Scintillation Efficiency And Ionization Yield Of Low Energy Nuclear Recoils In Liquid-xenon Dark Matter Detection

Liquid xenon(LXe) detectors, among many direct detection experiments which have been proposed and run in the last two decades, have shown particular promising in the detection of weakly interacting massive particles(WIMPs), an attractive candidate for Dark Matter(DM), by observing the atomic recoils after WIMPs’ elastic scattering on nuclei. The elastic scattering will produce a recoiling xenon atom, also called nuclear recoil, with kinetic energy up to a few tens keV. It excites and ionizes xenon atoms, giving rise to scintillation signals S1 through the de-excitation of excitons and recombination of electron-ion pairs, and ionization signals S2 through the electrons escaping from recombination, respectively.Two crucial properties of LXe detectors are the so-called relative scintillation ef-?ciency Leffand ionization yield Qy, which serve as bridges between the detected S1 and S2 signals and the deposited energy of the WIMPs in LXe detector. Leffor Qy,together with the detected S1 or S2 signals, is used to reconstruct and calibrate the initial nuclear recoils energy, hence study the properties of the WIMPs. The biggest challenge for experiments to measure Leffand Qylies in very low energy nuclear recoils, particularly at the detection threshold, where most of the recoiling events will be if the mass of WIMPs is around several GeV/c2. In this thesis,the LXe scintillation and ionization process are analyzed in details and a state-of-art theoretical analysis of the Leff and Qy in the very low energy region has been performed.Based on Lindhard’s basic integral equation and the binary collision approximation, a computer program, which re?ects our understanding on the slowing down process of the recoiling nucleus in liquid xenon, is developed to calculate how much nuclear recoil energy is dissipated to electrons in the medium, hence produces scintillation and ionization signals at last, which is the so-called nuclear quenching factor qncor Lindhard factor. To obtain an accurate nuclear quenching factor at low energy region,existing theoretical models and experimental data for electronic energy dissipation:electronic stopping power Se, are reviewed and analyzed. We improve transport cross section method and re-calculate the electronic stopping power at low energy region.The theoretical prediction for Sein liquid xenon agrees with the experimental data very well. To evaluate the electron-ion pair recombination rate, the different behaviors for the electron-ion pair recombination process regarding electron recoils, alpha recoils, and nuclear recoils, are studied and the Thomas-Imel box model is generalized to describe the recombination behaviors regarding nuclear recoils. At last the recombination rate can be expressed as a function of nuclear recoil energy and the applied electric ?eld.Combining the electronic energy dissipation from the computer simulation and the generalized Thomas-Imel box model, we predict the Leffand Qyat low energy region. The predictions from this work agree well with the measured Leffand Qyfrom the neutron scattering experiments. The predicted Leffsuggests a rapid drop when the recoiling energy comes below 3 keV where authors have pointed out the liquid xenon scintillation response should drop steadily at low energy. The predicted Qyincreases with the decreasing of the recoiling energy and reaches the maximum value at 2～3 keV,which may be examined by experiment in the future and lower the energy threshold for nuclear recoils to ~1 keV.