9 tegrated rate of neutron interactions in XENON10 is cal- culated to be < 0 . 015 neutron/month in the dark matter region of interest. The muon flux in the cavern was mea- sured to be 1 . 06 ± 0 . 03 muon /m $^{2}$/hour [40]; from this, and the muon-induced neutron yield in rock and in Pb, the expected rate for cosmogenic neutrons is expected to be about 13 times higher [41] than from natural radioac- tivity. The internal shield cavity is 107 . 5 × 90 × 90 cm, of which the detector occupies about 30% of the volume. Care was taken not to leave any trapped air pockets between the lead and polyethylene, or anywhere else in the struc- ture. An additional 15 cm HDPE is also present be- low the shield structure. All seams were coated with a low-radioactivity silicon-gel, and a 10 mm thick rubber seal runs in a 5 mm deep trench along the perimeter of the polyethylene on the door; thus the cavity is able to sustain a slight over-pressure, and boil-off N$_{2}$ from a low-pressure dewar is passed continuously into the top of the cavity at a rate of 1 . 5 ± 0 . 1 standard liter/min, and allowed to exit at the bottom. The concentration of $^{222}$Rn in the shield cavity, typically about 130 Bq/m$^{3}$, was reduced by the N$_{2}$ purge to a value of < 5 . 5 Bq/m 3 (consistent with the sensitivity floor of the RAD-7 radon measuring device [42]). The purge process occurred in about 30 hours. The expected contribution to the elec- tron recoil background ( E < 25 keVee) from this level of Rn activity is 7 × 10 − 3 dru in the inner fiducial volume (Fig. 13). The shield structure is secured by steel panels along the outer walls, with bolts passing through the HDPE sheets. The XENON10 detector is supported by stainless steel I-beams on a single wall, which also serves as a door and can be moved laterally on rails. All cryogenic and electronic feed-throughs pass through this wall, using a Z-shape where possible, and as far off-center as possible, to minimize line-of-sight. The overall diameter of each opening was also minimized, and gaps in the Pb (after placing the cables) were filled with low-radioactivity Pb shavings. Silicon-gel was used to fill holes in the HDPE, also ensuring air-tightness in these locations. During the construction all components were cleaned and prepped with Ethanol before mounting them on the shield struc- ture. no cables or pipes have to be disconnected when opening the door to access the cryostat. To maximize the information available from time struc- ture and amplitude of the primary (S1) and secondary (S2) pulses, the signal from 89 PMTs are individu- ally digitized by an ADC array at 105 MHz (about 10 ns/sample). The S1 pulses from the direct scintilla- tion light of a typical background event in the energy re- gion of interest have 4-27 p.e. distributed over a width of < 200 ns (with a characteristic decay time of ∼ 27 ns [43]). IV. THE XENON10 DATA ACQUISITION SYSTEM FIG. 14: (Color online) S1 and S2 pulses for 2.3 keVee gamma event in XENON10 background data. Signals from each PMT hit are plotted separately. An enlarged view of the S1 pulse (5 p.e.) is also shown. See [40] for more details. The digitizers are Struck 3301 fast ADC (VME bus) modules [44], with sampling frequency 105 MHz and 14- bit ADC resolution (+0.1 V → -1.9 V). The signal from each PMT is amplified using Phillips PS776 × 10 am- plifiers, and a single-pole RC filter (30 MHz) at the in- put to the ADC prevents aliasing. The filter also has a slight shaping effect; because a single photo-electron pulse width is similar to the inverse sample rate of the digitizer, this improves the single photoelectron sensitiv- ity (see Fig. 14). An on-line software baseline-flattening algorithm sets small fluctuations in the baseline ( < ± 8 digitizer bins) to zero, allowing for very efficient lossless data compression by a factor of ∼ 20 using open source software (gzip). The data is not further modified or pro- cessed before being recorded to disk. The sum of the 30 central PMTs of the top array pro- vides the global S2-sensitive event trigger (see Fig. 15). The summed signal is amplified × 10 by a CAEN N968 with integration time constant 1 µ s; this value was se- lected based on the width of the S2 signal. The shaped signal is then passed to a CAEN voltage discriminator. The S2 trigger was verified to have 100% efficiency for a square pulse voltage input of width 1 µ s and a step size 16 mV. Since a typical photoelectron from a PMT with 2 × 10 6 gain has an integrated area of 16 mV · ns, this is equivalent to 100 photoelectrons. Considering that about 55% of S2 scintillation is recorded by the top PMT ar- ray, it is inferred that the trigger efficiency is 100% for 10 20 30 40 50 60 70 80 90 100 0 5 10 15 20 25 30 µ s phe/sample ←  0.20  µ s  → phe x10 2573 phe 5 phe The S2 pulses from the secondary proportional scintilla- tion have × 200 the number of photoelectrons distributed over about 1 µ s. The distance between the S1 pulse and the S2 pulse vary from 0 to 80 µ s, the maximum drift time for electrons extracted in the liquid. Events with multiple scatters in the active Xe volume will have more than one S2 pulse. To guarantee that the S1 pulse and all S2 pulses associated with an event are digitized, a full waveform of 160 µ s is recorded for each event.