178 nm. They are compact (only 3.5 cm tall), metal channel PMTs, with 10 multiplication stages and a total amplification of a few 10 $^{6}$. They are certified for operation at -100 $^{◦}$C and up to 5 bar. The radioactivity of these PMTs was mea- sured with a ultra sensitive HPGe detector [32] at the Gran Sasso Low Background facility [33]. Their contam- ination in $^{238}$U/$^{232}$Th/$^{40}$K/$^{60}$Co was measured as low as (0.25 ± 0.04)/(0.21 ± 0/05)/(9.3 ± 1.1)/(0.59 ± 0.05) mBq/PMT. A photo of the top array, mounted inside the “diving bell” structure which defines the liquid level, is shown in Fig. 7. The custom-made cathode HV feedthrough and the level meter are also visible. The high voltage for the PMTs is provided by CAEN 1833 power supply boards [29]; this system featuring single channel control, allowed to easily equalize the gain of the PMTs, by ad- justing individual voltages. The HV divider for the PMTs was mounted on a Cirlex substrate, with surface mount components. The Cirlex boards for each of the two arrays are mounted on cop- per plates with PTFE spacers for insulation. Despite the small mass, the ceramic of the resistors and capacitors contribute the largest fraction of the radioactive back- ground of the HV divider boards. Heating of the LXe by the resistive dividers is kept to a minimum by choos- ing high resistance values (10 MΩ), allowed because of the low event rate and low light level during dark matter search data-taking. The typical power consumption of the XENON10 PMTs was about 5 mW per PMT. C. PMTs and Calibration System