17 FIG. 30: log$_{10}$( S 2 /S 1) vs. S 1 distribution from neutron in- teractions in the XENON10 detector. The data show two different regions: region a) which corresponds to recoils from elastic scatters and defines the nuclear recoil band, and region b) which corresponds to recoils from inelastic scatters. FIG. 29: (Color online) Number of scintillation photons ( N$_{ph}$ ) and ionization electrons ( N$_{e}$ ) per unit energy from gamma rays interaction in liquid xenon via photo-absorption for dif- ferent stopping power ( dE/dx ) at the operating field (0.73 kV/cm) of XENON10. Values of dE/dx corresponding to dif- ferent energy of electrons (from photo-absorption of gamma rays) can be obtained from the ESTAR database [55]. The to- tal number of quanta (photons plus electrons) per unit energy does not depend on the energy of gamma rays. FIG. 28: (Color online) Energy dependence of resolutions in liquid xenon for S 1 (scintillation) only, S 2 (ionization) only and combined (scintillation plus ionization) energy spectra, obtained for gamma rays below 1 MeV in XENON10. C. Nuclear/Electron Recoil Discrimination The success of XENON10 as a dark matter detec- tor hinges in large part upon its ability to discriminate electronic recoils from nuclear recoils, which in turn re- quires adequate definition of the detector response to such events based on calibration data. In addition to the formation of excitons, recoiling particles will produce a population of ionized electrons, many of which promptly recombine with their parent ions. Under an applied elec- tric field, the relative number of recombining electrons decreases. However, because nuclear recoils have a char- acteristically higher ionization density than electronic re- coils, fewer electrons escape recombination from recoiling nuclei than electrons, for a given energy and drift field. Electrons which recombine contribute to the prompt scintillation signal (S1), while those which escape recom- bination are drifted to the anode in the gas and produce the proportional signal (S2). The relative strength of re- combination for a given event can be measured by the ratio S2/S1, and hence this parameter can be used to discriminate between recoiling species. Figure 31 shows the behavior of Log$_{10}$(S2 / S1) as a function of energy , in unit of keV electron-equivalent (keVee), for popula- tions of both recoil types, called the electronic and nu- clear recoil bands, or ER and NR bands, respectively. The main purpose of such ER and NR calibrations is to identify a region in Log$_{10}$(S2 / S1) vs. S1 space, called the WIMP acceptance window, which should be nearly free of ER events while covering a significant portion of the NR band. The lower bound of this window along the horizontal axis is determined by the detector’s S1 threshold, and the corresponding upper bound is chosen to maximize the potential integrated WIMP rate while minimizing the effects of anomalous background events which occur mostly at higher energies (see section VII B). The choice of bounds along the vertical axis are discussed here. Data were taken with the $^{137}$Cs source throughout all of November 2006, and intermittently from Decem- ber 1 through February 14, 2007, accumulating a to- tal of ∼ 2100 events (after quality and fiducial cuts) in the WIMP acceptance energy window, 4.4 p.e. < S1 < 26.4 p.e. Fluctuations in Log$_{10}$(S2 / S1) over most of Energy [keV] 0 100 200 300 400 500 600 700 800 900 1000 /E) [%] ! Resolution ( 2 4 6 8 10 12 14 16 18 20  [%] E R = 43.0/   + 6.0 [%] E R = 109.7/   [%] E R = 120.0/  S1 only S2 only S1&S2 combined σ$_{E}$ E = 3 ▷ 5 √ E◁ MeV + 6 ▷ 0[%] σ$_{E}$ E = 3 ▷ 8 √ E◁ MeV [%] σ$_{E}$ E = 1 ▷ 4 √ E◁ MeV [%] (S2/S1) [p.e./p.e.] 10 log 0 0.5 1 1.5 2 2.5 3 3.5 4 a) b) S1 [keVee] (2.2 p.e./keVee) 20 40 60 80 100 120