10 TABLE I: Results from radioactive measurements for relevant shield components (values in mBq/kg). FIG. 15: (Color online) Schematics for the XENON10 Data Acquisition System electronics. V. THE XENON10 DATA PROCESSING AND ANALYSIS A. S1 & S2 Pulse Identification $^{238}$U $^{232}$Th $^{210}$Pb $^{235}$U $^{40}$K $^{137}$Cs $^{60}$Co $^{54}$Mn Pb (inner) < 3 . 9 < 6 . 8 (17 ± 5) × 10 3 < 20 < 28 < 0 . 85 < 0 . 19 Pb (outer) < 1 . 6 < 5 . 7 (560 ± 90) × 10 3 < 51 14 ± 6 < 2 . 1 < 1 . 1 Polyethylene < 5 . 2 < 6 . 6 < 3 . 2 < 64 < 2 . 6 Steel (I-beams) 7 . 8 ± 3 . 2 < 4 . 1 < 3 . 1 < 15 < 1 . 4 170 ± 3 3 . 3 ± 0 . 6 Steel (side panels) < 3 . 5 < 4 . 7 < 1 . 8 < 26 < 2 . 2 2 . 4 ± 0 . 7 Steel (ceiling plate) < 8 . 3 < 8 . 7 < 5 . 7 < 42 < 2 . 8 2 . 9 ± 1 . 2 2 . 0 ± 0 . 8 S2 events of 182 phe or greater. Because a single ionized electron extracted from LXe typically yields 24 ± 5 pho- toelectrons (after taking into account light collection and PMTs quantum efficiency), 182 photoelectrons in the S2 signal corresponds to 7 − 8 ionized electrons. This is the expected charge from an event with about 1 keV nu- clear recoil equivalent energy [4]. The trigger signal is distributed from the discriminator to all ADC modules simultaneously using a Fan-Out chain. The S2 trigger provides a lower threshold than is possible with an S1- based trigger (as is evident from Fig. 14); moreover, an S2-trigger allows for elimination of S1-only events due to scatters in the charge-insensitive regions of the detector, thus reducing the trigger rate. A trigger hold-off is used to prevent the system from triggering on after-pulsing following a large S2 pulse. The hold-off requires that no S2 trigger candidates occur for the previous 200 µ s be- fore allowing a trigger to start acquisition. The hold-off time is included in computing the acquistion dead time. For gamma calibration data runs, a high energy S1 veto is added to reduce the trigger rate. The veto is based on the sum of the 9 center bottom array PMTs and eliminates events with an S1 > 150 keVee. Using the S2-trigger and no high energy veto, the acquisition rate during WIMP search mode is ∼ 2.5 Hz, leading to a dead time of 7%. The raw data for an event consist of 88 waveforms of 16350 samples and is reduced to physical parameters us- ing two fully separate and parallel analysis chains. One is written in ROOT [45], and the other uses the commer- cial MATLAB software; both were specifically developed for the XENON10 data analysis. The high-level struc- ture of the data reduction is similar in both cases, and proceeds in three stages: i) preprocessing the waveforms, ii) searching for pulses, and finally iii) computing the re- duced quantities associated with each scintillation pulse. In the preprocessing stage, the samples of each PMT waveform are zeroed if they are below a threshold of ∼ 1 mV, which is much lower than the single photoelectron level and which depends on the noise characteristics of the channel. The waveforms of all channels are then added into a total waveform that is used to search for S1 and S2 pulses. The pulse searching stage operates in two steps: it first looks for S2-like pulses in the entire waveform and then looks for S1-like pulses that precede the first S2-like pulse. Fig. 16 shows a typical events with S 1 and S 2 and their hit patterns on the PMTs. The S2 pulse finding algorithm starts by applying a digital filter to the entire waveform to smooth out the high frequency components in order to facilitate the de- tection of the extent of pulses. It then searches the fil- tered waveform for regions where the signal exceeds a threshold of 10 mV for at least 0 . 6 µs , a time interval large enough to contain at least one S2 pulse, and for which the preceding and following 0 . 2 µs have an average signal less than 5% of the maximum within the interval. Because of the long afterpulsing tails that follow large S2 pulses the interval above threshold will often contain mul- tiple pulses. The algorithm then recursively searches for S2 like pulses within that interval. This is done by com- puting the extent of any potential pulse by starting from its maximum sample in the interval and going backward in the trace until either the signal drops below 0.1% of its maximum or the slope of the signal changes sign. This defines the left boundary of this pulse. The same pro- cedure is repeated going forward in the trace to find the right boundary. If the pulse found has a FWHM larger