Scintillation Efficiency of Nuclear Recoils in Liquid Xenon T. Wongjirad, L. Kastens, A. Manzur, K. Ni, and D.N. McKinsey Yale University
Scintillation Efficiency! By Definition: Ratio of light produced by a nuclear recoil to the light produced by an electron recoil of the same energy In Practice: Convert signal -- calibrated to electron recoils -- into nuclear recoil energy S1 signal in pe Light Yield for 122 kev electron recoils in pe/ kevee (with field) Relative scintillation efficiency of NR to 122 kev gammas at zero field E nr = S1/L y /(L eff S nr /S er ) Reduction in light due to electric field for electron recoils (er) and nuclear recoils (nr)
Motivation WIMP interacts via nuclear recoil The scintillation efficiency for nuclear recoils is not well understood at energies below 10 kev - within the range of energies probed by the XENON10 experiment Effect of electric field not measured extensively The uncertainty in the Scintillation Efficiency is the dominant contribution to the uncertainty in the XENON10 WIMP-nucleon cross-section limit An accurately determined Scintillation Efficiency at low energies is crucial for calculating the WIMP sensitivity for future dark matter experiments (e.g. XENON100 and LUX)
Recent World Values (zero field) XENON10 SEARCH WINDOW* VALUE USED BY XENON10 19% Arneodo 2000 Akimov 2002 Aprile 2006 Chepel 2006 *USING 0.19
STAINLESS MESH APPLY E-FIELD Experimental Setup! 3 INCHES SCINTILLATOR COINCIDENCE DEFINES RECOIL ANGLE AND THUS ENERGY 1 meter MESH
122 kev GAMMA HIGH QE PMTS: HAMAMATSU 9869MOD (~37%) LIGHT YIELD ~11 phe/kevee
Purity measured in two-phase
Monte Carlo: GEANT4 MONTE CARLO SPECTRUM: DETECTOR ENERGY RESOLUTION AND SCINTILLATION EFFICIENCY APPLIED 35 DEGREES (7.7 kevr) SINGLE ELASTIC RECOIL IN LXE INELASTIC LXE RECOIL kevee MULTIPLE ELASTIC LXE RECOILS PURE SINGLE ELASTIC LXE RECOIL TIME OF FLIGHT CUT
DATA: 110 DEGREES (~56.9 kevr) RECOIL ENERGY TIME OF FLIGHT SCINTILLATOR PULSE SHAPE VS PULSE AREA CUT Cuts Used: SCINTILLATOR PULSE SHAPE CUT QUALITY CUTS (SATURATION, NOISE) TIME OF FLIGHT CUT
DATA: 35 DEGREES (7.7 kevr) RECOIL ENERGY TIME OF FLIGHT SCINTILLATOR PULSE SHAPE VS PULSE AREA CUT Cuts Used: SCINTILLATOR PULSE SHAPE CUT QUALITY CUTS (SATURATION, NOISE) TIME OF FLIGHT CUT
Data/Monte Carlo Analysis MONTE CARLO SPECTRUM CONSTRUCTED WITH A SCINTILLATION EFFICIENCY EQUAL TO 0.158 35 DEGREES (7.7 kevr) DATA MONTE CARLO MONTE CARLO PURE SINGLE ELASTICS χ 2 reduced =0.99
Results: No Field (preliminary) XENON10 SEARCH WINDOW* INCLUDES SYSTEMATIC AND STATISTICAL UNCERTAINTY VALUE USED BY XENON10 19% THIS WORK Aprile 2006 Chepel 2006 *USING 0.19
Results: with Field (preliminary) S n =0.955 ± 0.036 S n =0.950 ± 0.048 (0.73 kv/cm) (1.50 kv/cm) INCLUDES SYSTEMATIC AND STATISTICAL UNCERTAINTY ZERO FIELD 0.73 kv/cm 1.5 kv/cm
Sources of Error Statistical Uncertainty: <10% (On the order of several hundreds of events per angle) Calibration uncertainty from PMT gain and light yield drifting: Variation in the measured Light Yield over a period of a couple of months is <5% Background events: Cut with TOF, Use MC to help pick cut Error in Scintillator Position Threshold Efficiency: Relevant for low energy nuclear recoils (below 6 kevr) Analysis is on-going
Future Directions Currently taking data in two-phase mode Triggering on proportional light to test threshold effect Verify single-phase data Measure charge yield for mono-energetic nuclear recoils: look for anti-correlation between light and charge
Monte Carlo Procedure (back up) Generate events with GEANT4.9.0.p01 Convert energy of points to kevee using different Scintillation efficiencies Also added energy resolution Compared to data using Chi-2 Best fit gives Scintilation Efficiency
Monte Carlo: GEANT4 25 DEGREES (~4 kevr) SINGLE ELASTIC RECOIL IN LXE INELASTIC LXE RECOIL MULTIPLE ELASTIC LXE RECOILS PURE SINGLE ELASTIC LXE RECOIL (1 MINUTE) TIME OF FLIGHT CUT
Monte Carlo/Data Comparison: Decent agreement with different Time Of Flight Cut (back up) TOF 15-21 TOF 21-26 TOF 27-32 TOF 32-40 35 DEGREES
Monte Carlo: Understanding Efficiency (back up) Use Monte Carlo at higher energies to help tune the applied threshold efficiency 125 DEGREES UNTUNED TUNED
Monte Carlo/Data Comparison:Light Efficiency Uncertainty (back up) 125 DEGREES BEST FIT LIGHT EFFICIENCY = 18.8% +1.2% -1.4%
15.00 Measured Mean (kevee) Vs. Recoil Energy (kevr) 11.25 7.50 3.75 0 17.5 35.0 52.5 70.0 Peak Energy (kevee)
Monte Carlo: Understanding Efficiency (back up) Use Monte Carlo to characterize threshold efficiency Curve mostly determined by Trigger level PMT gain dispersion