Simultaneous Measurement of Ionization and Scintillation from Nuclear Recoils in Liquid Xenon for a Dark Matter Experiment

We report the first measurements of the absolute ionization yield of nuclear recoils in liquid xenon, as a function of energy and electric field. Independent experiments were carried out with two dual-phase time-projection chamber prototypes, developed for the XENON dark matter project. We find that the charge yield increases with decreasing recoil energy, and exhibits only a weak field dependence. These results are the first unambiguous demonstration of the capability of dual-phase xenon detectors to discriminate between electron and nuclear recoils down to 20 keV, a key requirement for a sensitive dark matter search.

Figure 1

Columbia detector response to AmBe neutron and ${}^{137}\mathrm{Cs}$ gamma sources, at $2.0\mathrm{kV}/\mathrm{cm}$ drift field.

Figure 2

Case detector response to ${}^{252}\mathrm{Cf}$ neutron and ${}^{133}\mathrm{Ba}$ gamma sources at $1.0\mathrm{kV}/\mathrm{cm}$ drift field.

Figure 3

Field dependence of scintillation and ionization yield in LXe for 122 keV electron recoils (ER), 56.5 keVr nuclear recoils (NR) and alphas.

Figure 4

Energy dependence of nuclear recoil ionization yield in LXe at different drift fields.

Figure 5

Predicted electronic stopping power, $dE/dx$, for different particles in LXe. The circles indicate the particle energies discussed in the text.