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background data, a central fiducial volume was chosen to minimize background while maintaining the largest pos- sible target mass. The fiducial volume was pre-defined for the blinded analysis with a radius of 80 mm and a drift time between 15 and 65 µ s, corresponding to a fidu- cial mass of 5.4 kg. Figure 37 shows the fiducial volume overlaid on the background rate distribution of single re- coil events between 2-30 keVee (2.2 pe/keVee). Within the chosen fiducial volume, the overall background rate is reduced by almost a factor of 10, as shown in Figure 38. 

A comparison between the measured background rate and the projected background from detailed Monte Carlo simulations, shown in Figure 39, reveals a dom- inant contribution from the stainless steel vessels, fol- lowed by approximately equal contributions from PMTs, feedthroughs, and Teflon. The contribution from intrin- sic contamination of $^{85}$Kr, Rn, $^{136}$Xe in the liquid xenon, or nuclear recoils from ( α ,n) reaction or fission of $^{238}$U in the surrounding environment is subdominant or neg- ligible in the entire WIMP search run. Details of the simulations and understanding of background contribu- tions are discussed in Appendix A. 

## B. Definition of anomalous events and cuts 

During the analysis of both gamma calibration data and unblinded background data from XENON10, a class of anomalous single scatter events with smaller S 2 /S 1 ratio than that of normal single scatter electron recoil events were observed. These events produce a non- Gaussian tail extending into the acceptance region for nuclear recoils and to even lower S 2 /S 1 ratios. Most of these anomalous events are likely due to the coinci- dence of two gamma-ray interactions, where a single in- teraction occurs in the sensitive volume and at least one other interaction occurs in a region of the detector that 

is light-sensitive but not charge-sensitive. Such events, dubbed Gamma-X , may comprise two Compton scatters of a high-energy gamma-ray, or, e.g., coincident gamma- rays from a cascade, such as $^{60}$Co from the stainless steel cryostat. There are a few regions of the XENON10 detec- tor that are light-sensitive but not charge-sensitive, such as the region between cathode and bottom PMTs or the 

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